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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 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 with the same Instance IDwants 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 ID, the group coordinator replaces the old member with by the new member and gives back its current assignment.

If the leaving member . The new member can continue from where it left offdoes 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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When the member has got a new target assignment, the group coordinator will notify him 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.

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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.
• UNRELEASED_INSTANCE_ID - The instance 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": "SubscribedTopicIdsSubscribedTopicNames", "type": "[]uuidstring", "versions": "0+", "nullableVersions": "0+", "default": "null",
        "about": "null if it didn't change since the last heartbeat; the subscribed topic idsnames 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": "MetadataVersion", "type": "int16", "versions": "0+",
            "about": "The version of the metadata." },
          { "name": "MetadataBytes", "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." }
      ]}
  ]
}

...

• 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
  "fields": // - UNRELEASED_INSTANCE_ID
  // - 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": "MemberId", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "The member id generated by the coordinator. Only provided when the member joins with MemberEpoch == 0." }, 
    { "name": "MemberEpoch", "type": "int32", "versions": "0+",
      "about": "The member epoch." },
    { "name": "ShouldComputeAssignment", "type": "bool", "versions": "0+",
      "about": "True if the member should compute the assignment for the group." },
    { "name": "HeartbeatIntervalMs", "type": "int32", "versions": "0+",
      "about": "The heartbeat interval in milliseconds." }, 
    { "name": "Assignment", "type": "Assignment", "versions": "0+", "nullableVersions": "0+", "default": "null",
	  "about": "null if not provided; the assignment otherwise.", "fields": [
        { "name": "Error", "type": "int8", "versions": "0+",
          "about": "The assigned error." },
        { "name": "AssignedTopicPartitions", "type": "[]TopicPartitionTopicPartitions", "versions": "0+",
          "about": "The partitions assigned to the member that can be used immediately." },
        { "name": "PendingTopicPartitions", "type": "[]TopicPartitionTopicPartitions", "versions": "0+",
          "about": "The partitions assigned to the member that cannot be used because they are not released by their former owners yet." },
        { "name": "MetadataVersion", "type": "int16", "versions": "0+",
          "about": "The version of the metadata." },
        { "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": "Partitions", "type": "[]int32", "versions": "0+",
          "about": "The partitions." }
    ]}
  ]
}

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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.

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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.

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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+", "entityType"nullableVersions": "0+", "default": "groupIdnull",
          "about": "The grouptop-level ID stringerror 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": "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": "SubscribedTopicIdsSubscribedTopicNames", "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." },
          { "name": "TargetAssignment",  "fieldstype": [
  "Assignment", "versions": "0+",
            "about": "The target assignment." },
      { "name": "TopicIdAuthorizedOperations", "type": "uuidint32", "versions": "0+3+", "default": "-2147483648",
        "about": "32-bit bitfield to represent authorized operations for this  "about": "The topic ID." }group." }
    ]}
  ],
  "commonStructs": [
    { "name": "Assignment", "versions": "0+", "fields": [
      { "name": "Partitions", "type": "[]int32TopicPartitions", "versions": "0+",
                  "about": "The assigned topic-partitions." }
to the member.",            ]},"fields": [
            { "name": "VersionTopicId", "type": "int32uuid", "versions": "0+",
              "about": "The assignortopic metadata versionID." },
            { "name": "MetadataTopicName", "type": "bytesstring", "versions": "0+",
              "about": "The assignortopic metadata bytesname." }, 
        {  ]},
          { "name": "TargetAssignment", ""name": "Partitions", "type": "Assignment[]int32", "versions": "0+",
            "about": "The target assignmentpartitions.", }
            "fields": [
      ]},
      { "name": "PartitionsError", "type": "[]TopicPartitionint8", "versions": "0+",
              "about": "The assigned topic-partitions to the membererror.",
              "fields": [
    }, 
             { "name": "TopicIdMetadataVersion", "type": "uuidint32", "versions": "0+",
                  "about": "The topicassignor metadata IDversion." },
                { "name": "PartitionsMetadataBytes", "type": "[]int32bytes", "versions": "0+",
                  "about": "The partitionsassignor metadata bytes." }
           ]}
   ]
},
            { "name": "Version", "type": "int32", "versions": "0+",
              "about": "The assignor metadata version." }
            { "name": "Metadata", "type": "bytes", "versions": "0+",
              "about": "The assignor metadata bytes." }
          ]

Response Handling

Nothing particular.

ListGroups API

The existing ListGroups API will be extended to support the notion of group types and to support the new group states.

Request Schema

The TypesFilter field is introduced. It allows listing groups of certain types.

{
  "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": "AuthorizedOperationsTypesFilter", "type": "int32[]string", "versions": "35+", "default": "-2147483648",
        "about": "32-bitThe bitfieldtypes toof representthe authorizedgroups operationswe forwant thisto group." }
    ]list. If empty all groups are returned" }
  ]
}

Response Handling

Nothing particular.

ListGroups API

The existing ListGroups API will be extended to support the notion of group types and to support the new group states.

Request Schema

Required ACL

• Describe Cluster

Request Validation

No particular changes.

Request Handling

The new types filter is handled.

Response Schema

The GroupType The TypesFilter field is introduced. It allows listing groups of certain typesrepresents the type of the group.

{
  "apiKey": 16,
  "type": "requestresponse",
  "listeners": ["zkBroker", "broker"],
  "name": "ListGroupsRequestListGroupsResponse",
  // Version 1 and 2 are the same as version 0adds 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 StatesFilterGroupState field (KIP-518).
  //
  // Version 5 adds the TypesFilterGroupType field (KIP-848).
  "validVersions": "0-5",
  "flexibleVersions": "3+",
  "fields": [
    { "name": "StatesFilterThrottleTimeMs", "type": "[]stringint32", "versions": "4+"1+", 
      "ignorable": true,
      "about": "The states of duration in milliseconds for which the groupsrequest was wethrottled wantdue to list. If empty all groups are returned with their state a quota violation, or zero if the request did not violate any quota." },
    { "name": "TypesFilterErrorCode", "type": "[]stringint16", "versions": "50+",
      "about": "The error typescode, ofor the0 groupsif wethere wantwas tono listerror." If},
 empty all groups are{ returned"name" }
  ]
}

Required ACL

• Describe Cluster

Request Validation

No particular changes.

Request Handling

The new types filter is handled.

Response Schema

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

{
  "apiKey": 16,
 : "Groups", "type": "[]ListedGroup", "versions": "0+",
      "about": "Each group in the response.", "fields": [
      { "name": "GroupId", "type": "responsestring",
  "nameversions": "ListGroupsResponse0+",
  //  Version 1 adds the throttle time."entityType": "groupId",
  //
   // 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"about": "The group ID." },
      { "name": "ProtocolType", "type": "string", "versions": "0+",
        "about": "The group protocol type." },
      { "name": "GroupState", "type": "int32string", "versions": "14+", 
      "ignorable": true,
        "about": "The durationgroup in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quotastate name." },
      { "name": "ErrorCodeGroupType", "type": "int16string", "versions": "05+", "ignorable": true,
        "about": "The errorgroup code, or 0 if there was no errortype name." },
    { "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 state name." }
    ]}
  ]
}

Response Handling

No changes.

OffsetCommit 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

]}
  ]
}

Response Handling

No changes.

OffsetCommit 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

{
  "apiKey": 8,
  "type": "request",
  "listeners": ["zkBroker", "broker"],
  "name": "OffsetCommitRequest",
  // Version 1 adds timestamp and group membership information, as well as the commit timestamp.
  //
  // Version 2 adds retention time.  It removes the commit timestamp added in version 1.
  //
  // Version 3 and 4 are the same as version 2. 
  //
  // Version 5 removes the retention time, which is now controlled only by a broker configuration.
  //
  // Version 6 adds the leader epoch for fencing.
  //
  // version 7 adds a new field called groupInstanceId to indicate member identity across restarts.
  //
  // Version 8 is the first flexible version.
  //
  // Version 9 adds TopicId field (KIP-848).
  "validVersions": "0-9",
  "flexibleVersions": "8+",
  "fields": [
    { "name": "GroupId", "type": "string", "versions": "0+", "entityType": "groupId",
      "about": "The unique group identifier." },
    // Renamed field.
    { "name": "GenerationIdOrMemberEpoch", "type": "int32", "versions": "1+", "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": "1+", "ignorable": true,
      "about": "The member ID assigned by the group coordinator." },
    { "name": "GroupInstanceId", "type": "string", "versions": "7+",
      "nullableVersions": "7+", "default": "null

{
  "apiKey": 8,
  "type": "request",
  "listeners": ["zkBroker", "broker"],
  "name": "OffsetCommitRequest",
  // Version 1 adds timestamp and group membership information, as well as the commit timestamp.
  //
  // Version 2 adds retention time.  It removes the commit timestamp added in version 1.
  //
  // Version 3 and 4 are the same as version 2. 
  //
  // Version 5 removes the retention time, which is now controlled only by a broker configuration.
  //
  // Version 6 adds the leader epoch for fencing.
  //
  // version 7 adds a new field called groupInstanceId to indicate member identity across restarts.
  //
  // Version 8 is the first flexible version.
  //
  // Version 9 adds TopicId field (KIP-848).
  "validVersions": "0-9",
  "flexibleVersions": "8+",
  "fields": [
    { "name": "GroupId", "type": "string", "versions": "0+", "entityType": "groupId",
      "about": "The unique groupidentifier identifier." },
    // Renamed field.of the consumer instance provided by end user." },
    { "name": "GenerationIdOrMemberEpochRetentionTimeMs", "type": "int32int64", "versions": "1+2-4", "default": "-1", "ignorable": true,
      "about": "The generationtime ofperiod thein groupms ifto the generic group protocol or the member epoch if the consumer protocolretain the offset." },
    { "name": "MemberIdTopics", "type": "string[]OffsetCommitRequestTopic", "versions": "10+", "ignorable": true,
      "about": "The member ID assigned by the group coordinator." },
topics to commit offsets for.", "fields": [
      // Updated field.
      { "name": "GroupInstanceIdName", "type": "string", "versions": "70+",
      "nullableVersions": "79+", "default": "null", "entityType": "topicName",
        "about": "The unique identifier of the consumer instance provided by end user." },
topic name."},
      // New field.
      { "name": "RetentionTimeMsTopicId", "type": "int64uuid", "versions": "2-49+", "default": "-1", "ignorable": true,
      "about": "The timeunique period in ms to retain the offset.topic ID" },
      { "name": "TopicsPartitions", "type": "[]OffsetCommitRequestTopicOffsetCommitRequestPartition", "versions": "0+",
        "about": "TheEach topicspartition to commit offsets for.", "fields": [
      // Updated field.
      { "name": "NamePartitionIndex", "type": "stringint32", "versions": "0+",
 "nullableVersions": "9+", "default": "null", "entityType": "topicName",
        "about": "The topicpartition nameindex." },
      // New field.
      { "name": "TopicIdCommittedOffset", "type": "uuidint64", "versions": "90+",
          "about": "The message offset uniqueto topicbe IDcommitted." },
        { "name": "PartitionsCommittedLeaderEpoch", "type": "[]OffsetCommitRequestPartitionint32", "versions": "06+",
        "aboutdefault": "Each partition to commit offsets for.-1", "fieldsignorable": [true,
        {  "nameabout": "PartitionIndex", "type": "int32", "versions": "0+"The leader epoch of this partition." },
        //  "about": "The partition index." },CommitTimestamp has been removed from v2 and later.
        { "name": "CommittedOffsetCommitTimestamp", "type": "int64", "versions": "1", "default": "0+-1",
          "about": "The messagetimestamp offsetof tothe be committedcommit." },
        { "name": "CommittedLeaderEpochCommittedMetadata", "type": "int32string", "versions": "60+", "defaultnullableVersions": "-10+", "ignorable": true,
          "about": "The leader epoch of this partitionAny associated metadata the client wants to keep." },
      ]}
  // CommitTimestamp has]}
 been removed from v2 and later.
        { "name": "CommitTimestamp", "type": "int64", "versions": "1", "default": "-1",
          "about": "The timestamp of the commit." },
        { "name": "CommittedMetadata", "type": "string", "versions": "0+", "nullableVersions": "0+",
          "about": "Any associated metadata the client wants to keep." }
      ]}
    ]}
  ]
}

Required ACL

• Read Group

Request Validation

...

{
  "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 SchemaResponse 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 the group. The records for the members are written first and followed by the assignment metadata. When a single record.

...

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": "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+", 
    ]
}

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": "[]Memberdata", "fields": [
        	{ 
    "name": "MemberIdConsumerGroupTargetAssignmentMemberKey", 
    "versionsvalidVersions": "0+7", 
    "typeflexibleVersions": "stringnone" },
                "fields": [
      	{ "name": "ErrorGroupId", "versionstype": "0+string", "typeversions": "int87" },
             { "name": "TopicPartitionsMemberId", "versionstype": "0+string",
          	  "typeversions": "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": "VersionMetadataVersion", "versions": "0+", "type": "int16" },
        	{ "name": "MetadataMetadataBytes", "versions": "0+", "type": "bytes" }
        ]
    ]
}

Current Member Assignment

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 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 it is written to the partition.

...

{
    "type": "data",
    "name": "ConsumerGroupCurrentMemberAssignmentKey",
    "validVersions": "78",
    "flexibleVersions": "none",
    "fields": [
      	{ "name": "GroupId", "type": "string", "versions": "78" },
      	{ "name": "MemberId", "type": "string", "versions": "78" },
    ]
}

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": "VersionMetadataVersion", "versions": "0+", "type": "int16" },
        { "name": "MetadataMetadataBytes", "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 }
  ]
}

...

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

        /**
         * The current target partitions of the member.
         */
        List<TopicPartition>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.
         */
        List<TopicPartition>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

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.

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.

...

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 set of topics that the member is subscribed to.
         */
        List<String>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.
         */
        List<TopicPartition>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 {
        /**
         * The target partitions assigned to this member.
         */
        List<TopicPartition>Collection<TopicPartition> targetPartitions;

 		/**
		 * The metadata.
		 */
		Metadata metadata;
    }

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

 		/**
		 * The metadata reported by the assignor.
		 */
		Metadata metadata;
    }

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

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

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

    /**
     * Return assignor metadata that will be sent to the assignor.
     */
    AssignorMetadata metadata();

    /**
     * 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);

    /**
     * Callback which 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 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 onAssignment(byte error, MemberAssignment assignment, ConsumerGroupMetadata consumerGroupMetadata);
}

...

...

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

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

    /**
     * The target assignment of the member. Provided only for consumer group.
     */
    public Optional<MemberAssignment>public class ConsumerGroupDescription {
    public String type() {
      return type;
    }
}

public class MemberDescription {
    // Current Assignment
    public MemberAssignment assignment() {}

    // Target Assignment
    public MemberAssignment targetAssignment() {}
}

public class MemberAssignmentMetadata {
	    /**
	     * The metadata reasonversion.
 reported by the assignor.
	 */
	byteint errorversion; 

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

 metadata bytes.
     */
	ByteBuffer bytes;
} 

public class MemberAssignment {
	/**
     * The partitions.
     */
    Set<TopicPartition> topicPartitions;

 	/**
	 * The custom metadata provided by the assignorerror reported 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>Optional<Metadata> ownedPartitionsmetadata;
}

Admin#incrementalAlterConfigs and Admin#describeConfigs

...

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.

...

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

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

...

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. 

...

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.

...

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

...

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.

...

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.

...

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.

...

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

Member Failure

...

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

...

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

...

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=[]

...

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=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 recreatedis 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.

...

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.