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).

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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.

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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.

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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:

• A client side assignor is used if possible. This means that a client side assignor must be supported by all the members. If multiple are, it will respect the precedence defined by the members when they advertise their supported client side assignors.
• A The server side assignor is used if any member specified oneotherwise. 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 If a member does not provide an assignor, the group coordinator will use the assignor which is supported by all the members in the group and, if multiple are, it will respect the precedence defined by the members when they advertise their supposed default to the first one in group.consumer.assignors.

Server Side Mode

The server side assignor is 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.

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• 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.

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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.

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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. 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.

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Static membership, introduced in KIP-345, is still supported by this new rebalance protocol. When a member rejoins 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

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The group configurations are stored in the controller like all the other dynamic configurations in the cluster. This allows configurations to be installed independently from wether 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.

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• It has to setup the session timeouts for all the members (like today).
• It has to check wether whether the topic-partition metadata has changed and potentially trigger a rebalance for the group if it has.
• It has to check wether whether new topics match the regex subscriptions and trigger a rebalance for the group if new topic do.
• It has to check wether 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.

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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.

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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

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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

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. 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 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

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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:

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Consumer#enforceRebalance will be deprecated and will throw an IllegalStateException be a no-op if used when the new protocol is enabledenable. 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.

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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 its assignments. This process is detailed in the next sub-chapters:

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• FENCED_MEMBER_EPOCH - The member epoch 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.
• COMPUTEUNRELEASED_INSTANCE_ASSIGNMENT ID - The member has been selected by the coordinator to compute the new target assignment of the groupinstance ID is still used by another member. The member must leave first.
• UNSUPPORTED_ASSIGNOR - The assignor used by the member or its version range are not supported by the group.

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{
  "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 if not provided or if it didn't change since the last heartbeat; the instance Id otherwise." },
      { "name": "RackId", "type": "string", "versions": "0+",  "nullableVersions": "0+", "default": "null",
        "about": "null if not provided or if it didn't change since the last heartbeat; the rack ID of consumer otherwise." },
       { "name": "RebalanceTimeoutMs", "type": "int32", "versions": "0+", "default": -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

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• 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.

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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 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. 

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{
  "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
  "fields// - 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 epoch id 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 not provided; the assignment otherwise."
      "fields": [member should compute the assignment for the group." },
     	{ "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": "Error", "type": "int8", "versions": "0+",
          "fieldsabout": ["The assigned error." },
        	{ "name": "TopicIdAssignedTopicPartitions", "type": "uuid[]TopicPartitions", "versions": "0+",
              "about": "The topic ID partitions assigned to the member that can be used immediately." },
	        { "name": "PartitionsPendingTopicPartitions", "type": "[]int32TopicPartitions", "versions": "0+",
              "about": "The partitions." }
      	] assigned to the member that cannot be used because they are not released by their former owners yet." },
        { "name": "VersionMetadataVersion", "type": "int16", "versions": "0+",
          "about": "The version of the metadata." },
           { "name": "MetadataMetadataBytes", "type": "bytes", "versions": "0+",
          "about": "The assigned metadata." }
	]}
  ]
}

Response Handling

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

,
  "commonStructs": [
    { "name": "TopicPartitions", "versions": "0+", "fields": [
        { "name": "TopicId", "type": "uuid", "versions": "0+",
          "about": "The topic ID." },
        { "name": "Partitions", "type": "[]int32", "versions": "0+",
          "about": "The partitions." }
    ]}
  ]
}

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 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 errorIf 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

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When the group coordinator handle a ConsumerGroupPrepareAssignmentRequest request:

1. Checks wether whether 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, STALE_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. Returns the group state of the group.

...

{
  "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": "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": "GroupEpoch", "type": "int32", "versions": "0+",
      "about": "The group epoch." },
    { "name": "AssignorName", "type": "string", "versions": "0+",
      "about": "The selected assignor." },
    { "name": "Members", "type": "[]Member", "versions": "0+",
      "about": "The members.", "fields": [
      { "name": "MemberId", "type": "string", "versions": "0+",
        "about": "The member ID." },
      { "name": "MemberEpoch", "type": "int32", "versions": "0+",
        "about": "The member epoch." },
      { "name": "InstanceId", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null",
           "about": "The member instance ID." },
      { "name": "RackId", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null", 
        "about": "The member instance ID." },
        { "name": "SubscribedTopicIds", "type": "[]uuid", "versions": "0+",
        "about": "The subscribed topic IDs." },
      { "name": "Assignor", "type": "Assignor", "versions": "0+",
        "about": "The information of the selected assignor",
        "fields": [ 
        { "name": "VersionReason", "type": "int16int8", "versions": "0+",
          "about": "The versionreason of the metadata update." }, 
        { "name": "ReasonMetadataVersion", "type": "int8int16", "versions": "0+",
          "about": "The reasonversion of the metadata update." }, 
        { "name": "MetadataMetadataBytes", "type": "bytes", "versions": "0+",
          "about": "The assignor 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": "Partitions", "type": "[]int32", "versions": "0+",
            "about": "The partitions." }
      ]}
    ]},
    { "name": "Topics", "type": "[]TopicMetadata", "versions": "0+",
      "about": "The topic-partition metadata.",
      "fields": [
        { "name": "TopicId", "type": "uuid", "versions": "0+",
          "about": "The topic ID." },
        { "name": "TopicName", "type": "string", "versions": "0+",
          "about": "The topic name." },
        { "name": "NumPartitions", "type": "int32", "versions": "0+",
          "about": "The number of partitions." }
    ]}    
  ]
}
 

Response Handling

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

...

{
  "apiKey": TBD,
  "type": "request",
  "listeners": ["zkBroker", "broker"],
  "name": "ConsumerGroupInstallAssignmentRequest",
  "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 assigned by the group coordinator." },
    { "name": "MemberEpoch", "type": "int32", "versions": "0+",
      "about": "The member epoch." },
    { "name": "GroupEpoch", "type": "int32", "versions": "0+",
      "about": "The group epoch." },
    { "name": "Error", "type": "int8", "versions": "0+",
      "about": "The assignment error; or zero if the assignment is successful." },
    { "name": "Members", "type": "[]Member", "versions": "0+",
      "about": "The members.", "fields": [
      { "name": "MemberId", "type": "string", "versions": "0+",
        "about": "The member ID." },
      { "name": "Partitions", "type": "[]TopicPartition", "versions": "0+",
        "about": "The assigned topic-partitions to the member.",
        "fields": [
          { "name": "TopicId", "type": "uuid", "versions": "0+",
            "about": "The topic ID." },
          { "name": "Partitions", "type": "[]int32", "versions": "0+",
            "about": "The partitions." }
        ]},
      { "name": "VersionMetadataVersion", "type": "int32", "versions": "0+",
        "about": "The metadata version." }
      { "name": "MetadataMetadataBytes", "type": "bytes", "versions": "0+",
        "about": "The metadata bytes." }
    ]}
  ]
}

...

When the group coordinator handle a ConsumerGroupInstallAssignmentRequest request:

1. Checks wether whether 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, STALE_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.

...

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

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

...

When the group coordinator handle a 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+", "nullableVersions": "0+", "entityTypedefault": "groupIdnull",
          "about": "The group ID string top-level error message, or null if there was no error." },
           { "name": "GroupStateGroupId", "type": "string", "versions": "0+", "entityType": "groupId",
          "about": "The group state string, 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": "RackId", "type": "string", "versions": "0+",  "nullableVersions": "0+", "default": "null", 
            "about": "The member rack ID." },
          { "name": "MemberEpoch", "type": "int32", "versions": "0+",
            "about": "The current member epoch." },
          { "name": "ClientId", "type": "string", "versions": "0+",
            "about": "The client ID." },
          { "name": "ClientHost", "type": "string", "versions": "0+",
            "about": "The client host." },
          { "name": "SubscriptionsSubscribedTopicNames", "type": "[]uuidstring", "versions": "0+",
            "about": "The subscribed topic IDsnames." },
          { "name": "AssignmentSubscribedTopicRegex", "type": "Assignmentstring", "versions": "0+",
            "about"nullableVersions": "0+", "default": "The current assignment.null",
            "fieldsabout": [
"the subscribed topic regex otherwise or null of not provided." },
          { "name": "PartitionsAssignment", "type": "[]TopicPartitionAssignment", "versions": "0+",
              "about": "The assigned topic-partitions to the membercurrent assignment." },
              "fields": [
                { "name": "TopicIdTargetAssignment", "type": "uuidAssignment", "versions": "0+",
                  "about": "The topictarget IDassignment." },
                { "name{ "name": "PartitionsAuthorizedOperations", "type": "[]int32", "versions": "03+",
 "default": "-2147483648",
        "about": "32-bit bitfield to represent authorized operations for "about": "The partitionsthis group." }
    ]}
  ],
  "commonStructs": [
    {  ]},
      "name": "Assignment", "versions": "0+", "fields": [
      { "name": "VersionPartitions", "type": "int32[]TopicPartitions", "versions": "0+",
              "about": "The assignor metadata versionassigned topic-partitions to the member." }
 , "fields": [
           { "name": "MetadataTopicId", "type": "bytesuuid", "versions": "0+",
              "about": "The assignortopic metadata bytesID." }
          ]},
          { "name": "TargetAssignmentTopicName", "type": "Assignmentstring", "versions": "0+",
            "about": "The targettopic assignmentname." },
            "fields": [
            { "name": "Partitions", "type": "[]TopicPartitionint32", "versions": "0+",
              "about": "The assigned topic-partitions to the member.",." }
              "fields": [
   ]},
             { "name": "TopicIdError", "type": "uuidint8", "versions": "0+",
                  "about": "The topicassigned IDerror." },
                { "name": "Partitions", "type": "[]int32", "versions": "0+",
                  "about": "The partitions." }
              ]},
            { "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

Response Handling

Nothing Nothing particular.

ListGroups API

...

{
  "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

...

{
  "apiKey": 9,
  "type": "request",
  "listeners": ["zkBroker", "broker"],
  "name": "OffsetFetchRequest",
  // 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": "0-7", "entityType": "groupId",
      "about": "The group to fetch offsets for." },
    // New fields.
    { "name": "GenerationIdOrMemberEpoch", "type": "int32", "versions": "9+", "default": "-1", "ignorable": true,
      "about": "The generation of the group." },
    { "name": "MemberId", "type": "string", "versions": "9+", "ignorable": true,
        "about": "The member ID assigned by the group coordinator." },
    // End of new fields.
    { "name": "Topics", "type": "[]OffsetFetchRequestTopic", "versions": "0-7", "nullableVersions": "2-7",
      "about": "Each topic we would like to fetch offsets for, or null to fetch offsets for all topics.", "fields": [
      { "name": "Name", "type": "string", "versions": "0-7", "entityType": "topicName",
        "about": "The topic name."},
      { "name": "PartitionIndexes", "type": "[]int32", "versions": "0-7",
        "about": "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": "groupId", "type": "string", "versions": "8+", "entityType": "groupId",
        "about": "The group ID."},
      { "name": "Topics", "type": "[]OffsetFetchRequestTopics", "versions": "8+", "nullableVersions": "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": "8+", "nullableVersions": "9+", "default": "null", "entityType": "topicName",
          "about": "The topic name."},
        // New field.
        { "name": "TopicId", "type": "uuid", "versions": "9+", "about": "The unique topic ID" },
        { "name": "PartitionIndexes", "type": "[]int32", "versions": "8+",
          "about": "The partition indexes we would like to fetch offsets for." }
      ]}
    ]},
    { "name": "RequireStable", "type": "bool", "versions": "7+", "default": "false",
      "about": "Whether broker should hold on returning unstable offsets but set a retriable error code for the partitions."}
  ]
}

...

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

{
  "apiKey": 9,
  "type": "response",
  "name": "OffsetFetchResponse",
  // 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": "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": "[]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." }
    ]}
  ]
}

...

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

...

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

...

{
    "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" }
          ]}
    ], 
}

...

The target assignment is stored in N + 1 records where N is the number of members in a single record.

ConsumerGroupTargetAssignmentKey

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"

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

...

ConsumerGroupTargetAssignmentMetadataValue

{
    "type": "data",
    "name": "ConsumerGroupTargetAssignmentValueConsumerGroupTargetAssignmentMetadataValue",
    "validVersions": "0",
    "flexibleVersions": "0+",
    "fields": [
        { "name": "AssignmentEpoch", "versions": "0+", "type": "int32" },
        { "name": "Members", "versions": "0+", "type": "[]Member", "fields": [
        	{ "name": "MemberId", "versions": "0+", "type": "string" },
            { "name": "Error", "versions": "0+",     ]
}

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": "int8data" },
            {     "name": "TopicPartitionsConsumerGroupTargetAssignmentMemberKey", 
    "versionsvalidVersions": "0+7",
          	  "type    "flexibleVersions": "[]TopicPartitionnone", 
    "fields": [
                  	{ "name": "TopicIdGroupId", "versionstype": "0+string", "typeversions": "uuid7" },
            	{ "name": "Partitions", "versions": "0+", "type": "[]int32" }
        	]},
        	        { "name": "VersionMemberId", "versions": "0+", "type": "int16" },
        	{ "name": "Metadata"string", "versions": "0+7", "type": "bytes" }
         ]
      ]
}

The AssignmentEpoch corresponds to the group epoch used to compute the assignment. It is not necessarily the last one.

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

ConsumerGroupTargetAssignmentMemberValue

{
    "type": "data",
    "name": "ConsumerGroupCurrentMemberAssignmentKeyConsumerGroupTargetAssignmentMemberValue",
    "validVersions": "70",
    "flexibleVersions": "none0+",
    "fields": [
      	        { "name": "GroupIdError", "typeversions": "string0+", "versionstype": "7int8" },
      	        { "name": "MemberIdTopicPartitions", "typeversions": "string0+", "versions": "7" },
    ]
}

ConsumerGroupCurrentMemberAssignmentValue

{
    
       	  "type": "data[]TopicPartition",
     "namefields": "ConsumerGroupCurrentMemberAssignmentValue",
    "validVersions": "0",
    "flexibleVersions": "0+",
    "fields": [
        [
      	  { "name": "MemberEpochTopicId", "versions": "0+", "type": "int32uuid" },
		          { "name": "ErrorPartitions", "versions": "0+", "type": "int8[]int32" }
        ]},
        { "name": "TopicPartitionsMetadataVersion", "versions": "0+",
           "type": "[]TopicPartitionint16", "fields": [
             },
        { "name": "TopicIdMetadataBytes", "versions": "0+", "type": "uuidbytes" }
    ]
}

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
            { "name": "Partitionsdata", 
    "versionsname": "0+ConsumerGroupCurrentMemberAssignmentKey", 
    "typevalidVersions": "[]int32" }
        ]},
        8",
    "flexibleVersions": "none",
    "fields": [
      	{ "name": "VersionGroupId", "versionstype": "0+string", "typeversions": "int168" },
              	{ "name": "MetadataMemberId", "versionstype": "0+string", "typeversions": "bytes8" },
    ], 
}

...

ConsumerGroupCurrentMemberAssignmentValue

{
      "type": "data",
      "name": "OffsetCommitValueConsumerGroupCurrentMemberAssignmentValue",
      "validVersions": "0-4",
      "flexibleVersions": "40+",
      "fields": [
            { "name": "offsetMemberEpoch", "typeversions": "int640+", "versionstype": "0+int32" },
    		{ "name": "leaderEpochError", "type": "int32", "versions": "30+", "defaulttype": -1, "ignorableint8": true },
        { "name": "metadataTopicPartitions", "typeversions": "string0+", 
          "versionstype": "0+" },
    []TopicPartition", "fields": [
            { "name": "commitTimestampTopicId", "typeversions": "int640+", "versionstype": "0+uuid" },
                { "name": "expireTimestampPartitions", "typeversions": "int640+", "versionstype": "1[]int32", "default": -1, "ignorable": true  }
        ]},
    //  Adds TopicId field.
    { "name{ "name": "MetadataVersion", "versions": "0+", "type": "topicIdint16" },
        { "typename": "uuidMetadataBytes", "versions": "40+", "ignorabletype": true"bytes" }
      ], 
}

Broker API

...

Offsets

OffsetCommitValue

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

        /**
         * 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.

{
  "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
• Group count by type
• Group count by state
• Rebalance Rate
• Thread utilisation in percent

Broker Configurations

New properties in the broker configuration.

Group Configurations

New dynamic group properties.

...

New PartitionAssignor interface

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

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

Code Block
language java linenumbers true
package org.apache.kafka.clients.consumer;   public interface PartitionAssignor { class Metadata { /** * The metadata version. */ int version; /** * The metadata bytes. */ ByteBuffer bytes; } class AssignmentSpec
Code Block
language java linenumbers true
package org.apache.kafka.clients.consumer; public interface PartitionAssignor {     class Group {         /**                   * The members keyed by member id.          */         List<GroupMember> Map<String, AssignmentMemberSpec> members;         /**                   * The topics' metadata. keyed by topic id          */         List<TopicMetadata> Map<Uuid, AssignmentTopicMetadata> topics;      }     class GroupMember {         /**          * The member ID.          */         String memberId; class AssignmentMemberSpec {         /**          * The instance ID if provided.          */         Optional<String> Optional<String> instanceId;         /**         * The rack ID if provided.          */        Optional<String> Optional<String> rackId;        /**          * The set of topic IDstopics that the member is subscribed to.          */         List<Uuid> subscribedTopicIds Collection<String> subscribedTopics; /** * The reason reported by the member. */ byte reason; /** * The version of the metadata encodedreported inby {{@link GroupMember#metadata()}}the member. */ int version Metadata metadata;         /**                   * The customcurrent metadatatarget providedpartitions byof the member as defined          * by {{@link PartitionAssignor#metadata()}}.          */         ByteBuffer metadata;         /**          * The partitions owned by the member at the current epoch.          */         List<TopicIdPartition> ownedPartitions;      Collection<TopicPartition> targetPartitions; } class TopicMetadataAssignmentTopicMetadata { /** * The topic IDname. */ Uuid topicId String topicName; /** * The number of partitions. */ int numPartitions; }     class Assignment class GroupAssignment { /** * The assignment error. */ byte error;         /**                   * The member assignmentassignments keyed by member id.          */         List<MemberAssignment>*/ Map<String, MemberAssignment> members;     }     class class MemberAssignment {         /**          * The member ID.          */         String memberId;         /**          * The assigned partitionstarget partitions assigned to this member.          */         List<TopicIdPartition> partitions; /** * The error reported by the assignor. */ byte error; Collection<TopicPartition> targetPartitions; /** * The version of the metadata encoded in {{@link GroupMember#metadata()}}. */ intMetadata versionmetadata;         /**          * The custom metadata provided by the assignor.          */         ByteBuffer metadata;     } class MetadataAssignorMetadata { /** * The reason reported by the assignor. */ byte reason; /** * The version of the metadata encodedreported inby {{@link Metadata#metadata()}}the assignor. */ int version;         /**          * The custom metadata provided by the assignor.          */         ByteBuffer metadata; Metadata metadata; }     /**      * Unique name for this assignor.      */     String String name();     /**      * The minimum version.      */     int int minimumVersion();     /**      * The maximum version.      */     int int maximumVersion();     /**      * Return serializedassignor datametadata that will be sent to the assignor.      */     Metadata AssignorMetadata metadata();     /**      * Perform the group assignment given the current members and      * topic metadata.      *      * @param groupassignmentSpec The groupassignment statespec.      * @return The new assignment for the group.      */     Assignment GroupAssignment assign(GroupAssignmentSpec groupassignmentSpec);     /**      * Callback which is invoked when the member received a new assignment      * assignment from the assignor/group coordinator* from the assignor/group coordinator. This is called once per epoch * and contains the target partitions for this members. This means that * partitions may not be assigned to the member yet. The rebalance * listener must be used to know this. * * @param byte The error reported by the assignor. * @param assignment The assignment computed by the assignor. * @param consumerGroupMetadata The group metadata.      */     void void onAssignment(byte error, MemberAssignment assignment, ConsumerGroupMetadata consumerGroupMetadata); }

New SubscriptionPattern class

...

...

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 metadata version.
     */
	int version;

    /**
	     * The metadata reasonbytes.
 reported by the assignor.
	 */
	byteByteBuffer errorbytes;
} 

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 partitions owned byassigned metadata. Provided only if the membergroup atis theConsumerGroup current epochtype.
         */
    List<TopicIdPartition> ownedPartitions   Optional<Metadata> metadata;
}

Admin#incrementalAlterConfigs and Admin#describeConfigs

...

kafka-consumer-groups.sh -–bootstrap-server localhost:9092 -–validate-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=0)
• Empty
• Target Assignment (epoch=0)
• Empty
• Member Assignment
• Empty

...

Member A joins the group. The coordinator bumps the group epoch to 1, adds A to the group, and creates an empty member assignment.

• 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-2-2], pending-partitions=[]
• B - epoch=0, partitions=[], pending-partitions=[]

At this point B can transitions to epoch 2 but cannot get foo-2 until A revokes it.Note that the persisted target assignment of B already includes foo-2 here but B does not get it yet because foo-2 is not free.

• 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=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=[], 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 him it to epoch 3 because B has no partitions to revoke. It persists the change and reply. 

• 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=0, partitions=[], pending-partitions=[]

When C heartbeats, it transitions to epoch 3 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=[], 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 3 and 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=3, 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], foopending-2partitions=[]
• 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, 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, =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], pending-partitions=[]

B heartbeats and gets his its assignment.

Compatibility, Deprecation, and Migration Plan

...

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 recreatedrecreated.

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.

...

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 KIP.