Status

Current state: Under Discussion Accepted

Discussion thread: Thread 1 and Thread 2here

JIRA: here

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Note this is a joint worked work proposed by David Jacot, guozhang Wang and Jason Gustafson.

...

When a client side assignor is used, the group coordinator requests the assignment from one group member by notifying him it via the heartbeat protocol. The chosen member uses the ConsumerGroupPrepareAssignment API and the ConsumerGroupInstallAssignment API to respectively get the current state of the group and to install the computed assignment. Thanks to this, the input of the client side assignor is entirely driven by the group coordinator. The consumer is no longer responsible for maintaining any state besides its assigned partitions.

The new protocol’s RPCs are specified in the details in the public interfaces section of this document while the details of the rebalance logic is described in the next chapter.

Group Coordinator

Implementing the new rebalance protocol in the current group coordinator is not appropriate in our opinion because its requires many changes anyway in the current protocol to make it interoperable. Therefore, this KIP proposes to rewrite the group coordinator from scratch in Java. The new This KIP proposes to evolve the group coordinator to rely on an event loop. The rational of using an event loop is that 1) it simplifies the concurrency and 2) enables simulation testing. The group coordinator will have a replicated state machine per __consumer_offsets partitions, where each replicated state machine is modelled as an event loop. Those replicated state machines will be executed in group.coordinator.threads threads.

...

...

The rebalance timeout is provided by the member when it joins the group. It is basically the max poll interval configured on the client side. The timer starts ticking when the heartbeat response request is sent out processed by the group coordinator.

...

The group coordinator has to determine which assignment strategy must be used for the group. The group's members may not have exactly the same assignors at any given point in time - e.g. they may migrate from an assignor to another one for instance. The group coordinator will chose the assignor as follow:

• The server A client side assignor is used if any member specified one. If multiple server side assignors are specified in the group, the group coordinator uses the most common one.The client side assignor is used otherwise. The group coordinator will use the assignor which is possible. This means that a client side assignor must be supported by all the members in the group and, if . If multiple are, it will respect the precedence defined by the members when they advertise their supposed supported client side assignors.

Server Side Mode

...

• A server side assignor is used otherwise. If multiple server side assignors are specified in the group, the group coordinator uses the most common one. If a member does not provide an assignor, the group coordinator will default to the first one in group.consumer.assignors.

Server Side Mode

The server side assignor is pluggable and the client pluggable and the client can choose the one that it wants to use by providing its name in the heartbeat request. If the selected assignor does not exist, the group coordinator will reject the heartbeat with an UNSUPPORTED_ASSIGNOR error. The list of supported assignors will be configured in the broker configuration.

We will support two assignors out of the box for Apache Kafka:

• range - org.apache.kafka.server.group.consumer.RangeAssignor - An assignor which co-partitions topics.
• uniform - org.apache.kafka.server.group.consumer.UniformAssignor - An assignor which uniformly assign partitions amongst the members. This is somewhat similar to the existing "sticky" assignor.

...

• The group coordinator selects a member to run the assignment logic. The selection is explained later in this chapter.
• The group coordinator notifies the member to compute the new assignment by returning the COMPUTE_ASSIGNMENT error setting the ShouldComputeAssignment field in its next heartbeat response.
• When the member receives this error, it is expected to call the ConsumerGroupPrepareAssignment API to get the current group metadata and the current target assignment.
• The member computes the new assignment with the relevant assignor.
• The member calls the ConsumerGroupInstallAssignment API to install the new assignment. The group coordinator validates it and persists it.

...

The chosen member is expected to complete the assignment process within the rebalance timeout. The time on the coordinator side starts ticking when the member is notified. If the process is not completed within the rebalance timeout, the group coordinator picks up another member to run the assignment. Note that the previous chosen member is not fenced here because the fencing is only done based on the session.

Metadata Version Handling (KIP-268)

...

• All partitions are assigned.
• A partition is assigned only once.
• All members existsmember ids are valid. They must correspond to members in the group.

Note that this validation is made with regarding to the metadata used to compute the assignment. The group may have already advanced to a newer group epoch - e.g. a member could have left during the assignment computation.

...

The member uses the ConsumerGroupHeartbeat API to establish a session between him and the with the group coordinator. The member is expected to heartbeat every group.consumer.heartbeat.interval.ms in order to keep its session opened. If it does not heartbeat at least once within the group.consumer.session.timeout.ms, the group coordinator will kick him the member out from the group. group.consumer.heartbeat.interval.ms is defined on the server side and the member is told about it in the heartbeat response. The group.consumer.session.timeout.ms is also defined on the server side.

...

The member joins the group by sending an heartbeat with no Member ID and a member epoch equals to 0. He can rejoins the group with a with a member epoch equals to 0. He can leaves the group by using a member epoch equals to -1. The member must commit its offsets before leaving.

Fencing

The group coordinator ensures that requests comes from a known Member ID. Any request is rejected with the UNKNOWN_MEMBER_ID error otherwise. It also ensures that the Member Epoch matches the expected member epoch. If not, the request is rejected with the FENCED_MEMBER_EPOCH error. Details for every API are given in the Public Interfaces section.In this case, the member is expected to immediately gives up all its partitions and rejoin the group with the same member ID and a member epoch equals to zero. Details for every API are given in the Public Interfaces section.

Static Membership Static Membership (KIP-345)

Static membership, introduced in KIP-345, is still supported by this new rebalance protocol. When a member wants to leave temporary – e.g. while being bounced – it should send an heartbeat with a member epoch equals to -2. This signals to the group coordinator that the member left but will rejoin within the session timeout. When the member rejoins with the same Instance instance ID, the group coordinator replaces the old member with by the new member . The new member can continue from where it left offand gives back its current assignment.

If the leaving member does not rejoin within the session timeout, the group coordinator kicks it out from the group. If a new member joins with an existing instance ID before the previous member left, the new member is rejected with a UNRELEASED_INSTANCE_ID error as long as the previous member is still present.

Consumer Group States

EMPTY

...

The new rebalance protocol relies on server side configurations such as group.consumer.heartbeat.interval.ms and group.consumer.session.timeout.ms. Our goal is to give administrator the ability to use and tweak those settings for their entire consumers fleet. However, it may not always be possible to have values fitting all workloads. Therefore, we propose to extend the IncrementalAlterConfigs and the DescribeConfigs API to support a new resource type called GROUP. This allows users to override the default defined by the administrators. The dynamic group configurations are described in the Public Interfaces section.

The group coordinator is responsible for storing those group configurations in order to keep their lifecycle tight to their group. When a group is deleted, we want the configuration to be deleted as well. This assumes that IncrementalAlterConfigs and the DescribeConfigs API will be routed to the group coordinator owning the group they are acting uponconfigurations are stored in the controller like all the other dynamic configurations in the cluster. This allows configurations to be installed independently from whether the group exists or not. Configurations are also preserved if the group is deleted. In ZK mode, the dynamic group configurations will be store in the /config/groups znode. In KRaft mode, they are stored in the ConfigRecord.

Regex Based Subscription

...

The new group coordinator will rely on the metadata.version or IBP to enable the new protocol. The exact version will be determined when the feature is ready.

Persistence

Fail Over

When the Group Coordinator fails over, the newly elected coordinator will load the state from the We will introduce a new set of records to persist the new consumer group type in the existing __consumer_offsets topic. The records are detailed in the public interfaces section of this document.

Consumer

The semantics of the consumer will remain unchanged after this proposal is implemented. The goal is to swap the implementation of the group membership/assignment protocol by the new one.

Feature Flag

partition(s). When it is done with this, it has a few other duties for the newly loaded groups:

• It has to setup the session timeouts for all the members (like today).
• It has to check whether the topic-partition metadata has changed and potentially trigger a rebalance for the group if it has.
• It has to check whether new topics match the regex subscriptions and trigger a rebalance for the group if new topic do.
• It has to check whether a new assignment is required for the group (group epoch != assignment epoch). If it is the case, the group coordinator can directly compute it using the server side assignor or can trigger a client side assignment computation.

Persistence

We will introduce a new set of records to persist the new consumer group type in the existing __consumer_offsets topic. The records are detailed in the public interfaces section of this document.

Consumer

The semantics of the consumer will remain unchanged after this proposal is implemented. The goal is to swap the implementation of the group membership/assignment protocol by the new one.

Feature Flag

A new configuration A new configuration setting will be used to determine whether the new protocol should be used or not. The feature flag allows the user to control when he starts using or migrating to the new protocol for its application. This is also required by our migration path as we will require to have the software on a specific version which is compatible with the new protocol.

...

The rebalance process in the consumer is basically the opposite of the process that was described earlier in this document. The consumer will know at any point in time its current epoch and the list of partitions that it owns. There are a few cases to consider:

Fenced

If the member is fenced by the group coordinator, it will immediately abandon all its partitions and call ConsumerRebalanceListener#onPartitionsLost. It will rejoin the group as a new member afterwards.

...

Revocation

When the member has got a new target assignment, the group coordinator will notify it that it has to revoke partitions if any partitions must be revoked. The member can determine the revoked partitions by computing the difference between its

...

current local assignment and the assignment received from the group coordinator. The consumer stops fetching from those revoked partitions and, If auto commit is enabled, commits their offsets

...

. Finally, it calls ConsumerRebalanceListener#onPartitionsRevoked and inform the group coordinator that the revocation process in its next heartbeat.

Assignment

When the member transitions to its next epoch, it will receives the its epoch, its assigned partitions and its pending partitions from the group coordinator. When the member transitions to its new epoch, PartitionAssignor#onAssignment is called if a customer assignor assignor is setup. PartitionAssignor#onAssignment receives the new metadata and the new target assignment for the member. The metadata is final for this epoch. The target assignment is also final for this epoch and contains partitions that are not revoked yet by other members. Therefore, the assignor does not know which of them are being fetched or not. It must rely on the ConsumerRebalanceListener for this. Finally, ConsumerRebalanceListener#onPartitionsAssigned is called with the assigned partitions and the consumer starts fetching those partitions.

Whenever new partitions are assigned to the member within the current epoch, the consumer calls ConsumerRebalanceListener#onPartitionsAssigned with those. This means that ConsumerRebalanceListener#onPartitionsAssigned can be called multiple times within an epoch. At most, it will be called N times where N is the number of assigned partitions in the current epoch.

Client-Side Assignor

By default, the consumer will entirely rely on the group coordinator but it will allow specifying a customer assignor on the client-side as already explained in this document. For this purpose, we propose to introduce a new and optional assignor interface in the Consumer called

...

Note that the process is dependent on Consumer#poll being called like with the current protocol. There is a parallel effort to this design to redesign the threading model of the consumers. This will very likely change how/when those callbacks are called.

Client-Side Assignor

By default, the consumer will entirely rely on the group coordinator but it will allow specifying a customer assignor on the client-side as already explained in this document. For this purpose, we propose to introduce a new and optional assignor interface in the Consumer called PartitionAssignor. The interface is specified in the public interfaces section of this document. The current assignor interface is strongly tied to the current group membership/assignment protocol so reusing it is not appropriate for two reasons:

• The new protocol does not really fit in the current interface and its semantic is different; and
• It seems preferable to let us evolve the current protocol independently if the need arises.

Deprecate Enforcing Rebalances

Consumer#enforceRebalance will be deprecated and will throw an IllegalStateException if used when the new protocol is enabled. Enforcing a rebalance with the new protocol does not make any sense. Instead, power users will have the ability to trigger a reassignment by either providing a non-zero reason or by updating the assignor metadata.

Streams

Kafka Streams remains a power user of the consumer so it will continue extending the consumer by providing an implementation of the new assignor interface. Streams will also rely on a feature flag to enable the new rebalance protocol.

Member Metadata & Assignment Metadata

Member Metadata refers to the metadata provided by a given member from its assignor. Assignment Metadata refers to the metadata computed by the assignor for the member. The Version, MinimumVersion, MaximumVersion, Reason and Error fields are now first class citizen in the rebalance protocol so Stream does not have the specify them in the metadata anymore. The schemas for respectively the assignor metadata and the assignment metadata are detailed in the Public Interfaces section.

Note that Streams may take this opportunity to do further changes to its metadata. We may extend this KIP or do a follow-up KIP in the future for this.

Assignor Behavior

The assignor behavior remains similar to the existing assignor. The major difference is that the assignor must serialize the assignment metadata of each member with the correct version used by the member. Another difference is that the new assignor must be able to handle the old metadata format as well during the upgrade from the old to the new protocol. This upgrade path is detailed in the upgrade section of this document.

Member Behavior

Upon receiving the assignment, each member would respectively create, close, or recycle tasks as indicated and update the global assignment information, like today. We explained earlier that partitions are incrementally assigned to the member when they are revoked by the others. This means that the assignment metadata may already reference partitions which are not assigned to the member yet. The Streams assignor must consider the assigned partitions as the source of truth in this case.

Each member encodes the lag of its standby tasks in its metadata. We can not update the lag in every heartbeat request because that would constantly trigger reassignment in the group. Instead, when a) the task lag has been reduced within the acceptable.recovery.lag threshold or b) the task lag is consistently increasing for some time, the member should consider triggering a rebalance by sending its next heartbeat with the appropriate encoded reason and the updated task lags. 

Supporting Online Consumer Group Upgrade

Upgrading to the new protocol or downgrading from it is possible by rolling the consumers, assuming that the new protocol is enabled on the server side, with the correct group.protocol. When the first consumer using the new rebalance protocol joins the group, the group is converted from a generic group to a consumer group. When the last consumer using the new rebalance protocol leaves the group, the group is converted back to a generic group. Note that the group epoch starts at the current group generation. During the migration, all the JoinGroup, SyncGroup and Heartbeat calls from the non-upgraded consumers are translated to the new protocol. How? The idea is to basically reconcile each member individually with the old protocol APIs. 

Before explaining how that will work, let's recapitulate how the current protocol workflow. First, the members join or re-join the group with the JoinGroup API. The JoinGroup request contains the subscriptions, the owned partitions, etc. When all the members are have joined, the group coordinator picks a leader for the group and sends back the JoinGroup response to all the members. The leader is responsible for computing the assignment. Second, all the members collects their assignment - computed by the leader - by using the SyncGroup API. In parallel, the members heartbeat with the Heartbeat API in order to maintain their session. The Heartbeat API is also used by the group coordinator to inform the members about an ongoing rebalance. All those interactions are synchronized on the generation of the group. It is important to note that the consumer does not make any assumption about the generation id. It basically uses what it receives from the group coordinator. At least, this is how the Java client works. The current rebalance protocol supports two modes: Eager and Cooperative. In the eager mode, the consumer revokes all its partitions before rejoining the group during a rebalance. In the cooperative mode, the consumer does not revoke any partitions before rejoining the group. However, it revokes the partitions that it does not own anymore when it receives it new assignment and rejoins immediately if he had to revoke any partitions.

The new protocol relies on the ConsumerGroupHeartbeat API to do all the above. Concretely, the API updates the group state, provides the partitions owned by the consumer, gets back the assignment, and updates the session. We can remap those to the old protocol as follow: the JoinGroup API updates the group state and provides the partitions owned, the SyncGroup API gets back the assignment, and the Heartbeat API updates the session. The main difference here is that the JoinGroup and SyncGroup does not run continuously. The group coordinator has to trigger it when it is need by returning the REBALANCE_IN_PROGRESS error in the heartbeat response.

The idea is to manage each members individually while relying on the new engine to do the synchronization between them. Each member will use rebalance loops to update the group coordinator and collect their assignment. The group coordinator will ensure that a rebalance is triggered when one needs to update his assignments. This process is detailed in the next sub-chapters:

Member States

Let's start by defining a state machine for each member:

• PrepareRebalance - The member enters this state when a rebalance is required. The member must rejoin within the rebalance timeout or the coordinator kicks it out from the group. We will discuss later when a rebalance is required.
• CompletingRebalance - The member enters this state when the join group has been received. The member must sync within the rebalance timeout or the coordinator kicks it out from the group.
• Stable - The member enters this state when it has completed the rebalance loop.

We use the terminology already use in the current protocol intentionally here in order to facilitate the reasoning.

JoinGroup Handling

The JoinGroup API is handled as follow:

Topic ID and Topic Recreation

With KIP-516, the Consumer already uses topic Ids on the fetch patch and on the metadata path. However, it does not use them for the assignment and the committed offsets. This KIP closes that gap. The new consumer rebalance protocol works only with topic ids and the OffsetFetch and OffsetCommit APIs are updated to use topic ids as well. This strengthen the semantic of the Consumer. The consumer will be able to correctly handle topic recreation. When a topic is deleted, the Consumer will re-resolve the name to get the new topic id. Then, it updates it subscriptions with the new topic id. The rebalance protocol will resign the old topic id and assign the new one to members so members will consider the topic as a new/different topic. Adding the topic id to the OffsetFetch and OffsetCommit ensures that the Consumer won't use the old committed offsets with the new topic as well. The processing of the new topic with start fresh based on the offset reset policy.

Note that we don't plan to update the Consumer's public API to expose topic ids at this stage so end users won't be able to detect the recreations yet. We might consider doing this in a future KIP.

Deprecate Enforcing Rebalances

Consumer#enforceRebalance will be deprecated and will be a no-op if used when the new protocol is enable. A warning will be logged in this case. Enforcing a rebalance with the new protocol does not make any sense. Instead, power users will have the ability to trigger a reassignment by either providing a non-zero reason or by updating the assignor metadata.

Streams

Kafka Streams remains a power user of the consumer so it will continue extending the consumer by providing an implementation of the new assignor interface. Streams will also rely on a feature flag to enable the new rebalance protocol.

Member Metadata & Assignment Metadata

Member Metadata refers to the metadata provided by a given member from its assignor. Assignment Metadata refers to the metadata computed by the assignor for the member. The Version, MinimumVersion, MaximumVersion, Reason and Error fields are now first class citizen in the rebalance protocol so Stream does not have the specify them in the metadata anymore. The schemas for respectively the assignor metadata and the assignment metadata are detailed in the Public Interfaces section.

Note that Streams may take this opportunity to do further changes to its metadata. We may extend this KIP or do a follow-up KIP in the future for this.

Assignor Behavior

The assignor behavior remains similar to the existing assignor. The major difference is that the assignor must serialize the assignment metadata of each member with the correct version used by the member. Another difference is that the new assignor must be able to handle the old metadata format as well during the upgrade from the old to the new protocol. This upgrade path is detailed in the upgrade section of this document.

Member Behavior

Upon receiving the assignment, each member would respectively create, close, or recycle tasks as indicated and update the global assignment information, like today. We explained earlier that partitions are incrementally assigned to the member when they are revoked by the others. This means that the assignment metadata may already reference partitions which are not assigned to the member yet. The Streams assignor must consider the assigned partitions as the source of truth in this case.

Each member encodes the lag of its standby tasks in its metadata. We can not update the lag in every heartbeat request because that would constantly trigger reassignment in the group. Instead, when a) the task lag has been reduced within the acceptable.recovery.lag threshold or b) the task lag is consistently increasing for some time, the member should consider triggering a rebalance by sending its next heartbeat with the appropriate encoded reason and the updated task lags. 

Supporting Online Consumer Group Upgrade

Upgrading to the new protocol or downgrading from it is possible by rolling the consumers, assuming that the new protocol is enabled on the server side, with the correct group.protocol. When the first consumer using the new rebalance protocol joins the group, the group is converted from a generic group to a consumer group. When the last consumer using the new rebalance protocol leaves the group, the group is converted back to a generic group. Note that the group epoch starts at the current group generation. During the migration, all the JoinGroup, SyncGroup and Heartbeat calls from the non-upgraded consumers are translated to the new protocol. How? The idea is to basically reconcile each member individually with the old protocol APIs. 

Before explaining how that will work, let's recapitulate how the current protocol workflow. First, the members join or re-join the group with the JoinGroup API. The JoinGroup request contains the subscriptions, the owned partitions, etc. When all the members are have joined, the group coordinator picks a leader for the group and sends back the JoinGroup response to all the members. The leader is responsible for computing the assignment. Second, all the members collects their assignment - computed by the leader - by using the SyncGroup API. In parallel, the members heartbeat with the Heartbeat API in order to maintain their session. The Heartbeat API is also used by the group coordinator to inform the members about an ongoing rebalance. All those interactions are synchronized on the generation of the group. It is important to note that the consumer does not make any assumption about the generation id. It basically uses what it receives from the group coordinator. At least, this is how the Java client works. The current rebalance protocol supports two modes: Eager and Cooperative. In the eager mode, the consumer revokes all its partitions before rejoining the group during a rebalance. In the cooperative mode, the consumer does not revoke any partitions before rejoining the group. However, it revokes the partitions that it does not own anymore when it receives it new assignment and rejoins immediately if he had to revoke any partitions.

The new protocol relies on the ConsumerGroupHeartbeat API to do all the above. Concretely, the API updates the group state, provides the partitions owned by the consumer, gets back the assignment, and updates the session. We can remap those to the old protocol as follow: the JoinGroup API updates the group state and provides the partitions owned, the SyncGroup API gets back the assignment, and the Heartbeat API updates the session. The main difference here is that the JoinGroup and SyncGroup does not run continuously. The group coordinator has to trigger it when it is need by returning the REBALANCE_IN_PROGRESS error in the heartbeat response.

The idea is to manage each members individually while relying on the new engine to do the synchronization between them. Each member will use rebalance loops to update the group coordinator and collect their assignment. The group coordinator will ensure that a rebalance is triggered when one needs to update its assignments. This process is detailed in the next sub-chapters:

Member States

Let's start by defining a state machine for each member:

• PrepareRebalance - The member enters this state when a rebalance is required. The member must rejoin within the rebalance timeout or the coordinator kicks it out from the group. We will discuss later when a rebalance is required.
• CompletingRebalance - The member enters this state when the join group has been received. The member must sync within the rebalance timeout or the coordinator kicks it out from the group.
• Stable - The member enters this state when it has completed the rebalance loop.

We use the terminology already use in the current protocol intentionally here in order to facilitate the reasoning.

JoinGroup Handling

The JoinGroup API is handled as follow:

• The basic validations are applied. The request is rejected with the appropriate error if the validation fails.
  • The group must exist.
  • The member must exist or be created.
  • The embedded protocol must be on version >= 3.
  • The generation id must match the current member epoch. The generation id is provided in the embedded protocol.
• The group state is updated if needed.
  • The group epoch is incremented if a new assignment is required. See "Group Epoch - Trigger a rebalance" chapter for details about the rebalance triggers.
  • The Topics provided in the embedded consumer protocol are used to update the SubscribedTopicNames.
  • The Protocols are converted to Assignors
• The basic validations are applied. The request is rejected with the appropriate error if the validation fails.
  • The group must exist.
  • The member must exist or be created.
  • The embedded protocol must be on version >= 3.
  • The generation id must match the current member epoch. The generation id is provided in the embedded protocol.
• The group state is updated if needed.
  • The group epoch is incremented if a new assignment is required.
  • The Topics provided in the embedded consumer protocol are used to update the SubscribedTopicNames.
  • The Protocols are converted to Assignors where the UserData become the metadata and the version is set to -1.
• The current member assignment is updated if needed:
  • If the member has revoked all its partitions or the required partitions, the member can transition to its next epoch. The current target assignment become becomes the current assignment. The group coordinator replies with the new member epoch as the generation id.
  • If the member has to revoke partitions, the group coordinator replies with the current member epoch as the generation id.
• The member transitions to CompletingRebalance state.

...

• FENCED_MEMBER_EPOCH - The member epoch does not correspond to the member epoch expected by the coordinator.is fenced by the coordinator. The member must abandon all its partitions and rejoins.
• STALE_MEMBER_EPOCH - The member epoch is stale. The member must retry after receiving its updated member epoch via the ConsumerGroupHeartbeat API.
• UNRELEASED_INSTANCE_ID - The instance ID is still used by another member. The member must leave firstCOMPUTE_ASSIGNMENT - The member has been selected by the coordinator to compute the new target assignment of the group.
• UNSUPPORTED_ASSIGNOR - The assignor used by the member or its version range are not supported by the group.

...

{
  "apiKey": TBD,
  "type": "request",
  "listeners": ["zkBroker", "broker"],
  "name": "ConsumerGroupHeartbeatRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
      { "name": "GroupId", "type": "string", "versions": "0+", "entityType": "groupId",
        "about": "The group identifier." },
      { "name": "MemberId", "type": "string", "versions": "0+",
        "about": "The member id generated by the servercoordinator. The member id must be kept during the entire lifetime of the member." },
      { "name": "MemberEpoch", "type": "int32", "versions": "0+",
 "default": "-1",
      "about": "The current member epoch; 0 to join the group; -1 to leave the group." },
      { "name": "InstanceId", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null",
        "about": "null itif not provided or if it didn't change since the last heartbeat; the instance Id otherwise." },
      { "name": "RebalanceTimeoutMsRackId", "type": "int32string", "versions": "0+",  "nullableVersions": "0+", "default": -1"null",
        "about": "-1null if not provided or if it didn't chancechange since the last heartbeat; the maximumrack timeID inof milliseconds that the coordinator will wait on the member to revoke its partitions consumer otherwise." },
      { "name": "SubscribedTopicNamesRebalanceTimeoutMs", "type": "[]stringint32", "versions": "0+", "nullableVersionsdefault": "0+", "-1,
        "about": "-1 if it didn't chance since the last heartbeat; the maximum time in milliseconds that the coordinator will wait on the member to revoke its partitions otherwise." },
      { "name": "SubscribedTopicNames", "type": "[]string", "versions": "0+", "nullableVersions": "0+", "default": "null",
        "about": "null if it didn't change since the last heartbeat; the subscribed topic names otherwise." },
      { "name": "SubscribedTopicRegex", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null",
        "about": "null if it didn't change since the last heartbeat; the subscribed topic regex otherwise" },
      { "name": "ServerAssignor", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null",
        "about": "null if not used or if it didn't change since the last heartbeat; the server side assignor to use otherwise." }, 
      { "name": "ClientAssignors", "type": "[]Assignor", "versions": "0+", "nullableVersions": "0+", "default": "null",
        "about": "null if not used or if it didn't change since the last heartbeat; the list of client-side assignors otherwise.",
      "fields": [
          { "name": "Name", "type": "string", "versions": "0+",
            "about": "The name of the assignor." },
          { "name": "MinimumVersion", "type": "int16", "versions": "0+",
            "about": "The minimum supported version for the metadata." },
          { "name": "MaximumVersion", "type": "int16", "versions": "0+",
            "about": "The maximum supported version for the metadata." },
          { "name": "Reason", "type": "int8", "versions": "0+",
            "about": "The reason of the metadata update." }, 
          { "name": "VersionMetadataVersion", "type": "int16", "versions": "0+",
            "about": "The version of the metadata." },
             { "name": "MetadataMetadataBytes", "type": "bytes", "versions": "0+",
            "about": "The metadata." }
      ]},
      { "name": "TopicPartitions", "type": "[]TopicPartitionTopicPartitions", "versions": "0+", "nullableVersions": "0+", "default": "null",
        "about": "null if it didn't change since the last heartbeat; the partitions owned by the member.",
      "fields": [
          { "name": "TopicId", "type": "uuid", "versions": "0+",
            "about": "The topic ID." },
          { "name": "Partitions", "type": "[]int32", "versions": "0+",
            "about": "The partitions." }
      ]}
  ]
}

Required ACL

• Read Group

Request Validation

...

• GroupId must be non-empty.
• MemberId must be non-empty.
• MemberEpoch must be >= -1.
• InstanceId, if provided, must be non-empty.
• RebalanceTimeoutMs must be larger than zero in the first heartbeat request.
• SubscribedTopicNames and SubscribedTopicRegex cannot be used together.SubscribedTopicNames or SubscribedTopicRegex must be, at minimum, in the first heartbeat requestheartbeat request when member epoch is 0.
• SubscribedTopicRegex must be a valid regular expression.
• ServerAssignor and ClientAssignors cannot be used together.
• Assignor.Name must be non-empty.
• Assignor.MinimumVersion must be >= -1.
• Assignor.MaximumVersion must be >= 0 and >= Assignor.MinimumVersion.
• Assignor.Version must be in the >= Assignor.MinimumVersion and <= Assignor.MaximumVersion.

...

1. Lookups the group or creates it.
2. Creates the member should the member epoch be zero or checks whether it exists. If it does not exist, UNKNOWN_MEMBER_ID is returned.
3. Checks wether whether the member epoch matches the member epoch if in its current assignment. FENCED_MEMBER_EPOCH is returned otherwise. The member is also removed from the group.
   • There is an edge case here. When the group coordinator transitions a member to its target epoch, the heartbeat response with the new member epoch may be lost. In this case, the member will retry with the member epoch that he knows about and his its request will be rejected with a FENCED_MEMBER_EPOCH. This is not optimal. Instead, the group coordinator could accept the request if the partitions owned by the members are a subset of the target partitions. If it is the case, it is safe to transition the member to its target epoch again.
4. Updates the members informations if any. The group epoch is incremented if there is any change. See "Group Epoch - Trigger a rebalance" chapter for details about the rebalance triggers.
5. Reconcile the member assignments as explained earlier in this document. 

...

The group coordinator will only set the Assignment field when until the member epoch is smaller than the target assignment epochacknowledges that it has converged to the desired assignment. This is done to ensure that the members converge to the target assignment.

{
  "apiKey": TBD,
  "type": "response",
  "name": "ConsumerGroupHeartbeatResponse",
  "validVersions": "0",
  "flexibleVersions": "0+",
  // Supported errors:
  // - GROUP_AUTHORIZATION_FAILED
  // - NOT_COORDINATOR
  // - COORDINATOR_NOT_AVAILABLE
  // - COORDINATOR_LOAD_IN_PROGRESS
  // - INVALID_REQUEST
  // - UNKNOWN_MEMBER_ID
  // - FENCED_MEMBER_EPOCH
  // - UNSUPPORTED_ASSIGNOR
  // - COMPUTEUNRELEASED_INSTANCE_ASSIGNMENTID
  // - GROUP_MAX_SIZE_REACHED
  "fields": [
    { "name": "ThrottleTimeMs", "type": "int32", "versions": "0+",
      "about": "The duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The top-level error code, or 0 if there was no error" },
    { "name": "ErrorMessage", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "The top-level error message, or null if there was no error." },
    { "name": "MemberEpochMemberId", "type": "int32string", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "The member epochid generated by the coordinator. Only provided when the member joins with MemberEpoch == 0." }, 
    { "name": "HeartbeatIntervalMsMemberEpoch", "type": "int32", "versions": "0+",
      "about": "The heartbeat interval in millisecondsmember epoch." }, 
    { "name": "AssignmentShouldComputeAssignment", "type": "Assignmentbool", "versions": "0+",
 "nullableVersions": "0+", "default": "null",
	  "about": "nullTrue if the member notshould provided;compute the assignment for the otherwisegroup."
      "fields": [ },
     	{ "name": "ErrorHeartbeatIntervalMs", "type": "int8int32", "versions": "0+",
          "about": "The heartbeat interval assignedin errormilliseconds." }, 
        { "name": "TopicPartitionsAssignment", "type": "[]TopicPartitionAssignment", "versions": "0+",
 "nullableVersions": "0+", "default": "null",
	      "about": "Thenull assignedif topic-partitions tonot provided; the assignment memberotherwise.",
          "fields": [
        	{ "name": "TopicIdError", "type": "uuidint8", "versions": "0+",
              "about": "The topicassigned IDerror." },
	        { "name": "PartitionsAssignedTopicPartitions", "type": "[]int32TopicPartitions", "versions": "0+",
              "about": "The partitions." }
      	] assigned to the member that can be used immediately." },
        { "name": "VersionPendingTopicPartitions", "type": "int16[]TopicPartitions", "versions": "0+",
          "about": "The partitions versionassigned ofto the metadata member that cannot be used because they are not released by their former owners yet." },
           { "name": "MetadataMetadataVersion", "type": "bytesint16", "versions": "0+",
          "about": "The version of assignedthe metadata." },
	]
  ]
}

Response Handling

If the response contains no error, the member will reconcile its current assignment towards its new assignment. It does the following:

1. It updates its member epoch.
2. It computes the difference between the old and the new assignment to determine the revoked partitions and the newly assignment partitions. There should be either revoked partitions or newly assignment partitions. The protocol never does both together.
   1. It revokes the partitions, commit all the offsets, and calls ConsumerRebalanceListener#onPartitionsRevoked.
   2. It assigns the new partitions, calls PartitionAssignor#onAssignment if one is defined and calls ConsumerRebalanceListener#onPartitionsAssigned.
3. After a revocation, It sends the next heartbeat immediately to acknowledge it. 

Upon receiving the COMPUTE_ASSIGNMENT error, the consumer starts the assignment process.

Upon receiving the UNKNOWN_MEMBER_ID or FENCED_MEMBER_EPOCH error, the consumer abandon all its partitions and rejoins with the same member id and the epoch 0.

ConsumerGroupPrepareAssignment API

The ConsumerGroupPrepareAssignment API will be used by the consumer to get the information to feed its client-side assignor.

Request Schema

{
  "apiKey": TBD,
  "type": "request",
  "listeners": ["zkBroker", "broker"],
  "name": "ConsumerGroupPrepareAssignmentRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
        { "name": "MetadataBytes", "type": "bytes", "versions": "0+",
          "about": "The assigned metadata." }
	]}
  ],
  "commonStructs": [
    { "name": "TopicPartitions", "versions": "0+", "fields": [
        { "name": "TopicId", "type": "uuid", "versions": "0+",
          "about": "The topic ID." },
        { "name": "GroupIdPartitions", "type": "string[]int32", "versions": "0+", "entityType": "groupId",

          "about": "The group identifierpartitions." },
    { "name": "MemberId", "type": "string", "versions": "0+",
      "about": "The member id assigned by the group coordinator." },
    { "name": "MemberEpoch", "type": "int32", "versions": "0+",
      "about": "The member epoch." }
  ]
}

Required ACL

• Read Group

Request Validation

INVALID_REQUEST is returned should the request not obey to the following invariants:

• GroupId must be non-empty.
• MemberId must be non-empty.
• MemberEpoch must be >= 0.

Request Handling

When the group coordinator handle a ConsumerGroupPrepareAssignmentRequest request:

1. Checks wether the group exists. If it does not, GROUP_ID_NOT_FOUND is returned.
2. Checks wether the member exists. If it does not, UNKNOWN_MEMBER_ID is returned.
3. Checks wether the member epoch matches the current member epoch. If it does not, FENCED_MEMBER_EPOCH is returned.
4. Checks wether the member is the chosen one to compute the assignment. If it does not, UNKNOWN_MEMBER_ID is returned.
5. Returns the group state of the group.

...

]}
  ]
}

Response Handling

If the response contains no error, the member will reconcile its current assignment towards its new assignment. It does the following:

1. It updates its member epoch.
2. It computes the difference between the old and the new assignment to determine the revoked partitions and the newly assignment partitions. There should be either revoked partitions or newly assignment partitions. The protocol never does both together.
   1. It revokes the partitions, commit all the offsets, and calls ConsumerRebalanceListener#onPartitionsRevoked.
   2. It assigns the new partitions, calls PartitionAssignor#onAssignment if one is defined and calls ConsumerRebalanceListener#onPartitionsAssigned.
3. After a revocation, It sends the next heartbeat immediately to acknowledge it. 

If the response has ShouldComputeAssignment field set to true, the consumer starts the assignment process.

Upon receiving the UNKNOWN_MEMBER_ID or FENCED_MEMBER_EPOCH error, the consumer abandon all its partitions and rejoins with the same member id and the epoch 0.

Upon receiving the UNRELEASED_INSTANCE_ID error, the consumer should fail.

ConsumerGroupPrepareAssignment API

The ConsumerGroupPrepareAssignment API will be used by the consumer to get the information to feed its client-side assignor.

Request Schema

{
  "apiKey": TBD,
  "type": "response"request",
  "listeners": ["zkBroker", "broker"],
  "name": "ConsumerGroupPrepareAssignmentResponseConsumerGroupPrepareAssignmentRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  // Supported errors"fields": [
  //  - GROUP_AUTHORIZATION_FAILED
  // - NOT_COORDINATOR
  // - COORDINATOR_NOT_AVAILABLE
  // - COORDINATOR_LOAD_IN_PROGRESS
  // - INVALID_REQUEST
  // - INVALID_GROUP_ID
  // - GROUP_ID_NOT_FOUND
  // - UNKNOWN_MEMBER_ID
  // - FENCED_MEMBER_EPOCH
  "fields": [
    { "name": "ThrottleTimeMs" { "name": "GroupId", "type": "string", "versions": "0+", "entityType": "groupId",
      "about": "The group identifier." },
    { "name": "MemberId", "type": "int32string", "versions": "0+",
      "about": "The durationmember inid millisecondsassigned forby which the request was throttled due to a quota violation, or zero if the request did not violate any quotagroup coordinator." },
    { "name": "ErrorCodeMemberEpoch", "type": "int16int32", "versions": "0+",
      "about": "The top-level error code, or 0 if there was no errormember epoch." },
  ]
}

Required ACL

• Read Group

Request Validation

INVALID_REQUEST is returned should the request not obey to the following invariants:

• GroupId must be non-empty.
• MemberId must be non-empty.
• MemberEpoch must be >= 0.

Request Handling

When the group coordinator handle a ConsumerGroupPrepareAssignmentRequest request:

1. Checks whether the group exists. If it does not, GROUP_ID_NOT_FOUND is returned.
2. Checks whether the member exists. If it does not, UNKNOWN_MEMBER_ID is returned.
3. Checks whether the member epoch matches the current member epoch. If it does not, STALE_MEMBER_EPOCH is returned.
4. Checks whether the member is the chosen one to compute the assignment. If it does not, UNKNOWN_MEMBER_ID is returned.
5. Returns the group state of the group.

Response Schema

{
  "apiKey": TBD,
  "type": "response",
  "name": "ConsumerGroupPrepareAssignmentResponse",
  "validVersions": "0",
  "flexibleVersions": "0+",
  // Supported errors: 
  // - GROUP_AUTHORIZATION_FAILED
  // - NOT_COORDINATOR
  // - COORDINATOR_NOT_AVAILABLE
  // - COORDINATOR_LOAD_IN_PROGRESS
  // - INVALID_REQUEST
  // - INVALID_GROUP_ID
  // - GROUP_ID_NOT_FOUND
  // - UNKNOWN_MEMBER_ID
  // - STALE_MEMBER_EPOCH
  "fields": [  { "name": "ErrorMessage", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "The top-level error message, or null if there was no error." },
    { "name": "GroupEpochThrottleTimeMs", "type": "int32", "versions": "0+",
      "about": "The group epoch duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "AssignorNameErrorCode", "type": "stringint16", "versions": "0+",
      "about": "The selected assignor." },
    top-level error code, or 0 if there was no error" },
    { "name": "MembersErrorMessage", "type": "[]Memberstring", "versions": "0+",
      "aboutnullableVersions": "The members.0+", "fieldsdefault": ["null",
      { "nameabout": "MemberId", "type": "string", "versions": "0+",
        "about": "The member IDThe top-level error message, or null if there was no error." },
      { "name": "MemberEpochGroupEpoch", "type": "int32", "versions": "0+",
        "about": "The membergroup epoch." },
      { "name": "InstanceIdAssignorName", "type": "string", "versions": "0+",
        "about": "The memberselected instance IDassignor." },
      { "name": "SubscribedTopicIdsMembers", "type": "[]uuidMember", "versions": "0+",
        "about": "The subscribed topic IDs." },members.", "fields": [
      { "name": "AssignorMemberId", "type": "Assignorstring", "versions": "0+",
        "about": "The informationmember of the selected assignor"ID." },
        "fields": [ 
        { "name": "VersionMemberEpoch", "type": "int16int32", "versions": "0+",
          "about": "The version of the metadatamember epoch." },
        { "name": "ReasonInstanceId", "type": "int8string", "versions": "0+",
     "nullableVersions": "0+", "default": "null",
        "about": "The reasonmember of the metadata updateinstance ID." }, 
        { "name": "MetadataRackId", "type": "bytesstring", "versions": "0+",
 "nullableVersions": "0+", "default": "null", 
        "about": "The assignormember instance metadataID." },
      { "name": "SubscribedTopicIds", "type": "[]uuid", "versions": "0+",
        "about": "The subscribed topic IDs." },
      { "name": "TopicPartitionsAssignor", "type": "[]TopicPartitionAssignor", "versions": "0+",
        "about": "The target topic-partitionsinformation of the selected member.assignor",
        "fields": [ 
          { "name": "TopicIdReason", "type": "uuidint8", "versions": "0+",
            "about": "The topic ID reason of the metadata update." }, 
          { "name": "PartitionsMetadataVersion", "type": "[]int32int16", "versions": "0+",
            "about": "The partitions." }
      ]}version of the metadata." },
    ]},
    { "name": "TopicsMetadataBytes", "type": "[]TopicMetadatabytes", "versions": "0+",
          "about": "The topic-partition metadataassignor metadata." }
      ]},
      { "name": "TopicPartitions", "type": "[]TopicPartition", "versions": "0+",
        "about": "The target topic-partitions of the member.",
        "fields": [
          { "name": "TopicId", "type": "uuid", "versions": "0+",
            "about": "The topic ID." },
          { "name": "NumPartitionsPartitions", "type": "[]int32", "versions": "0+",
            "about": "The number of partitions." }
      ]}
    ]},
    { "name": "Topics", "type": "[]
}

Response Handling

If the response contains no error, the member calls the client side assignor with the group state.

Upon receiving the UNKNOWN_MEMBER_ID error, the consumer abandon the process.

Upon receiving the FENCED_MEMBER_EPOCH error, the consumer retries when receiving its next heartbeat response with its member epoch.

ConsumerGroupInstallAssignment API

The ConsumerGroupInstallAssignment API will be used by the consumer to install a new assignment for the group. The new assignment is the result of the client-side assignor.

Request Schema

{
  "apiKey": TBD,
  "type": "request",
  "listeners": ["zkBroker", "broker"],
  "name": "ConsumerGroupInstallAssignmentRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
TopicMetadata", "versions": "0+",
      "about": "The topic-partition metadata.",
      "fields": [
        { "name": "TopicId", "type": "uuid", "versions": "0+",
          "about": "The topic ID." },
        { "name": "GroupIdTopicName", "type": "string", "versions": "0+", "entityType": "groupId",

          "about": "The grouptopic identifiername." },
        { "name": "MemberIdNumPartitions", "type": "stringint32", "versions": "0+",
          "about": "The membernumber idof partitions." }
    ]}    
  ]
} 

Response Handling

If the response contains no error, the member calls the client side assignor with the group state.

Upon receiving the UNKNOWN_MEMBER_ID error, the consumer abandon the process.

Upon receiving the STALE_MEMBER_EPOCH error, the consumer retries when receiving its next heartbeat response with its member epoch.

ConsumerGroupInstallAssignment API

The ConsumerGroupInstallAssignment API will be used by the consumer to install a new assignment for the group. The new assignment is the result of the client-side assignor.

Request Schema

{
  "apiKey": TBD,
  "type": "request",
  "listeners": ["zkBroker", "broker"],
  "name": "ConsumerGroupInstallAssignmentRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [assigned by the group coordinator." },
    { "name": "MemberEpoch", "type": "int32", "versions": "0+",
      "about": "The member epoch." },
    { "name": "GroupEpochGroupId", "type": "int32string", "versions": "0+",
 "entityType": "groupId",
      "about": "The group epochidentifier." },
    { "name": "ErrorMemberId", "type": "int8string", "versions": "0+",
      "about": "The assignmentmember error;id orassigned zero ifby the assignmentgroup is successfulcoordinator." },
     { "name": "MembersMemberEpoch", "type": "[]Memberint32", "versions": "0+",
      "about": "The member membersepoch.", "fields": [ },
      { "name": "MemberIdGroupEpoch", "type": "stringint32", "versions": "0+",
        "about": "The membergroup IDepoch." },
      { "name": "PartitionsError", "type": "[]TopicPartitionint8", "versions": "0+",
        "about": "The assigned topic-partitions to the member.",
        "fields": [
          assignment error; or zero if the assignment is successful." },
    { "name": "TopicIdMembers", "type": "uuid[]Member", "versions": "0+",
            "about": "The topic IDmembers." },
   "fields": [
       { "name": "PartitionsMemberId", "type": "[]int32string", "versions": "0+",
            "about": "The partitionsmember ID." }
        ]},
      { "name": "VersionPartitions", "type": "int32[]TopicPartition", "versions": "0+",
        "about": "The metadata version." }
 assigned topic-partitions to the member.",
       { "namefields": "Metadata[
          { "name": "TopicId", "type": "bytesuuid", "versions": "0+",
            "about": "The metadatatopic bytesID." },
          { "name": "Partitions", "type": "[]int32", "versions": "0+",
            "about": "The partitions." }
        ]
}

Required ACL

• Read Group

Request Validation

INVALID_REQUEST is returned should the request not obey to the following invariants:

• GroupId must be non-empty.
• MemberId must be non-empty.
• MemberEpoch must be >= 0.

Request Handling

When the group coordinator handle a ConsumerGroupInstallAssignmentRequest request:

},
      { "name": "MetadataVersion", "type": "int32", "versions": "0+",
        "about": "The metadata version." }
      { "name": "MetadataBytes", "type": "bytes", "versions": "0+",
        "about": "The metadata bytes." }
    ]}
  ]
}

Required ACL

• Read Group

Request Validation

INVALID_REQUEST is returned should the request not obey to the following invariants:

• GroupId must be non-empty.
• MemberId must be non-empty.
• MemberEpoch must be >= 0.

Request Handling

When the group coordinator handle a ConsumerGroupInstallAssignmentRequest request:

1. Checks whether Checks wether the group exists. If it does not, GROUP_ID_NOT_FOUND is returned.
2. Checks wether whether the member exists. If it does not, UNKNOWN_MEMBER_ID is returned.
3. Checks wether whether the member epoch matches the current member epoch. If it does not, FENCEDSTALE_MEMBER_EPOCH is returned.
4. Checks wether whether the member is the chosen one to compute the assignment. If it does not, UNKNOWN_MEMBER_ID is returned.
5. Validates the assignment based on the information used to compute it. If it is not valid, INVALID_ASSIGNMENT is returned.
6. Installs the new target assignment.

...

{
  "apiKey": TBD,
  "type": "response",
  "name": "ConsumerGroupInstallAssignmentResponse",
  "validVersions": "0",
  "flexibleVersions": "0+",
  // Supported errors: 
  // - GROUP_AUTHORIZATION_FAILED
  // - NOT_COORDINATOR
  // - COORDINATOR_NOT_AVAILABLE
  // - COORDINATOR_LOAD_IN_PROGRESS
  // - INVALID_REQUEST
  // - INVALID_GROUP_ID
  // - GROUP_ID_NOT_FOUND
  // - UNKNOWN_MEMBER_ID
  // - FENCEDSTALE_MEMBER_EPOCH  
  // - INVALID_ASSIGNMENT  
  "fields": [
    { "name": "ThrottleTimeMs", "type": "int32", "versions": "0+",
      "about": "The duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The top-level error code, or 0 if there was no error" },
    { "name": "ErrorMessage", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "The top-level error message, or null if there was no error." }
  ]
}

...

If the response contains no error, the member is done with the assignment process.

Upon receiving the FENCEDSTALE_MEMBER_EPOCH error, the consumer retries when receiving its next heartbeat response with its member epoch.

...

{
  "apiKey": TBD,
  "type": "request",
  "listeners": ["zkBroker", "broker"],
  "name": "ConsumerGroupDescribeConsumerGroupDescribeRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "GroupIds", "type": "[]string", "versions": "0+", "entityType": "groupId",
      "about": "The names of the groups to describe" },
    { "name": "IncludeAuthorizedOperations", "type": "bool", "versions": "0+",
      "about": "Whether to include authorized operations." }
  ]
}

...

When the group coordinator handle a ConsumerGroupPrepareAssignmentRequest ConsumerGroupDescribeRequest request:

• Checks wether whether the group ids exists. If it does not, GROUP_ID_NOT_FOUND is returned.
• Looks up the groups and returns the response.

...

{
  "apiKey": 71,
  "type": "response",
  "name": "ConsumerGroupDescribeResponse",
  "validVersions": "0",
  "flexibleVersions": "0+",
  // Supported errors: 
  // - GROUP_AUTHORIZATION_FAILED
  // - NOT_COORDINATOR
  // - COORDINATOR_NOT_AVAILABLE
  // - COORDINATOR_LOAD_IN_PROGRESS
  // - INVALID_REQUEST
  // - INVALID_GROUP_ID
  // - GROUP_ID_NOT_FOUND
  "fields": [
    { "name": "ThrottleTimeMs", "type": "int32", "versions": "0+",
      "about": "The duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "Groups", "type": "[]DescribedGroup", "versions": "0+",
      "about": "Each described group.",
      "fields": [
        { "name": "ErrorCode", "type": "int16", "versions": "0+",
          "about": "The describe error, or 0 if there was no error." },
        { "name": "GroupIdErrorMessage", "type": "string", "versions": "0+", "entityTypenullableVersions": "groupId0+",
   "default": "null",
          "about": "The top-level error message, or null if there was no error." }
        { "name": "GroupId", "type": "string", "versions": "0+", "entityType": "groupId",
          "about": "The group ID string." },
        { "name": "GroupState", "type": "string", "versions": "0+",
          "about": "The group state string, or the empty string." },
        { "name": "GroupEpoch", "type": "int32", "versions": "0+",
          "about": "The group epoch." },
        { "name": "AssignmentEpoch", "type": "int32", "versions": "0+",
          "about": "The assignment epoch." },
        { "name": "AssignorName", "type": "string", "versions": "0+",
          "about": "The selected assignor." },
        { "name": "Members", "type": "[]Member", "versions": "0+",
          "about": "The members.",
          "fields": [
          { "name": "MemberId", "type": "uuid", "versions": "0+",
            "about": "The member ID." },
          { "name": "InstanceId", "type": "string", "versions": "0+",
 "nullableVersions": "0+", "default": "null", 
             "about": "The member instance ID." },
        	  { "name": "MemberEpochRackId", "type": "int32string", "versions": "0+",
 "nullableVersions": "0+", "default": "null", 
             "about": "The currentmember memberrack epochID." },
              { "name": "ClientIdMemberEpoch", "type": "stringint32", "versions": "0+",
            "about": "The current clientmember IDepoch." },
          { "name": "ClientHostClientId", "type": "string", "versions": "0+",
            "about": "The client hostID." },
          { "name": "SubscriptionsClientHost", "type": "[]uuidstring", "versions": "0+",
            "about": "The subscribedclient topic IDshost." },
          { "name": "AssignmentSubscribedTopicNames", "type": "Assignment[]string", "versions": "0+",
            "about": "The subscribed currenttopic assignmentnames.",
            "fields": [
   },
          { "name": "PartitionsSubscribedTopicRegex", "type": "[]TopicPartitionstring", "versions": "0+",
              "about"nullableVersions": "0+", "default": "The assigned topic-partitions to the member.null",
              "fieldsabout": [
"the subscribed topic regex otherwise or null of not provided." },
          { "name": "TopicIdAssignment", "type": "uuidAssignment", "versions": "0+",
                  "about": "The topiccurrent IDassignment." },
                { "name": "PartitionsTargetAssignment", "type": "[]int32Assignment", "versions": "0+",
                  "about": "The target partitionsassignment." }
              ]},
            { "name": "VersionAuthorizedOperations", "type": "int32", "versions": "03+",
      "default": "-2147483648",
        "about": "The assignor metadata version32-bit bitfield to represent authorized operations for this group." }
    ]}
  ],
  "commonStructs": [
    { "name": "MetadataAssignment", "typeversions": "bytes0+", "versionsfields": "0+",[
      { "name": "Partitions", "type":     "about"[]TopicPartitions", "versions": "0+"The,
 assignor metadata bytes." }
    "about": "The assigned topic-partitions to the ]}member.",
 "fields": [
        { "name": "TargetAssignmentTopicId", "type": "Assignmentuuid", "versions": "0+",
            "about": "The targettopic assignmentID." },
            "fields": [
            { "name": "PartitionsTopicName", "type": "[]TopicPartitionstring", "versions": "0+",
              "about": "The assigned topic-partitions to the member name.",
              "fields": [
         }, 
        { "name": "TopicIdPartitions", "type": "uuid[]int32", "versions": "0+",
                  ""about": "The topic IDpartitions." },
         ]},
       { "name": "PartitionsError", "type": "[]int32int8", "versions": "0+",
                  "about": "The partitionsassigned error." }
              ]},
  , 
          { "name": "VersionMetadataVersion", "type": "int32", "versions": "0+",
              "about": "The assignor metadata version." }
            { "name": "MetadataMetadataBytes", "type": "bytes", "versions": "0+",
              "about": "The assignor metadata bytes." }
          ]},
      { "name": "AuthorizedOperations", "type": "int32", "versions": "3+", "default": "-2147483648",
        "about": "32-bit bitfield to represent authorized operations for this group." }
    ]}
  ]
}

Response Handling

Nothing particular.

...

{
  "apiKey": 16,
  "type": "request",
  "listeners": ["zkBroker", "broker"],
  "name": "ListGroupsRequest",
  // Version 1 and 2 are the same as version 0.
  //
  // Version 3 is the first flexible version.
  //
  // Version 4 adds the StatesFilter field (KIP-518).
  //
  // Version 5 adds the TypesFilter field (KIP-848).
  "validVersions": "0-5",
  "flexibleVersions": "3+",
  "fields": [
    { "name": "StatesFilter", "type": "[]string", "versions": "4+",
      "about": "The states of the groups we want to list. If empty all groups are returned with their state." },
    { "name": "TypesFilter", "type": "[]string", "versions": "5+",
      "about": "The types of the groups we want to list. If empty all groups are returned" }
  ]
}

Required ACL

• Describe GroupCluster

Response Schema

Request Validation

No particular changes.

Request Handling

The new types filter is handled.

Response Schema

The The GroupType field is introduced. It represents the type of the group.

{
  "apiKey": 16,
  "type": "response",
  "name": "ListGroupsResponse",
  // Version 1 adds the throttle time.
  //
  // Starting in version 2, on quota violation, brokers send out
  // responses before throttling.
  //
  // Version 3 is the first flexible version.
  //
  // Version 4 adds the GroupState field (KIP-518).
  //
  // Version 5 adds the GroupType field (KIP-848).
  "validVersions": "0-5",
  "flexibleVersions": "3+",
  "fields": [
    { "name": "ThrottleTimeMs", "type": "int32", "versions": "1+", 
      "ignorable": true,
      "about": "The duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The error code, or 0 if there was no error." },
    { "name": "Groups", "type": "[]ListedGroup", "versions": "0+",
      "about": "Each group in the response.", "fields": [
      { "name": "GroupId", "type": "string", "versions": "0+",
        "entityType": "groupId",
        "about": "The group ID." },
      { "name": "ProtocolType", "type": "string", "versions": "0+",
        "about": "The group protocol type." },
      { "name": "GroupState", "type": "string", "versions": "4+", "ignorable": true,
        "about": "The group state name." },
      { "name": "GroupType", "type": "string", "versions": "5+", "ignorable": true,
        "about": "The group statetype name." }
    ]}
  ]
}

Response Handling

No changes.

OffsetCommit API

The version of the API is bumped to 9 .

Request Schema

We propose to rename GenerationId to GenerationIdOrMemberEpoch.

Required ACL

• Read Group

Request Handling

When the group id corresponds to a consumer group using the new rebalance protocol, the provided member epoch must match the expected member epoch.

Response Schema

The response can return FENCED_MEMBER_EPOCH.

OffsetFetch API

The version of the API is bumped to 9to add support for topic ids. The request can either use topic ids or topic names. The consumer will only use topic ids when they are available whereas the admin client will continue to use topic names as per its API.

Request Schema

{
  "apiKey": 98,
  "type": "request",
  "listeners": ["zkBroker", "broker"],
  "name": "OffsetFetchRequestOffsetCommitRequest",
  // In version 0, the request read offsets from ZK Version 1 adds timestamp and group membership information, as well as the commit timestamp.
  //
  // StartingVersion in2 versionadds 1,retention thetime. broker supports fetching offsets fromIt removes the internal __consumer_offsets topic.
  commit timestamp added in version 1.
  //
  // StartingVersion 3 inand version4 2,are the requestsame canas containversion a2. null
 topics array to indicate that offsets //
  // forVersion all5 topicsremoves shouldthe beretention fetched.time, Itwhich alsois returnsnow acontrolled toponly levelby errora code
  // for group or coordinator level errorsbroker configuration.
  //
  // Version 3, 4, and 5 are the same as version 26 adds the leader epoch for fencing.
  //
  // Versionversion 67 isadds thea firstnew flexiblefield version.
called groupInstanceId //
to indicate //member Versionidentity 7 is adding the require stable flagacross restarts.
  //
  // Version 8 is addingthe supportfirst for fetching offsets for multiple groups at a timeflexible version.
  //
  // Version 9 adds GenerationIdOrMemberEpoch and MemberId fieldsTopicId field (KIP-848).
  "validVersions": "0-9",
  "flexibleVersions": "68+",
  "fields": [
    { "name": "GroupId", "type": "string", "versions": "0-7+", "entityType": "groupId",
      "about": "The unique group to fetch offsets foridentifier." },
    // NewRenamed fieldsfield.
    { "name": "GenerationIdOrMemberEpoch", "type": "int32", "versions": "91+", "default": "-1", "ignorable": true,
      "about": "The generation of the group if the generic group protocol or the member epoch if the consumer protocol." },
    { "name": "MemberId", "type": "string", "versions": "91+", "ignorable": true,
      "about": "The member ID assigned by the group coordinator." },
    // End of new fields.
    { "name": "TopicsGroupInstanceId", "type": "[]OffsetFetchRequestTopicstring", "versions": "0-7+",
      "nullableVersions": "2-7+", "default": "null",
      "about": "EachThe topicunique weidentifier wouldof likethe toconsumer fetchinstance offsetsprovided for,by or null to fetch offsets for all topics.", "fields": [
  end user." },
    { "name": "NameRetentionTimeMs", "type": "stringint64", "versions": "02-74", "entityTypedefault": "topicName-1",
 "ignorable": true,
      "about": "The topic name." time period in ms to retain the offset." },
      { "name": "PartitionIndexesTopics", "type": "[]int32OffsetCommitRequestTopic", "versions": "0-7+",
        "about": "The partition indexes we would like topics to fetchcommit offsets for.", "fields": }[
    ]},
  // Updated field.
      { "name": "GroupsName", "type": "[]OffsetFetchRequestGroupstring", "versions": "0+", "nullableVersions": "89+", "default": "null", "entityType": "topicName",
        "about": "EachThe group we would like to fetch offsets for", "fields": [topic name."},
      // New field.
      { "name": "groupIdTopicId", "type": "stringuuid", "versions": "89+", "entityType": "groupId",
        "about": "The groupunique topic ID." },
      { "name": "TopicsPartitions", "type": "[]OffsetFetchRequestTopicsOffsetCommitRequestPartition", "versions": "80+", "nullableVersions": "8+",
        "about": "Each topic we would likepartition to fetch offsets for, or null to fetchcommit offsets for all topics.", "fields": [
        { "name": "NamePartitionIndex", "type": "stringint32", "versions": "80+", "entityType": "topicName",
          "about": "The topicpartition nameindex." },
        { "name": "PartitionIndexesCommittedOffset", "type": "[]int32int64", "versions": "80+",
          "about": "The partitionmessage indexesoffset weto wouldbe like to fetch offsets forcommitted." },
        { "name": "CommittedLeaderEpoch", "type": "int32", "versions": "6+", "default": "-1", "ignorable": true,
          "about": "The leader epoch of this partition." },
        //  ]}CommitTimestamp has been removed from v2 and later.
    ]},
    { "name": "RequireStableCommitTimestamp", "type": "boolint64", "versions": "7+1", "default": "false-1",
          "about": "WhetherThe timestamp brokerof shouldthe hold on returning unstable offsets but set a retriable error code for the partitions."}
  ]
}

Required ACL

• Describe Group

commit." },
        { "name": "CommittedMetadata", "type": "string", "versions": "0+", "nullableVersions": "0+",
          "about": "Any associated metadata the client wants to keep." }
      ]}
    ]}
  ]
}

Required ACL

• Read Group

Request Validation

INVALID_REQUEST is returned should the request not obey to the following invariants:

• A topic has both a name and an ID set.

Request Request Handling

The MemberId and the GenerationIdOrMemberEpoch are verified. FENCEDSTALE_MEMBER_EPOCH or UNKNOWN_MEMBER_ID is returned accordingly.

Response Schema

The response is the same. Only new FENCED_MEMBER_EPOCH or UNKNOWN_MEMBER_ID errors could be returned.

Response Handling

Upon receiving the FENCED_MEMBER_EPOCH error, the consumer retries when receiving its next heartbeat response with its member epoch.

DescribeConfigs API

Request Schema

The schema is the same but the ResourceType field can be set to GROUP (16).

Required ACL

• Describe Config on the group.

Response Schema

No changes.

AlterIncrementalConfigs API

The API is the same but supports a new resource type: GROUP (16). When GROUP is used, the resource name corresponds to the group id.

Request Schema

The schema is the same but the ResourceType field can be set to GROUP (16).

Required ACL

• Alter Config on the group.

Response Schema

No changes.

Records

This section describes the new record types required for the new protocol. The storage layout is based on the data model described earlier in this document.

As explained earlier, they will be persisted in the __consumer_offsets compacted topic. The compacted topic based storage requires a dedicated key type per record type in order for the compaction to work. The current protocol already uses versions from 0 to 2 (included) for the keys.

Group Metadata

Groups can be rather large so we propose to use several records to store a group in order to not be limited by the maximum batch size (1MB by default). Therefore we propose to store group metadata with two records types: the ConsumerGroupMetadata and the ConsumerGroupMemberMetadata.

A group with X members will be stored with X+2 records. One ConsumerGroupMemberMetadata per member, one ConsumerGroupPartitionMetadata, and one ConsumerGroupMetadata for the group at the end. Atomicity is not a concern here. All the records can be applied independently.

Moreover, the whole group does not necessarily have to be written for every epoch. Members who have not changed could be omitted as the compacted topic will retain their previous state anyway.

When a member is deleted, a tombstone for him is written to the partition.

ConsumerGroupMetadataKey

{
    "type": "data",
    "name": "ConsumerGroupMetadataKey",
    "validVersions": "3",
    "flexibleVersions": "none",
    "fields": [
      	{ "name": "GroupId", "type": "string", "versions": "3" }
    ]
}

ConsumerGroupMetadataValue

{
    "type": "data",
    "name": "ConsumerGroupMetadataValue",
    "validVersions": "0",
    "flexibleVersions": "0+",
    "fields": [
        { "name": "Epoch", "versions": "0+", "type": "int32" }
    ], 
}

ConsumerGroupPartitionMetadataKey

{
  "apiKey": 8,
  "type": "response",
  "name": "OffsetCommitResponse",
  // Versions 1 and 2 are the same as version 0.
  //
  // Version 3 adds the throttle time to the response.
  //
  // Starting in version 4, on quota violation, brokers send out responses before throttling.
  //
  // Versions 5 and 6 are the same as version 4.
  //
  // Version 7 offsetCommitRequest supports a new field called groupInstanceId to indicate member identity across restarts.
  //
  // Version 8 is the first flexible version.
  //
  // Version 9 adds TopicId field and can return STALE_MEMBER_EPOCH, UNKNOWN_MEMBER_ID
  // and UNKNOWN_TOPIC_ID errors (KIP-848).
  "validVersions": "0-9",
  "flexibleVersions": "8+",
  "fields": [
    { "name": "ThrottleTimeMs", "type": "int32", "versions": "3+", "ignorable": true,
      "about": "The duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "Topics", "type": "[]OffsetCommitResponseTopic", "versions": "0+",
      "about": "The responses for each topic.", "fields": [
      // Updated field.
      { "name": "Name", "type": "string", "versions": "0+", "nullableVersions": "9+", "default": "null", "entityType": "topicName",
        "about": "The topic name."},
      // New field.
      { "name": "TopicId", "type": "uuid", "versions": "9+", "about": "The unique topic ID" },
      { "name": "Partitions

{
    "type": "data",
    "name": "ConsumerGroupPartitionMetadataKey",
    "validVersions": "4",
    "flexibleVersions": "none",
    "fields": [
      	{ "name": "GroupId", "type": "string[]OffsetCommitResponsePartition", "versions": "40+" }
    ]
}

ConsumerGroupPartitionMetadataValue

{
    "type": "data",
    "name": "ConsumerGroupPartitionMetadataValue",
    "validVersions": "0",
    "flexibleVersions": "0+",
    ,
        "about": "The responses for each partition in the topic.",  "fields": [
                { "name": "EpochPartitionIndex", "versionstype": "0+int32", "typeversions": "int320+" },
        {          "nameabout": "TopicPartitionMetadata", "versions": "0+",
          "type": "[]TopicPartition", "fields": [
            The partition index." },
        { "name": "TopicIdErrorCode", "versionstype": "0+int16", "typeversions": "uuid0+" },
            { "name": "NumPartitions", "versions          "about": "0+", "type": "int32" }
          ]}
    ], 
}

...

The error code, or 0 if there was no error." }
      ]}
    ]}
  ]
}

Response Handling

Upon receiving the STALE_MEMBER_EPOCH error, the consumer retries when receiving its next heartbeat response with its member epoch.

OffsetFetch API

The version of the API is bumped to 9 to add support for topic ids. The request can either use topic ids or topic names. The consumer will only use topic ids when they are available whereas the admin client will continue to use topic names as per its API.

Request Schema

{
      "typeapiKey": "data"9,
      "nametype": "ConsumerGroupMemberMetadataKeyrequest",
      "validVersionslisteners": ["5zkBroker",
    "flexibleVersions": "nonebroker"],
    "fields": [
        { "name": "GroupIdOffsetFetchRequest",
 "type": "string", "versions": "5" },
        { "name": "MemberId", "type": "string", "versions": "5" }
    ]
}

ConsumerGroupMemberMetadataValue

{
    "type": "data",
    "name": "ConsumerGroupMemberMetadataValue",
    "validVersions": "0",
    "flexibleVersions": "0+",
    "fields": [
        { "name": "GroupEpoch", "versions": "0+", "type": "int32" },
        { "name": "InstanceId", "versions": "0+", "type": "string" },
        { "name": "ClientId", "versions": "0+", "type": "string" },
        { "name": "ClientHost", "versions": "0+", "type": "string" },
        { "name": "SubscribedTopicNames", "versions": "0+", "type": "[]string" },
        { "name": "SubscribedTopicRegex", "versions": "0+", "type": "string" },
        { "name": "Assignors", "versions": "0+",
          "type": "[]Assignor", "fields": [
            { "name": "Name", "versions": "0+", "type": "string" },
            { "name": "MinimumVersion", "versions": "0+", "type": "int16" },
            { "name": "MaximumVersion", "versions": "0+", "type": "int16" },
            { "name": "Reason", "versions": "0+", "type": "int8" },
			{ "name": "Version", "versions": "0+", "type": "int16" },
            { "name": "Metadata", "versions": "0+", "type": "bytes" }
          ]}
    ], 
}

Target Assignment

The target assignment is stored in a single record.

ConsumerGroupTargetAssignmentKey

{
    "type": "data",
    "name": "ConsumerGroupTargetAssignmentKey",
    "validVersions": "6",
    "flexibleVersions": "none",
    "fields": [
      	 // In version 0, the request read offsets from ZK.
  //
  // Starting in version 1, the broker supports fetching offsets from the internal __consumer_offsets topic.
  //
  // Starting in version 2, the request can contain a null topics array to indicate that offsets
  // for all topics should be fetched. It also returns a top level error code
  // for group or coordinator level errors.
  //
  // Version 3, 4, and 5 are the same as version 2.
  //
  // Version 6 is the first flexible version.
  //
  // Version 7 is adding the require stable flag.
  //
  // Version 8 is adding support for fetching offsets for multiple groups at a time
  //
  // Version 9 adds GenerationIdOrMemberEpoch, MemberId and TopicId fields (KIP-848).
  "validVersions": "0-9",
  "flexibleVersions": "6+",
  "fields": [
    { "name": "GroupId", "type": "string", "versions": "50-7" }
    ]
}

ConsumerGroupTargetAssignmentValue

{
    "type, "entityType": "datagroupId",
    "name      "about": "ConsumerGroupTargetAssignmentValue",
    "validVersionsThe group to fetch offsets for." },
    // New fields.
    { "name": "0GenerationIdOrMemberEpoch",
     "flexibleVersionstype": "0+int32",
     "fieldsversions": [
        {"9+", "namedefault": "AssignmentEpoch-1", "versionsignorable": "0+", "typetrue,
      "about": "int32The generation of the group." },
        { "name": "MembersMemberId", "versionstype": "0+string", "typeversions": "[]Member9+", "fieldsignorable": [
        	{true,
      "nameabout": "MemberId", "versions": "0+", "type": "stringThe member ID assigned by the group coordinator." },
               // End of new fields.
    { "name": "ErrorTopics", "type": "[]OffsetFetchRequestTopic", "versions": "0+-7", "typenullableVersions": "int82-7" },
            { "nameabout": "TopicPartitions", "versions": "0+",
          	  "type": "[]TopicPartition"Each topic we would like to fetch offsets for, or null to fetch offsets for all topics.", "fields": [
            	      { "name": "TopicIdName", "versionstype": "0+string", "typeversions": "uuid0-7" },
            	{ "nameentityType": "PartitionstopicName",
        "versionsabout": "0+", "type": "[]int32" }
        	]The topic name."},
         	{ "name": "VersionPartitionIndexes", "versionstype": "0+[]int32", "typeversions": "int160-7" },
        	{ "name": "Metadata", "versions": "0+", "type        "about": "bytes" }
        ]
    ]
}

Current Member Assignment

The current member assignment represents, as the name suggests, the current assignment of a given member.

When a member is deleted from the group, a tombstone for him is written to the partition.

ConsumerGroupCurrentMemberAssignmentKey

{
    "type": "data",
    "name": "ConsumerGroupCurrentMemberAssignmentKey",
    "validVersions": "7",
    "flexibleVersions": "none",
    The partition indexes we would like to fetch offsets for." }
    ]},
    { "name": "Groups", "type": "[]OffsetFetchRequestGroup", "versions": "8+",
      "about": "Each group we would like to fetch offsets for", "fields": [
      	      { "name": "GroupIdgroupId", "type": "string", "versions": "8+", "entityType": "groupId",
        "7about": "The group ID."},
      	      { "name": "MemberIdTopics", "type": "string[]OffsetFetchRequestTopics", "versions": "8+", "7nullableVersions": },
    ]
}

ConsumerGroupCurrentMemberAssignmentValue

{
    "type": "data",
    "name": "ConsumerGroupCurrentMemberAssignmentValue",
    "validVersions": "0",
    "flexibleVersions": "0+",
    "fields": [
        { "name": "MemberEpoch"8+",
        "about": "Each topic we would like to fetch offsets for, or null to fetch offsets for all topics.", "fields": [
        // Updated field.
        { "name": "Name", "type": "string", "versions": "08+", "typenullableVersions": "int329+" },
		{ "namedefault": "Errornull", "versionsentityType": "0+topicName",
          "typeabout": "int8" The topic name."},
               // New field.
        { "name": "TopicPartitionsTopicId", "versionstype": "0+uuid",
           "typeversions": "[]TopicPartition9+", "fieldsabout": [
             "The unique topic ID" },
        { "name": "TopicIdPartitionIndexes", "versionstype": "0+[]int32", "typeversions": "uuid8+" },
            {          "nameabout": "Partitions", "versions": "0+", "type": "[]int32The partition indexes we would like to fetch offsets for." }
              ]},
    ]},
    { "name": "VersionRequireStable", "versionstype": "0+bool", "typeversions": "int167+" },
        { "namedefault": "Metadatafalse",
      "versionsabout": "0+", "type": "bytes" }
    ], 
}

Group Configurations

GroupConfigurationKey

Whether broker should hold on returning unstable offsets but set a retriable error code for the partitions."}
  ]
}

Required ACL

• Describe Group

Request Validation

INVALID_REQUEST is returned should the request not obey to the following invariants:

• A topic has both a name and an ID set.

Request Handling

The MemberId and the GenerationIdOrMemberEpoch are verified. STALE_MEMBER_EPOCH, UNKNOWN_MEMBER_ID or ILLEGAL_GENERATION is returned accordingly.

The admin client is not expected to provide the MemberId nor the GenerationIdOrMemberEpoch.

Response Schema

{
      "typeapiKey": "data"9,
      "nametype": "GroupConfigurationKeyresponse",
      "validVersionsname": "8OffsetFetchResponse",
    "flexibleVersions": "none",
    "fields": [
     	{ "name": "GroupId", "type": "string", "versions": "8" }
    ]
}

GroupConfigurationValue

{
    "type": "data",
    "name": "GroupConfigurationValue",
    "validVersions": "0",
    "flexibleVersions": "0+",
    "fields": [
        { "name": "Configurations", "versions": "0+", "type": "[]Configuration",
          "fields": [
		     { "name": "Name", "type": "string", "versions": "0+",
      		   "about": "The name of the configuration key." },
    		  // Version 1 is the same as version 0.
  //
  // Version 2 adds a top-level error code.
  //
  // Version 3 adds the throttle time.
  //
  // Starting in version 4, on quota violation, brokers send out responses before throttling.
  //
  // Version 5 adds the leader epoch to the committed offset.
  //
  // Version 6 is the first flexible version.
  //
  // Version 7 adds pending offset commit as new error response on partition level.
  //
  // Version 8 is adding support for fetching offsets for multiple groups
  //
  // Version 9 adds TopicId field and can return STALE_MEMBER_EPOCH, UNKNOWN_MEMBER_ID,
  // ILLEGAL_GENERATION, and UNKNOWN_TOPIC_ID errors.
  "validVersions": "0-8",
  "flexibleVersions": "6+",
  "fields": [
    { "name": "ValueThrottleTimeMs", "type": "stringint32", "versions": "03+",
   "ignorable": true,
   		   "about": "The value of duration in milliseconds for which the configuration." }
		]}
    ] 
}

Broker API

The new PartitionAssignor interface will be introduced on the server side. Two implementations will be provided out of the box: RangeAssignor (range) and UniformAssignor (uniform).

package org.apache.kafka.server.group.consumer;

public interface PartitionAssignor {

    class Group {
        /**
         * The members.
         */
        List<GroupMember> members;

        /**
         * The topics' metadata.
         */
        List<TopicMetadata> topics;
    }

    class GroupMember {
        /**
         * The member ID.
         */
        String memberId;

        /**
         * The instance ID if provided.
         */
        Optional<String> instanceId;

        /**
         * The set of topic IDs that the member is subscribed to.
         */
        List<Uuid> subscribedTopicIds;

        /**
         * The partitions owned by the member at the current epoch.
         */
        List<TopicIdPartition> ownedPartitions;
    }

     class TopicMetadata {
      	/**
		 * The topic ID.
		 */
		Uuid topicId;

        /**
		 * The number of partitions.
		 */
		int numPartitions; 
    } 

    class Assignment {
        /**
         * The member assignment.
         */
        List<MemberAssignment> members;
    }

    class MemberAssignment {
        /**
         * The member ID.
         */
        String memberId;

        /**
         * The assigned partitions.
         */
        List<TopicIdPartition> partitions;
    }

    /**
     * Unique name for this assignor.
     */
    String name();

    /**
     * Perform the group assignment given the current members and
     * topic metadata.
     *
     * @param group The group state.
     * @return The new assignment for the group.
     */
    Assignment assign(Group group);
}

Broker Metrics

The set of new metrics is not clear at the moment. We plan to amend the KIP later on when progress on the implementation would have been made.

• Group count by type
• Group count by state
• Rebalance Rate

Broker Configurations

New properties in the broker configuration.

...

request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "Topics", "type": "[]OffsetFetchResponseTopic", "versions": "0-7",
      "about": "The responses per topic.", "fields": [
      { "name": "Name", "type": "string", "versions": "0-7", "entityType": "topicName",
        "about": "The topic name." },
      { "name": "Partitions", "type": "[]OffsetFetchResponsePartition", "versions": "0-7",
        "about": "The responses per partition", "fields": [
        { "name": "PartitionIndex", "type": "int32", "versions": "0-7",
          "about": "The partition index." },
        { "name": "CommittedOffset", "type": "int64", "versions": "0-7",
          "about": "The committed message offset." },
        { "name": "CommittedLeaderEpoch", "type": "int32", "versions": "5-7", "default": "-1",
          "ignorable": true, "about": "The leader epoch." },
        { "name": "Metadata", "type": "string", "versions": "0-7", "nullableVersions": "0-7",
          "about": "The partition metadata." },
        { "name": "ErrorCode", "type": "int16", "versions": "0-7",
          "about": "The error code, or 0 if there was no error." }
      ]}
    ]},
    { "name": "ErrorCode", "type": "int16", "versions": "2-7", "default": "0", "ignorable": true,
      "about": "The top-level error code, or 0 if there was no error." },
    { "name": "Groups", "type": "[]OffsetFetchResponseGroup", "versions": "8+",
      "about": "The responses per group id.", "fields": [
      { "name": "groupId", "type": "string", "versions": "8+", "entityType": "groupId",
        "about": "The group ID." },
      { "name": "Topics", "type": "[]OffsetFetchResponseTopics", "versions": "8+",
        "about": "The responses per topic.", "fields": [
        { "name": "Name", "type": "string", "versions": "8+", "nullableVersions": "9+", "default": "null", "entityType": "topicName",
          "about": "The topic name."},
        { "name": "TopicId", "type": "uuid", "versions": "9+", "about": "The unique topic ID" },
        { "name": "Partitions", "type": "[]OffsetFetchResponsePartitions", "versions": "8+",
          "about": "The responses per partition", "fields": [
          { "name": "PartitionIndex", "type": "int32", "versions": "8+",
            "about": "The partition index." },
          { "name": "CommittedOffset", "type": "int64", "versions": "8+",
            "about": "The committed message offset." },
          { "name": "CommittedLeaderEpoch", "type": "int32", "versions": "8+", "default": "-1",
            "ignorable": true, "about": "The leader epoch." },
          { "name": "Metadata", "type": "string", "versions": "8+", "nullableVersions": "8+",
            "about": "The partition metadata." },
          { "name": "ErrorCode", "type": "int16", "versions": "8+",
            "about": "The partition-level error code, or 0 if there was no error." }
        ]}
      ]},
      { "name": "ErrorCode", "type": "int16", "versions": "8+", "default": "0",
        "about": "The group-level error code, or 0 if there was no error." }
    ]}
  ]
}

Response Handling

Upon receiving the STALE_MEMBER_EPOCH error, the consumer retries when receiving its next heartbeat response with its member epoch.

DescribeConfigs API

Request Schema

The schema is the same but the ResourceType field can be set to GROUP (16).

Required ACL

• Describe Config on the group.

Request Validation

No changes.

Request Handling

The new GROUP resource type is handled.

Response Schema

No changes.

Response Handling

No changes.

AlterIncrementalConfigs API

The API is the same but supports a new resource type: GROUP (16). When GROUP is used, the resource name corresponds to the group id.

Request Schema

The schema is the same but the ResourceType field can be set to GROUP (16).

Required ACL

• Alter Config on the group.

Request Validation

No changes.

Request Handling

The new GROUP resource type is handled.

Response Schema

No changes.

Response Handling

No changes.

Records

This section describes the new record types required for the new protocol. The storage layout is based on the data model described earlier in this document.

As explained earlier, they will be persisted in the __consumer_offsets compacted topic. The compacted topic based storage requires a dedicated key type per record type in order for the compaction to work. The current protocol already uses versions from 0 to 2 (included) for the keys.

Group Metadata

Groups can be rather large so we propose to use several records to store a group in order to not be limited by the maximum batch size (1MB by default). Therefore we propose to store group metadata with two records types: the ConsumerGroupMetadata and the ConsumerGroupMemberMetadata.

A group with X members will be stored with X+2 records. One ConsumerGroupMemberMetadata per member, one ConsumerGroupPartitionMetadata, and one ConsumerGroupMetadata for the group at the end. Atomicity is not a concern here. All the records can be applied independently.

Moreover, the whole group does not necessarily have to be written for every epoch. Members who have not changed could be omitted as the compacted topic will retain their previous state anyway.

When a member is deleted, a tombstone for it is written to the partition.

ConsumerGroupMetadataKey

{
    "type": "data",
    "name": "ConsumerGroupMetadataKey",
    "validVersions": "3",
    "flexibleVersions": "none",
    "fields": [
      	{ "name": "GroupId", "type": "string", "versions": "3" }
    ]
}

ConsumerGroupMetadataValue

{
    "type": "data",
    "name": "ConsumerGroupMetadataValue",
    "validVersions": "0",
    "flexibleVersions": "0+",
    "fields": [
        { "name": "Epoch", "versions": "0+", "type": "int32" }
    ], 
}

Note that the Epoch is always the latest epoch of the group.

ConsumerGroupPartitionMetadataKey

{
    "type": "data",
    "name": "ConsumerGroupPartitionMetadataKey",
    "validVersions": "4",
    "flexibleVersions": "none",
    "fields": [
      	{ "name": "GroupId", "type": "string", "versions": "4" }
    ]
}

ConsumerGroupPartitionMetadataValue

{
    "type": "data",
    "name": "ConsumerGroupPartitionMetadataValue",
    "validVersions": "0",
    "flexibleVersions": "0+",
    "fields": [
        { "name": "Epoch", "versions": "0+", "type": "int32" },
        { "name": "Topics", "versions": "0+",
          "type": "[]TopicMetadata", "fields": [
            { "name": "TopicId", "versions": "0+", "type": "uuid" },
            { "name": "NumPartitions", "versions": "0+", "type": "int32" }
          ]}
    ], 
}

Note that the Epoch is always the latest epoch of the group.

ConsumerGroupMemberMetadataKey

{
    "type": "data",
    "name": "ConsumerGroupMemberMetadataKey",
    "validVersions": "5",
    "flexibleVersions": "none",
    "fields": [
        { "name": "GroupId", "type": "string", "versions": "5" },
        { "name": "MemberId", "type": "string", "versions": "5" }
    ]
}

ConsumerGroupMemberMetadataValue

{
    "type": "data",
    "name": "ConsumerGroupMemberMetadataValue",
    "validVersions": "0",
    "flexibleVersions": "0+",
    "fields": [
        { "name": "GroupEpoch", "versions": "0+", "type": "int32" },
        { "name": "InstanceId", "versions": "0+", "nullableVersions": "0+", "type": "string" },
        { "name": "RackId", "versions": "0+", "nullableVersions": "0+", "type": "string" },
        { "name": "ClientId", "versions": "0+", "type": "string" },
        { "name": "ClientHost", "versions": "0+", "type": "string" },
        { "name": "SubscribedTopicNames", "versions": "0+", "type": "[]string" },
        { "name": "SubscribedTopicRegex", "versions": "0+", "type": "string" },
        { "name": "Assignors", "versions": "0+",
          "type": "[]Assignor", "fields": [
            { "name": "Name", "versions": "0+", "type": "string" },
            { "name": "MinimumVersion", "versions": "0+", "type": "int16" },
            { "name": "MaximumVersion", "versions": "0+", "type": "int16" },
            { "name": "Reason", "versions": "0+", "type": "int8" },
			{ "name": "Version", "versions": "0+", "type": "int16" },
            { "name": "Metadata", "versions": "0+", "type": "bytes" }
          ]}
    ], 
}

Note that the GroupEpoch is always the latest epoch of the group.

Target Assignment

The target assignment is stored in N + 1 records where N is the number of members in the group. The records for the members are written first and followed by the assignment metadata. When a new assignment is computed, the group coordinator will compare it with the current assignment and only write the difference between the two assignments to the __consumer_offsets partition. The assignment must be atomic so the group coordinator will ensure that all the records are written in a single batch. This limit the size of the batch to 1MB (the default value). Given the incremental nature of the protocol, 1MB should be sufficient in most case here. 

ConsumerGroupTargetAssignmentMetadataKey

{
    "type": "data",
    "name": "ConsumerGroupTargetAssignmentMetadataKey",
    "validVersions": "6",
    "flexibleVersions": "none",
    "fields": [
      	{ "name": "GroupId", "type": "string", "versions": "6" }
    ]
}

ConsumerGroupTargetAssignmentMetadataValue

{
    "type": "data",
    "name": "ConsumerGroupTargetAssignmentMetadataValue",
    "validVersions": "0",
    "flexibleVersions": "0+",
    "fields": [
        { "name": "AssignmentEpoch", "versions": "0+", "type": "int32" },
    ]
}

The AssignmentEpoch corresponds to the group epoch used to compute the assignment. It is not necessarily the most recent group epoch because the assignment is computed asynchronously when a client-side assignor is used. When a client side assignor is used, the assignment is computed asynchronously. While it is computed for the group at epoch X, the group may have already advanced to epoch X+1 due to another event (e.g. new member joined). In this case, we have chosen to install the assignment computed for epoch X and to trigger a new assignment computation right away.

ConsumerGroupTargetAssignmentMemberKey

{
    "type": "data",
    "name": "ConsumerGroupTargetAssignmentMemberKey",
    "validVersions": "7",
    "flexibleVersions": "none",
    "fields": [
      	{ "name": "GroupId", "type": "string", "versions": "7" },
        { "name": "MemberId", "type": "string", "versions": "7" }
     ]
}

ConsumerGroupTargetAssignmentMemberValue

{
    "type": "data",
    "name": "ConsumerGroupTargetAssignmentMemberValue",
    "validVersions": "0",
    "flexibleVersions": "0+",
    "fields": [
        { "name": "Error", "versions": "0+", "type": "int8" },
        { "name": "TopicPartitions", "versions": "0+",
       	  "type": "[]TopicPartition", "fields": [
      	  { "name": "TopicId", "versions": "0+", "type": "uuid" },
          { "name": "Partitions", "versions": "0+", "type": "[]int32" }
        ]},
        { "name": "MetadataVersion", "versions": "0+", "type": "int16" },
        { "name": "MetadataBytes", "versions": "0+", "type": "bytes" }
    ]
}

Current Member Assignment

The current member assignment represents, as the name suggests, the current assignment of a given member.

When a member is deleted from the group, a tombstone for it is written to the partition.

ConsumerGroupCurrentMemberAssignmentKey

{
    "type": "data",
    "name": "ConsumerGroupCurrentMemberAssignmentKey",
    "validVersions": "8",
    "flexibleVersions": "none",
    "fields": [
      	{ "name": "GroupId", "type": "string", "versions": "8" },
      	{ "name": "MemberId", "type": "string", "versions": "8" },
    ]
}

ConsumerGroupCurrentMemberAssignmentValue

{
    "type": "data",
    "name": "ConsumerGroupCurrentMemberAssignmentValue",
    "validVersions": "0",
    "flexibleVersions": "0+",
    "fields": [
        { "name": "MemberEpoch", "versions": "0+", "type": "int32" },
		{ "name": "Error", "versions": "0+", "type": "int8" },
        { "name": "TopicPartitions", "versions": "0+",
          "type": "[]TopicPartition", "fields": [
            { "name": "TopicId", "versions": "0+", "type": "uuid" },
            { "name": "Partitions", "versions": "0+", "type": "[]int32" }
        ]},
        { "name": "MetadataVersion", "versions": "0+", "type": "int16" },
        { "name": "MetadataBytes", "versions": "0+", "type": "bytes" }
    ], 
}

Offsets

OffsetCommitValue

{
  "type": "data",
  "name": "OffsetCommitValue",
  "validVersions": "0-4",
  "flexibleVersions": "4+",
  "fields": [
    { "name": "offset", "type": "int64", "versions": "0+" },
    { "name": "leaderEpoch", "type": "int32", "versions": "3+", "default": -1, "ignorable": true },
    { "name": "metadata", "type": "string", "versions": "0+" },
    { "name": "commitTimestamp", "type": "int64", "versions": "0+" },
    { "name": "expireTimestamp", "type": "int64", "versions": "1", "default": -1, "ignorable": true },
    // Adds TopicId field.
    { "name": "topicId", "type": "uuid", "versions": "4", "ignorable": true }
  ]
}

Broker API

The new PartitionAssignor interface will be introduced on the server side. Two implementations will be provided out of the box: RangeAssignor (range) and UniformAssignor (uniform).

package org.apache.kafka.common.errors;

public class PartitionAssignorException extends ApiException {

    public PartitionAssignorException(String message) {
        super(message);
    }

    public PartitionAssignorException(String message, Throwable cause) {
        super(message, cause);
    }
}

package org.apache.kafka.server.group.consumer;

public interface PartitionAssignor {

    class AssignmentSpec {
        /**
         * The members keyed by member id.
         */
        Map<String, AssignmentMemberSpec> members;

        /**
         * The topics' metadata keyed by topic id
         */
        Map<Uuid, AssignmentTopicMetadata> topics;
    }

    class AssignmentMemberSpec {
        /**
         * The instance ID if provided.
         */
        Optional<String> instanceId;

        /**
         * The rack ID if provided.
         */
        Optional<String> rackId;

        /**
         * The topics that the member is subscribed to.
         */
        Collection<String> subscribedTopics;

        /**
         * The current target partitions of the member.
         */
        Collection<TopicPartition> targetPartitions;
    }

    class AssignmentTopicMetadata {
        /**
         * The topic name.
         */
        String topicName;

        /**
		 * The number of partitions.
		 */
		int numPartitions; 
    }

    class GroupAssignment {
        /**
         * The member assignments keyed by member id.
         */
        Map<String, MemberAssignment> members;
    }

    class MemberAssignment {
        /**
         * The target partitions assigned to this member.
         */
        Collection<TopicPartition> targetPartitions;
    }

    /**
     * Unique name for this assignor.
     */
    String name();

    /**
     * Perform the group assignment given the current members and
     * topic metadata.
     *
     * @param assignmentSpec The assignment spec.
     * @return The new assignment for the group.
     */
    GroupAssignment assign(AssignmentSpec assignmentSpec) throws PartitionAssignorException;
}

Broker Metrics

Existing generic group metrics have been migrated, with the same metric names except for

NumGroups which reported the number of generic groups. This metric changed to

kafka.server:type=group-coordinator-metrics,name=group-count,protocol={consumer|generic}

• number of groups based on type where type is the rebalance protocol

kafka.server:type=group-coordinator-metrics,name=partition-count,state={loading|active|failed}

• number of __consumer_offsets partitions based on state

kafka.server:type=group-coordinator-metrics,name=event-queue-size

• event accumulator queue size

kafka.server:type=group-coordinator-metrics,name=consumer-group-count,state={empty|assigning|reconciling|stable|dead}

• number of consumer groups based on state

consumer group rebalances sensor

• kafka.server:type=group-coordinator-metrics,name=consumer-group-rebalance-rate
• kafka.server:type=group-coordinator-metrics,name=consumer-group-rebalance-count

partition load sensor: __consumer_offsets partition load time

• kafka.server:type=group-coordinator-metrics,name=partition-load-time-max
• kafka.server:type=group-coordinator-metrics,name=partition-load-time-avg

thread idle ratio sensor: thread busy - idle ratio

• kafka.server:type=group-coordinator-metrics,name=thread-idle-ratio-min
• kafka.server:type=group-coordinator-metrics,name=thread-idle-ratio-avg

Broker Configurations

New properties in the broker configuration.

Group Configurations

New dynamic group properties.

Consumer API

New PartitionAssignor interface

The new PartitionAssignor interface will be introduced to replace the ConsumerPartitionAssignor interface. The interface is defined as follow:

package org.apache.kafka.clients.consumer;
  
public interface PartitionAssignor {

    class Metadata {
        /**
         * The metadata version.
         */
        int version;

        /**
         * The metadata bytes.
         */
        ByteBuffer bytes;
    }

    class AssignmentSpec {
        /**
         * The members keyed by member id.
         */
        Map<String, AssignmentMemberSpec> members;

        /**
         * The topics' metadata keyed by topic id
         */
        Map<Uuid, AssignmentTopicMetadata> topics;
    }

    class AssignmentMemberSpec {
        /**
         * The instance ID if provided.
         */
        Optional<String> instanceId;

        /**
         * The rack ID if provided.
         */
        Optional<String> rackId;

        /**
         * The topics that the member is subscribed to.
         */
        Collection<String> subscribedTopics;

   		/**
		 * The reason reported by the member.
		 */
		byte reason;  

		/**
		 * The metadata reported by the member.
		 */
        Metadata metadata;

        /**
         * The current target partitions of the member.
         */
        Collection<TopicPartition> targetPartitions;
    }

    class AssignmentTopicMetadata {
        /**
         * The topic name.
         */
        String topicName;

        /**
		 * The number of partitions.
		 */
		int numPartitions; 
    }

    class GroupAssignment {
        /**
         * The assignment error.
         */
		byte error;

        /**
         * The member assignments keyed by member id.
         */
        Map<String, MemberAssignment> members;
    }

    class MemberAssignment

Group Configurations

New dynamic group properties.

...

Consumer API

New PartitionAssignor interface

The new PartitionAssignor interface will be introduced to replace the ConsumerPartitionAssignor interface. The interface is defined as follow:

package org.apache.kafka.clients.consumer;

public interface PartitionAssignor {

    class Group {
        /**
         * The memberstarget partitions assigned to this member.
         */
        List<GroupMember> members        Collection<TopicPartition> targetPartitions;

         		/**
         		 * The topics' metadata.
         		 */
        List<TopicMetadata>		Metadata topicsmetadata;
        }

    class	class GroupMemberAssignorMetadata {
           		/**
         		 * The member ID.
          reason reported by the assignor.
		 */
        String		byte memberIdreason; 

         		/**
         		 * The instancemetadata reported IDby ifthe providedassignor.
         		 */
        Optional<String>		Metadata instanceId;metadata;
    }

            /**
              * TheUnique setname offor topic IDs that the member is subscribed to.
         this assignor.
     */
        List<Uuid> subscribedTopicIds    String name();

   		    /**
		      * The reason reported by the member.
		 minimum version.
     */
		byte reason;  

		    int minimumVersion();

    /**
		      * The maximum version of the metadata encoded in {{@link GroupMember#metadata()}}.
		 */
		int version;

        .
     */
    int maximumVersion();

    /**
              * TheReturn customassignor metadata providedthat bywill thebe membersent as defined
         * by {{@link PartitionAssignor#metadata()}}.
         */
        ByteBuffer metadata;

        to the assignor.
     */
    AssignorMetadata metadata();

    /**
              * Perform Thethe partitionsgroup ownedassignment bygiven the membercurrent at the current epoch.
         */
        List<TopicIdPartition> ownedPartitions;
    }

    class TopicMetadata {
      	/**
		 * The topic ID.
		 */
		Uuid topicId;
members and
     * topic metadata.
     *
     * @param assignmentSpec The assignment spec.
     * @return The new assignment for the group.
     */
    GroupAssignment    assign(AssignmentSpec assignmentSpec);

    /**
		      * TheCallback numberwhich of partitions.
		 */
		int numPartitions; 
    }

    class Assignment {
        /**is invoked when the member received a new assignment 
     * from the assignor/group coordinator. This is called once per epoch
     * and contains the target *partitions Thefor assignmentthis error.members. This means that
     * partitions may  */
		byte error;

        /**
         * Thenot be assigned to the member assignment.
         */
        List<MemberAssignment> members;
    }

    class MemberAssignment {
        /**
         * The member ID.
         */
        String memberId;

        /**
         * The assigned partitions.
         */
        List<TopicIdPartition> partitions;

  		/**
		 *yet. The rebalance
     * listener must be used to know this.
     * 
     * @param byte The error reported by the assignor.
		 */
		byte error; 

 		/**
		 * The version of the metadata encoded in {{@link GroupMember#metadata()}}.
		 */
		int version;

        /**
         * The custom metadata provided by the assignor.
         */
        ByteBuffer metadata;
    }

	class Metadata {
   		/**
		 * The reason reported by the assignor.
		 */
		byte reason; 

 		/**
		 * The version of the metadata encoded in {{@link Metadata#metadata()}}.
		 */
		int version;

        /**
         * The custom metadata provided by the assignor.
         */
        ByteBuffer metadata; 
    }

    /**
     * Unique name for this assignor.
     */
    String name();

    /**
     * The minimum version.
     */
    int minimumVersion();

    /**
     * The maximum version.
     */
    int maximumVersion();

    /**
     * Return serialized data that will be sent to the assignor.
     */
    Metadata metadata();

    /**
     * Perform the group assignment given the current members and
     * topic metadata.
     *
     * @param group The group state.
     * @return The new assignment for the group.
     */
    Assignment assign(Group group);

    /**
     * Callback which is invoked when the member received a new
     * assignment from the assignor/group coordinator.
     */
    void onAssignment(MemberAssignment assignment);
}

Deprecate Consumer#enforceRebalance and Consumer#enforceRebalance(String)

The enforceRebalance methods are no longer necessary and will be deprecated in a future release.

...

     * @param assignment The assignment computed by the assignor.
     * @param consumerGroupMetadata The group metadata.
     */
    void onAssignment(byte error, MemberAssignment assignment, ConsumerGroupMetadata consumerGroupMetadata);
}

New SubscriptionPattern class

We need to differentiate Google RE2/J regular expression from the java.util.regex.Pattern in our public APIs so we propose to introduce the SubscriptionPattern class for this purpose. This class is just a POJO as all the validation is on the server side.

package org.apache.kafka.clients.consumer;

/**
 * Represents a regular expression used to subscribe to topics. The pattern
 * must be a Google RE2/J compatible pattern.
 */
public class SubscriptionPattern {
    final private String pattern;

    public Pattern(final pattern) {
        this.pattern = pattern;
    }

    public String pattern() {
        return this.pattern;
    }
}

New Consumer methods

We introduce two new methods to subscribe with a SubscriptionPattern.

public interface Consumer<K, V> extends Closeable {
    ...

    /**
     * @see KafkaConsumer#subscribe(Pattern, ConsumerRebalanceListener)
     */
    void subscribe(SubscriptionPattern pattern, ConsumerRebalanceListener callback);

    /**
     * @see KafkaConsumer#subscribe(Pattern)
     */
    void subscribe(SubscriptionPattern pattern);
}

Deprecate Consumer methods

The following methods will be deprecated:

• Consumer#enforeRebalance
• Consumer#enforeRebalance(String)
• Consumer#subscribe(Pattern)
• Consumer#subscribe(Pattern, ConsumerRebalanceListener)

Deprecate ConsumerPartitionAssignor interface

The ConsumerPartitionAssignor interface will be deprecated in a future (major) release.

Deprecate Consumer configurations

The following configurations will be deprecated:

• partition.assignment.strategy
• session.timeout.ms
• heartbeat.interval.ms

Consumer Configurations

...

Assignment Metadata Errors

• None (0)
• Shutdown (1)
• AssignmentError (2)
InconsistentTopology (3
• (1)
• AssignmentError (2)
• InconsistentTopology (3)

Streams API

New Topology methods

All the Topology#addSource methods using java.util.regex.Pattern will get a corresponding overload using SubscriptionPattern.

Deprecated methods

The following methods will be deprecated:

• Topology#addSource(String, Pattern)
• Topology#addSource(AutoOffsetReset, String, Pattern)
• Topology#addSource(TimestampExtractor, String, Pattern)
• Topology#addSource(AutoOffsetReset, TimestampExtractor, String, Pattern)
• Topology#addSource(String, Deserializer, Deserializer, Pattern)
• Topology#addSource(AutoOffsetReset, String, Deserializer, Deserializer, Pattern)
• Topology#addSource(AutoOffsetReset, String, TimestampExtractor, Deserializer, Deserializer, Pattern)

Streams Configurations

...

public class ConsumerGroupDescription {
    public String type() {
      return type;
    }
}

public class MemberDescription {
    // Current Assignment**
     * The current assignment of the member. Provided for both generic group and consumer group.
     */
    public MemberAssignment assignment() {}

    // Target Assignment
    public MemberAssignment    /**
     * The target assignment of the member. Provided only for consumer group.
     */
    public Optional<MemberAssignment> targetAssignment() {}
}

public class MemberAssignmentMetadata {
	/**
	 * The reason reported by the assignor.
	 */
	byte error; 
    /**
     * The metadata version.
     */
	int version;

    /**
     * The metadata bytes.
     */
	ByteBuffer bytes;
} 

public class MemberAssignment {
	/**
	 * The version of the* metadata encoded in {{@link Metadata#metadata()}}The partitions.
	     */
	int    Set<TopicPartition> versiontopicPartitions;

 	/**
	 * The customerror metadata providedreported by the assignor. Provided only if the group is a ConsumerGroup type.
	 */
	ByteBufferbyte metadataerror;

	    /**
     * The assigned metadata. partitionsProvided ownedonly byif the membergroup atis theConsumerGroup current epochtype.
         */
    List<TopicIdPartition> ownedPartitions   Optional<Metadata> metadata;
}

Admin#incrementalAlterConfigs and Admin#describeConfigs

...

public final class ConfigResource {
    /**
     * Type of resource.
     */
    public enum Type {
         GROUP((byte) 16), BROKER_LOGGER((byte) 8), BROKER((byte) 4), TOPIC((byte) 2), UNKNOWN((byte) 0);
    }
}

kafka-consumer-groups

--type 

The kafka-consumer-group command line tool will be extended to support the –type filter which allows to list or to describe groups implementing a specific type.

kafka-consumer-groups.sh -–bootstrap-server localhost:9092 -–list -–type <comma separated list of types>

 types>

kafka-consumer-groups.sh -–bootstrap-server localhost:9092 -–describe -–type <comma separated list of types>

--validate-regex 

The kafka-consumer-group command line tool will be extended to support the –validate-regex which allows to verify the regular expression use by a group to subscribe to topics.

kafka-consumer-groups.sh -–bootstrap-server localhost:9092 -–describe –validate-–type <comma separated list of types>

...

regex <pattern>

Case Studies

All the examples shown in this chapter are based on the in-memory representation of the group coordinator.

Basic

Let’s look at a few examples to illustrate the rebalance logic. Let’s assume that the group is subscribed to the topic foo which has 3 partitions.

...

• Group (epoch=1)
• A
• Target Assignment (epoch=0)
• Empty
• Member Assignment
• A - epoch=0, partitions=[], pending-partitions=[]

The coordinator computes and installs the new target assignment. All the partitions are assigned to A.

• Group (epoch=1)
• A
• Target Assignment (epoch=1)
• A - partitions=[foo-0, foo-1, foo-2]
• Member Assignment
• A - epoch=0, partitions=[], pending-partitions=[]

When A heartbeats, the group coordinator transitions him it to its target epoch/assignment because it does not have any partitions to revoke. The group coordinator updates the member assignment and replies with the new epoch 1 and all the partitions.

• Group (epoch=1)
• A
• Target Assignment (epoch=1)
• A - partitions=[foo-0, foo-1, foo-2]
• Member Assignment
• A - epoch=1, partitions=[foo-0, foo-1, foo-2], pending-partitions=[]

Member B joins the group. The coordinator adds the member to the group and bumps the group epoch to 2.

• Group (epoch=2)
• A
• B
• Target Assignment (epoch=1)
• A - partitions=[foo-0, foo-1, foo-2]
• Member Assignment
• A - epoch=1, partitions=[foo-0, foo-1, foo-2], pending-partitions=[]
• B - epoch=0, partitions=[], pending-partitions=[]

The coordinator computes and installs the new target assignment.

• Group (epoch=2)
• A
• B
• Target Assignment (epoch=2)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-2]
• Member Assignment
• A - epoch=1, partitions=[foo-0, foo-1, foo-2], pending-partitions=[]
• B - epoch=10, partitions=[foo-2], pending-partitions=[]

At this point B can transitions to epoch 1 2 but cannot get foo-2 until A revokes it.

• Group (epoch=2)
• A
• B
• Target Assignment (epoch=2)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-2]
• Member Assignment
• A - epoch=1, partitions=[foo-0, foo-1, foo-2], pending-partitions=[]
• B - epoch=2, partitions=[], pending-partitions=[foo-2]

When A heartbeats, the group coordinator instructs him it to revoke foo-2.

When A heartbeats again and acknowledges the revocation, the group coordinator transitions him it to epoch 2 and releases foo-2.

• Group (epoch=2)
• A
• B
• Target Assignment (epoch=2)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-2]
• Member Assignment
• A - epoch=2, partitions=[foo-0, foo-1], pending-partitions=[]
• B - epoch=1, 2, partitions=[], pending-partitions=[foo-2]

When B heartbeats, he can now gets foo-2.

• Group (epoch=2)
• A
• B
• Target Assignment (epoch=2)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-2]
• Member Assignment
• A - epoch=2, partitions=[foo-0, foo-1]
• B - epoch=2, partitions=[foo-2], pending-partitions=[]

Member C joins the group. The coordinator adds the member to the group and bumps the group epoch to 3.

• Group (epoch=3)
• A
• B
• C
• Target Assignment (epoch=2)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-2]
• Member Assignment
• A - epoch=2, partitions=[foo-0, foo-1], pending-partitions=[]
• B - epoch=2, partitions=[foo-2], pending-partitions=[]
• C - epoch=0, partitions=[], partitions=[]pending-partitions=[]

The coordinator computes and installs the new target assignment.

• Group (epoch=3)
• A
• B
• C
• Target Assignment (epoch=3)
• A - partitions=[foo-0]
• B - partitions=[foo-2]
• C - partitions=[foo-1]
• Member Assignment
• A - epoch=2, partitions=[foo-0, foo-1], pending-partitions=[]
• B - epoch=2, partitions=[foo-2], pending-partitions=[]
• C - epoch=0, partitions=[], pending-partitions=[]

When B heartbeats, the group coordinator transitions it to epoch 3 because B has no partitions to revoke. It persists the change and reply. The coordinator computes and installs the new target assignment.

• Group (epoch=3)
• A
• B
• C
• Target Assignment (epoch=3)
• A - partitions=[foo-0]
• B - partitions=[foo-2]
• C - partitions=[foo-1]
• Member Assignment
• A - epoch=2, partitions=[foo-0, foo-1], pending-partitions=[]
• B - epoch=23, partitions=[foo-2], pending-partitions=[]
• C - epoch=0, partitions=[], pending-partitions=[]

When B C heartbeats, the group coordinator it transitions him to epoch 3 because B has no partitions to revoke. It persists the change and reply.but cannot get foo-1 yet. 

• Group (epoch=3)
• A
• B
• C
• Target Assignment (epoch=3)
• A - partitions=[foo-0]
• B - partitions=[foo-2]
• C - partitions=[foo-1]
• Member Assignment
• A - epoch=2, partitions=[foo-0, foo-1], pending-partitions=[]
• B - epoch=3, partitions=[foo-2], pending-partitions=[]
• C - epoch=3, partitions=[foo-1]

...

• , pending-partitions=[foo-1

...

• ]

When A heartbeats, the group coordinator instructs him it to revoke foo-1.

When A heartbeats again and acknowledges the revocation, the group coordinator transitions him it to epoch 2.When C heartbeats, the group coordinator transitions him to epoch 3 , persists the change, and replyand releases foo-1.

• Group (epoch=3)
• A
• B
• C
• Target Assignment (epoch=3)
• A - partitions=[foo-0]
• B - partitions=[foo-2]
• C - partitions=[foo-1]
• Member Assignment
• A - epoch=23, partitions=[foo-0], pending-partitions=[]
• B - epoch=3, partitions=[foo-2], pending-partitions=[]
• C - epoch=3, partitions=[foo-1], pending-partitions=[]

All the members have eventually advanced to the group epoch (3).

...

• Group (epoch=21)
• A
• B
• Target Assignment (epoch=21)
• A - partitions=[foo-0, foo-1, foo-2]
• B - partitions=[foo-3, foo-4, foo-5]
• Member Assignment
• A - epoch=21, partitions=[foo-0, foo-1, foo-2], pending-partitions=[]
• B - epoch=21, partitions=[foo-3, foo-4, foo-5], pending-partitions=[]

C joins the group. The group coordinator adds himit, bumps the group epoch, create the member assignment, and computes the target assignment.

• Group (epoch=22)
• A
• B
• C
• Target Assignment (epoch=22)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-3, foo-4]
• C - partitions=[foo-2, foo-5]
• Member Assignment
• A - epoch=21, partitions=[foo-0, foo-1, foo-2], pending-partitions=[]
• B - epoch=21, partitions=[foo-3, foo-4, foo-5], pending-partitions=[]
• C - epoch=0, , partitions=[], pending-partitions=[]

C heartbeats, the group coordinator transitions him it to epoch 22 but does not yet give him it any partitions because they are not revoked yet.

• Group (epoch=22)
• A
• B
• C
• Target Assignment (epoch=22)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-3, foo-4]
• C - partitions=[foo-2, foo-5]
• Member Assignment
• A - epoch=21, partitions=[foo-0, foo-1, foo-2], pending-partitions=[]
• B - epoch=21, partitions=[foo-3, foo-4, foo-5], pending-partitions=[]
• C - epoch=22, partitions=[], pending-partitions=[foo-2, foo-5]

A heartbeats, the group coordinator instructs him it to revoke foo-2.

B heartbeats, the group coordinator instructs him it to revoke foo-5.

C heartbeats, no changes for himit.

A heartbeats and acknowledges the revocation, the group coordinator transitions him it to epoch 22, release foo-2, persists and reply.

• Group (epoch=22)
• A
• B
• C
• Target Assignment (epoch=22)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-3, foo-4]
• C - partitions=[foo-2, foo-5]
• Member Assignment
• A - epoch=22, partitions=[foo-0, foo-1], pending-partitions=[]
• B - epoch=21, partitions=[foo-3, foo-4, foo-5], pending-partitions=[]
• C - epoch=22, partitions=[foo-2], pending-partitions=[foo-5]

C heartbeats, the group coordinator gives him it foo-2 which is now free but hold foo-5.

B heartbeats and acknowledges the revocation, the group coordinator transitions him it to epoch 22, releases foo-5, persists and reply.

• Group (epoch=22)
• A
• B
• C
• Target Assignment (epoch=22)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-3, foo-4]
• C - partitions=[foo-2, foo-5]
• Member Assignment
• A - epoch=22, partitions=[foo-0, foo-1], pending-partitions=[]
• B - epoch=22, partitions=[foo-3, foo-4], pending-partitions=[]
• C - epoch=22, partitions=[foo-2, foo-5], pending-partitions=[]

C heartbeats, the group coordinator gives him it foo-2 and foo-5.

Member Failure

...

• Group (epoch=22)
• A
• B
• C
• Target Assignment (epoch=22)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-3, foo-4]
• C - partitions=[foo-2, foo-5]
• Member Assignment
• A - epoch=22, partitions=[foo-0, foo-1], pending-partitions=[]
• B - epoch=22, partitions=[foo-3, foo-4], pending-partitions=[]
• C - epoch=22, partitions=[foo-2, foo-5], pending-partitions=[]

A fails to heartbeat. The group coordinator removes him it after the session timeout expires and bump the group epoch.

• Group (epoch=23)
  • B
  • C
• Target Assignment (epoch=22)
  • A - partitions=[foo-0, foo-1]
  • B - partitions=[foo-3, foo-4]
  • C - partitions=[foo-2, foo-5]
• Member Assignment
  
  • B - epoch=22, partitions=[foo-3, foo-4], pending-partitions=[]
  • C - epoch=22, partitions=[foo-2, foo-5], pending-partitions=[]

The group coordinator computes the new target assignment.

• Group (epoch=23)
  • B
  • C
• Target Assignment (epoch=23)
  
  • B - partitions=[foo-3, foo-4, foo-0]
  • C - partitions=[foo-2, foo-5, foo-1]
• Member Assignment
  
  • B - epoch=22, partitions=[foo-3, foo-4], pending-partitions=[]
  • C - epoch=22, partitions=[foo-2, foo-5], pending-partitions=[]

B and C heartbeat and transition to epoch 23.

• Group (epoch=23)
  • B
  • C
• Target Assignment (epoch=23)
  
  • B - partitions=[foo-3, foo-4, foo-0]
  • C - partitions=[foo-2, foo-5, foo-1]
• Member Assignment
  
  • B - epoch=23, partitions=[foo-3, foo-4, foo-0], pending-partitions=[]
  • C - epoch=23, partitions=[foo-2, foo-5, foo-1], pending-partitions=[]

Partition Added

Let's start with a group with two members and one partition.

• Group (epoch=22)
• A
• B
• Target Assignment (epoch=22)
• A - partitions=[foo-0]
• B - partitions=[]
• Member Assignment
• A - epoch=22, partitions=[foo-0], pending-partitions=[]
• B - epoch=22, partitions=[], pending-partitions=[]

A new partition foo-1 is created. The group coordinator detects it. It updates the group and bump the group epoch.

• Group (epoch=23)
• A
• B
• Target Assignment (epoch=22)
• A - partitions=[foo-0]
• B - partitions=[]
• Member Assignment
• A - epoch=22, partitions=[foo-0], pending-partitions=[]
• B - epoch=22, partitions=[], pending-partitions=[]

The group coordinator computes a new target assignment.

• Group (epoch=23)
• A
• B
• Target Assignment (epoch=23)
• A - partitions=[foo-0]
• B - partitions=[foo-1]
• Member Assignment
• A - epoch=22, partitions=[foo-0], pending-partitions=[]
• B - epoch=22, =22, partitions=[], pending-partitions=[]

B and C heartbeat and transition to epoch 23.

• Group (epoch=23)
• A
• B
• Target Assignment (epoch=23)
• A - partitions=[foo-0]
• B - partitions=[foo-1]
• Member Assignment
• A - epoch=23, partitions=[foo-0], pending-partitions=[]
• B - epoch=23, partitions=[foo-1], pending-partitions=[]

Online Migration

We starts with a generic group.

• Generic Group (generation=22)
• A
• B
• C
• Assignment
• A - partitions=[foo-0, foo-1], pending-partitions=[]
• B - partitions=[foo-3, foo-4], pending-partitions=[]
• C - partitions=[foo-2, foo-5], pending-partitions=[]

A leaves and rejoins with the new protocol enabled. The group is converted. The current generation becomes the group epoch. The target assignment and the member assignments are created based on the current assignment.

• Group (epoch=22)
• A (upgraded)
• B
• C
• Target Assignment (epoch=22)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-3, foo-4]
• C - partitions=[foo-2, foo-5]
• Member Assignment
• A - epoch=22, partitions=[foo-0, foo-1], pending-partitions=[]
• B - epoch=22, partitions=[foo-3, foo-4], pending-partitions=[]
• C - epoch=22, partitions=[foo-2, foo-5], pending-partitions=[]

A uses the new protocol. B and C still use the old protocol.

B leaves the group. The group coordinator removes him it and bumps the group epoch.

• Group (epoch=23)
• A (upgraded)
• C
• Target Assignment (epoch=22)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-3, foo-4]
• C - partitions=[foo-2, foo-5]
• Member Assignment
• A - epoch=22, partitions=[foo-0, foo-1], pending-partitions=[]
• C - epoch=22, partitions=[foo-2, foo-5], pending-partitions=[]

The group coordinator computes a new target assignment and installs it. It also triggers a rebalance for C. 

• Group (epoch=23)
• A (upgraded)
• C (PreparingRebalance)
• Target Assignment (epoch=23)
• A - partitions=[foo-0, foo-1, foo-3]
• C - partitions=[foo-2, foo-5, foo-4]
• Member Assignment
• A - epoch=22, partitions=[foo-0, foo-1], pending-partitions=[]
• C - epoch=22, partitions=[foo-2, foo-5], pending-partitions=[]

C heartbeats and is notified that a rebalance is required. C revokes all its partitions (assuming Eager protocol is used here) and sends a JoinGroup request.

The group coordinator sees that C does not own any partitions any more, so it can transition to epoch 23 and transition to CompletingRebalance. The transition to epoch 23 is important here because the new epoch must be given to the member in the JoinGroup response. This is the new generation of the group for it.

• Group (epoch=23)
• A (upgraded)
• C (CompletingRebalance)
• Target Assignment (epoch=23)
• A - partitions=[foo-0, foo-1, foo-3]
• C - partitions=[foo-2, foo-5, foo-4]
• Member Assignment
• A - epoch=22, partitions=[foo-0, foo-1], pending-partitions=[]
• C - epoch=23, partitions=[foo-2, foo-5, foo-4], pending-partitions=[]

In the meantime, A heartbeats and transitions to epoch 23 as well.

• Group (epoch=23)
• A (upgraded)
• C (CompletingRebalance)
• Target Assignment (epoch=23)
• A - partitions=[foo-0, foo-1, foo-3]
• C - partitions=[foo-2, foo-5, foo-4]
• Member Assignment
• A - epoch=23, partitions=[foo-0, foo-1, foo-3], pending-partitions=[]
• C - epoch=23, partitions=[foo-2, foo-5, foo-4], pending-partitions=[]

C sends the SyncGroup request and collects his its new assignment. All partitions are given because they are all free. C transitions to Stable.

• Group (epoch=23)
• A (upgraded)
• C (Stable)
• Target Assignment (epoch=23)
• A - partitions=[foo-0, foo-1, foo-3]
• C - partitions=[foo-2, foo-5, foo-4]
• Member Assignment
• A - epoch=23, partitions=[foo-0, foo-1, foo-3], pending-partitions=[]
• C - epoch=23, partitions=[foo-2, foo-5, foo-4], pending-partitions=[]

B rejoins the group with the new protocol. The group coordinator adds it and bumps the group epoch.

• Group (epoch=24)
• A (upgraded)
• B (upgraded)
• C (Stable)
• Target Assignment (epoch=23)
• A - partitions=[foo-0, foo-1, foo-3]
• C - partitions=[foo-2, foo-5, foo-4]
• Member Assignment
• A - epoch=23, partitions=[foo-0, foo-1, foo-3], pending-partitions=[]
• B A - epoch=0, partitions=[], pending-partitions=[]
• C - epoch=23, partitions=[foo-2, foo-5, foo-4], pending-partitions=[]

The group coordinator computes a new target assignment. A rebalance is triggered for C to revoke foo-4.

• Group (epoch=24)
• A (upgraded)
• B (upgraded)
• C (PreparingRebalance)
• Target Assignment (epoch=24)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-3, foo-4]
• C - partitions=[foo-2, foo-5]
• Member Assignment
• A - epoch=23, partitions=[foo-0, foo-1, foo-3], pending-partitions=[]
• B - epoch=0, partitions=[], pending-partitions=[foo-3, foo-4]
• C - epoch=23, partitions=[foo-2, foo-5, foo-4], pending-partitions=[]

A heartbeats and he is told to revoke foo-3.

...

The group coordinator sees that C does not own any partitions any more, so it can transition to epoch 24 and transition to CompletingRebalance. foo-4 is released.

• Group (epoch=24)
• A (upgraded)
• B (upgraded)
• C (CompletingRebalance)
• Target Assignment (epoch=24)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-3, foo-4]
• C - partitions=[foo-2, foo-5]
• Member Assignment
• A - epoch=23, partitions=[foo-0, foo-1, foo-3], pending-partitions=[]
• B - epoch=24, partitions=[foo-3, 4], pending-partitions=[foo-43]
• C - epoch=24, partitions=[foo-2, foo-5], pending-partitions=[]

C sends the SyncGroup request to collect its assignment. He transitions to Stable.

• Group (epoch=24)
• A (upgraded)
• B (upgraded)
• C (Stable)
• Target Assignment (epoch=24)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-3, foo-4]
• C - partitions=[foo-2, foo-5]
• Member Assignment
• A - epoch=23, partitions=[foo-0, foo-1, foo-3], pending-partitions=[]
• B - epoch=24, partitions=[foo-3, 4], pending-partitions=[foo-43]
• C - epoch=24, partitions=[foo-2, foo-5], pending-partitions=[]

A heartbeats. He confirms the revocation of foo-3. He transitions to epoch 24. foo-3 is released.

• Group (epoch=24)
• A (upgraded)
• B (upgraded)
• C (Stable)
• Target Assignment (epoch=24)
• A - partitions=[foo-0, foo-1]
• B - partitions=[foo-3, foo-4]
• C - partitions=[foo-2, foo-5]
• Member Assignment
• A - epoch=24, partitions=[foo-0, foo-1], pending-partitions=[]
• B - epoch=24, partitions=[foo-3, foo-4], pending-partitions=[]
• C - epoch=24, partitions=[foo-2, foo-5]

B heartbeats and gets his assignment.

Compatibility, Deprecation, and Migration Plan

Kafka Broker Migration

Upgrading the cluster to the new rebalance protocol is pretty strait-forward. First, the cluster must be first upgraded to the new group coordinator. This can be done by rolling-upgrading the broker to a software version which supports it. The migration is seamless. Note that group.coordinator.threads may require some tuning depending on your workload. Second, the new protocol must be enabled by setting an IBP/MetadataVersion which supports it with the kafka-features command line tool.

Downgrading the cluster will be possible to a certain extend. It is possible to downgrade the IBP/MetadataVersion to disable the new rebalance protocol. In this case, all the consumer groups using the new protocol will be lost. It will be possible to downgrade to an earlier version of Kafka that does not support the new group coordinator. In this case, we will only support specific versions. The issue is that the current group coordinator errors out when the __consumer_offsets topics contains unknown records.

Kafka Consumer Migration

Upgrading consumers can be done online. First, all the consumers must be upgraded to a software version which supports the online migration. The version is not defined yet but the consumer will need to support the consumer embedded protocol version 3 (introduced in KIP-792). There are also other considerations if a custom client side assignor is used. Second, the consumer must be rolled to enable the new rebalance protocol. This is done by setting group.protocol to consumer. group.remote.assignor may need to be adjusted as well. When a consumer joins with the new protocol, the group is automatically converted from a generic group to a consumer group if the upgrade requirements are met (e.g. consumer embedded protocol version >=3). If they are not, the consumer is rejected with an INVALID_REQUEST error. During the upgrade process, consumers will continue to use the old rebalance APIs. The group coordinator will translate the JoinGroup, SyncGroup and Heartbeat API to the new rebalance protocol. This translation is an implementation detail but it is explained earlier in this document, see the Supporting Online Consumer Group Upgrade chapter. If a customer client side assignor or regex based subscriptions are used, please pay attention to the details provided in the following sub-chapters.

• foo-2, foo-5], pending-partitions=[]

B heartbeats and gets its assignment.

Compatibility, Deprecation, and Migration Plan

Kafka Broker Migration

Upgrading the cluster to the new rebalance protocol is pretty strait-forward. First, the cluster must be first upgraded to the new group coordinator. This can be done by rolling-upgrading the broker to a software version which supports it. The migration is seamless. Note that group.coordinator.threads may require some tuning depending on your workload. Second, the new protocol must be enabled by setting an IBP/MetadataVersion which supports it with the kafka-features command line tool.

Downgrading the cluster will be possible to a certain extend. It is possible to downgrade the IBP/MetadataVersion to disable the new rebalance protocol. In this case, all the consumer groups using the new protocol will be lost. It will be possible to downgrade to an earlier version of Kafka that does not support the new group coordinator. In this case, we will only support specific versions. The issue is that the current group coordinator errors out when the __consumer_offsets topics contains unknown records.

Kafka Consumer Migration

Upgrading consumers can be done online. First, all the consumers must be upgraded to a software version which supports the online migration. The version is not defined yet but the consumer will need to support the consumer embedded protocol version 3 (introduced in KIP-792). There are also other considerations if a custom client side assignor is used. Second, the consumer must be rolled to enable the new rebalance protocol. This is done by setting group.protocol to consumer. group.remote.assignor may need to be adjusted as well. When a consumer joins with the new protocol, the group is automatically converted from a generic group to a consumer group if the upgrade requirements are met (e.g. consumer embedded protocol version >=3). If they are not, the consumer is rejected with an INVALID_REQUEST error. During the upgrade process, consumers will continue to use the old rebalance APIs. The group coordinator will translate the JoinGroup, SyncGroup and Heartbeat API to the new rebalance protocol. This translation is an implementation detail but it is explained earlier in this document, see the Supporting Online Consumer Group Upgrade chapter. If a customer client side assignor or regex based subscriptions are used, please pay attention to the details provided in the following sub-chapters.

Downgrading the consumers is the exact opposite process. The consumer must be rolled to disable the new rebalance protocol. This is done by setting the the group.protocol to generic. When the last consumer using the new rebalance protocol leaves the group, the group is automatically converted down from a consumer group to a generic group.

Regex Based Subscriptions

The new rebalance protocol relies on the group coordinator to track the metadata changes so the regular expressions is not used locally anymore but remotely. The group coordinator uses the Google RE2/J engine so the regular expression used with either of the methods must be compatible. Usage of methods subscribing to topics using java.util.regex.Pattern should be replaced by their homolog using SubscriptionPattern. By default, the java.util.regex.Pattern used to subscribe is toString'ed and passed to the group coordinator so simple regular expressions should keep working without any changes. If it is not compatible, the group coordinator will reject the ConsumerGroupHeartbeat request with an INVALID_REQUEST error. For simple regular expressions, we don't expect any changes to be required. It is recommended to test out the regex with the consumer-groups --verify-regex command line tool or with another group before migrating consumers.

Client Side Assignor

ConsumerPartitionAssignor cannot be used with the new rebalance protocol. Instead, the client side assignor must implement PartitionAssignor and configure group.local.assignors. The Javadoc of PartitionAssignor in the Public Interfaces section of this document explains how it works. During the migration, only the PartitionAssignor based assignor is used to compute the assignment for the group. That means that the new assignor must be able to deserialize the subscription metadata used by the old assignor and the serialize assignment metadata for the old members. The group coordinator cannot translate the metadata that it receives. However, it will signal them by using -1 as the version for it. This means that the new assignor must support version from -1 to X during the online migration. If a consumer using the new protocol joins a group with non-empty metadata, the group coordinator will ensure that the joining member's assignor supports -1 in its version range. It it does not, the group coordinator will reject it with an INVALID_REQUEST error.

Kafka Streams Migration

Upgrading streams instances can be done online. First, all the instances must be upgraded to a software version which supports the online migration. This version will be driven by the underlying consumer requirements. Second, the instances must be rolled to enable Downgrading the consumers is the exact opposite process. The consumer must be rolled to disable the new rebalance protocol. This is done by setting the the group.protocol to generic. When the last consumer using the new rebalance protocol leaves the group, the group is automatically converted down from a consumer group to a generic group.

Regex Based Subscriptions

The new rebalance protocol relies on the group coordinator to track the metadata changes so the regular expressions is not used locally anymore but remotely. The group coordinator uses the Google RE2/J engine so the regular expression used with either of the methods must be compatible. The java.util.Pattern used to subscribe is toString'ed and passed to the group coordinator. If it is not compatible, the group coordinator will reject the ConsumerGroupHeartbeat request with an INVALID_REQUEST error. For simple regular expressions, we don't expect any changes to be required. It is recommended to test out the regex with the consumer-groups --verify-regex command line tool or with another group before migrating consumers.

Client Side Assignor

ConsumerPartitionAssignor cannot be used with the new rebalance protocol. Instead, the client side assignor must implement PartitionAssignor and configure group.local.assignors. The Javadoc of PartitionAssignor in the Public Interfaces section of this document explains how it works. During the migration, only the PartitionAssignor based assignor is used to compute the assignment for the group. That means that the new assignor must be able to deserialize the subscription metadata used by the old assignor and the serialize assignment metadata for the old members. The group coordinator cannot translate the metadata that it receives. However, it will signal them by using -1 as the version for it. This means that the new assignor must support version from -1 to X during the online migration. If a consumer using the new protocol joins a group with non-empty metadata, the group coordinator will ensure that the joining member's assignor supports -1 in its version range. It it does not, the group coordinator will reject it with an INVALID_REQUEST error.

Kafka Streams Migration

Upgrading streams instances can be done online. First, all the instances must be upgraded to a software version which supports the online migration. This version will be driven by the underlying consumer requirements. Second, the instances must be rolled to enable the new rebalance protocol. This is done by setting group.protocol to consumer.

Downgrading the instances is the exact opposite process. The instances must be rolled to disable the new rebalance protocol. This is done by setting the the group.protocol to generic.

Test Plan

Our primary method for testing the implementation will be through Discrete Event Simulation (DES). DES allows us to test a large number of deterministically generated random scenarios which include various kinds of faults (such as network partitions). It allows us to define system invariants programmatically which are then checked after each step in the simulation. The protocol will be formally verified with a TLA+ model as well. Other than that, we will use the typical suite of unit/integration/system tests. System tests will be parameterised to run with both protocols.

Rejected Alternatives

An epoch per partition

We started this design by using an epoch per partition instead of relying on an epoch per member. Moving a partition from A to B would have require the following step: 1) revoke the partition from A; 2) bump the partition epoch; and 3) assign the partition to B. While this was very appealing at first, it was unpractical in the end for two reasons: 1) migrating from the current protocol is much more difficult without a member id; and 2) the metadata associated to the assignment (e.g. Streams metadata) is not tight to a particular partitions. We ended up using a member epoch with an incremental reassignment algorithm which is pretty close to this.

An epoch per member not aligned to the group epoch

The current design ensures that each member epoch eventually converged to the group epoch. In a previous iteration of the design, we considered incrementing member epoch only when the member's assignment changed. The benefit of this is that it reduce the number of writes to the __consumer_offsets partitions. The downside is that it is harder to debug/understand for users and operators because they cannot rely on the member epoch to know if the member has converged to the desired assignment. In the end, we decided to favour the debuggability. 

Not reusing the current coordinator

We considered not reusing the current group coordinator. Instead, the idea was to implement a brand new consumer group coordinator dedicated for the new rebalance protocol. The main benefits of this is that we could have moved away from the __consumer_offsets storage and use something more appropriate, perhaps closer to the KRaft metadata topic. This was rejected because migrating from a generic group to a consumer group would have been much more difficult.

No more client-side assignors, even for Kafka Streams

We considered removing the client-side assignor feature. From a consumer perspective, this is rarely used nowadays. Kafka Streams is its primary user so we thought about using a server side assignor in this case as well. We abandoned this for two reasons: 1) the Streams' assignor needs to know the entire Streams's topology so each member would have had to send it out to the server. The topology could be rather big (in MB) so this is not very practical; and 2) That would have introduced a strong dependency between the server version and the Streams version. Using new features in Streams would not be possible without upgrading the servers first. 

Future Work

Eventually, we aim at deprecating the current membership/rebalance API. In order to get to this point, we would need to first move all the use cases away from it.

Connect Rebalance Protocol

Kafka Connect is the second protocol type which is currently supported by Apache Kafka. We propose to use a similar approach that the one used by the current proposal for Connect in the future. We would introduce a new connect group type and introduce a new set of APIs for Connect. The rebalance protocol is very similar to the consumer rebalance protocol but works with different resource types.

Membership/Leader Election API

.

Downgrading the instances is the exact opposite process. The instances must be rolled to disable the new rebalance protocol. This is done by setting the the group.protocol to generic.

Test Plan

Our primary method for testing the implementation will be through Discrete Event Simulation (DES). DES allows us to test a large number of deterministically generated random scenarios which include various kinds of faults (such as network partitions). It allows us to define system invariants programmatically which are then checked after each step in the simulation. The protocol will be formally verified with a TLA+ model as well. Other than that, we will use the typical suite of unit/integration/system tests. System tests will be parameterised to run with both protocols.

Rejected Alternatives

An epoch per partition

We started this design by using an epoch per partition instead of relying on an epoch per member. Moving a partition from A to B would have require the following step: 1) revoke the partition from A; 2) bump the partition epoch; and 3) assign the partition to B. While this was very appealing at first, it was unpractical in the end for two reasons: 1) migrating from the current protocol is much more difficult without a member id; and 2) the metadata associated to the assignment (e.g. Streams metadata) is not tight to a particular partitions. We ended up using a member epoch with an incremental reassignment algorithm which is pretty close to this.

An epoch per member not aligned to the group epoch

The current design ensures that each member epoch eventually converged to the group epoch. In a previous iteration of the design, we considered incrementing member epoch only when the member's assignment changed. The benefit of this is that it reduce the number of writes to the __consumer_offsets partitions. The downside is that it is harder to debug/understand for users and operators because they cannot rely on the member epoch to know if the member has converged to the desired assignment. In the end, we decided to favour the debuggability. 

Not reusing the current coordinator

We considered not reusing the current group coordinator. Instead, the idea was to implement a brand new consumer group coordinator dedicated for the new rebalance protocol. The main benefits of this is that we could have moved away from the __consumer_offsets storage and use something more appropriate, perhaps closer to the KRaft metadata topic. This was rejected because migrating from a generic group to a consumer group would have been much more difficult.

No more client-side assignors, even for Kafka Streams

We considered removing the client-side assignor feature. From a consumer perspective, this is rarely used nowadays. Kafka Streams is its primary user so we thought about using a server side assignor in this case as well. We abandoned this for two reasons: 1) the Streams' assignor needs to know the entire Streams's topology so each member would have had to send it out to the server. The topology could be rather big (in MB) so this is not very practical; and 2) That would have introduced a strong dependency between the server version and the Streams version. Using new features in Streams would not be possible without upgrading the servers first. 

Storing dynamic group configuration in the Group Coordinator

We considered storing the dynamic group configurations in the group coordinator in order to have the ability to tight their lifecycles to their group. We discarded this approach for two reasons: 1) This pattern does not fit very well in the IncrementalAlterConfig API as it would require to send updates about groups to the coordinator whereas all the other updates go to the controller; and 2) It seems preferable to decouple the life cycle of the dynamic configurations from the life cycle of the groups. Users may want to create configurations before the group is created and users may want to keep their configurations if the group is recreated.

Client side generated Member ID

We considered letting the client generate its member ID (or UUID) instead of relying on the coordinator to generate one when the member joins the group. We ultimately rejected this because 1) it introduces extra dependencies on the client. For instance in C++, UUID are not natively supported so an extra library must be used; and 2) the client could not generate it correctly.

Future Work

Eventually, we aim at deprecating the current membership/rebalance API. In order to get to this point, we would need to first move all the use cases away from it.

Connect Rebalance Protocol

Kafka Connect is the second protocol type which is currently supported by Apache Kafka. We propose to use a similar approach that the one used by the current proposal for Connect in the future. We would introduce a new connect group type and introduce a new set of APIs for Connect. The rebalance protocol is very similar to the consumer rebalance protocol but works with different resource types.

Membership/Leader Election API

The group membership protocol is also used outside of Apache Kafka. For instance, the Confluent Schema Registry uses it for leader election. It is not clear whether we really want to suppose such cases in the future. If we do, we could also define a new set of APIs for it. That would be much cleaner in the long run.

Metadata Transactions

The KIP proposes the rely on the atomicity of the batch to write the assignment to the __consumer_offsets partitions. This means that the assignment or, to be precise, the delta between two assignments can not be larger than 1MB where 1MB is the default batch size. In the future, we could imagine doing something similar to KIP-868 Metadata Transactions in the group coordinator. The solution outlined in KIP-868 does not work in our context because the __consumer_offsets is compacted. However, we could imagine a similar approach. We will tackle this in the future if needed.

Upgrade / Downgrade

The KIP proposes to rely on the IBP/MetadataVersion to decide whether a record or an API could be used or not. We have discussed the idea to use a dedicate feature flag instead of relying on metadata.version. That would allow decoupling the group coordinator from the quorum controller during upgrades. We also need to flush out how to handle downgrades. We will do this in a future KIPThe group membership protocol is also used outside of Apache Kafka. For instance, the Confluent Schema Registry uses it for leader election. It is not clear whether we really want to suppose such cases in the future. If we do, we could also define a new set of APIs for it. That would be much cleaner in the long run.