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{
  "apiKey": TBD,
  "type": "request",
  "listeners": ["broker"],
  "name": "StreamsGroupHeartbeatRequest",
  "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 streams consumer. The member ID must be kept during the entire lifetime of the streams consumer process." },
    { "name": "MemberEpoch", "type": "int32", "versions": "0+",
      "about": "The current member epoch; 0 to join the group; -1 to leave the group; -2 to indicate that the static member will rejoin." },
    { "name": "CurrentAssignmentEpoch", "type": "int32", "versions": "0+",
      "about": "The current assignment epoch"},
    { "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 for static membership 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 the member otherwise." },
    { "name": "RebalanceTimeoutMs", "type": "int32", "versions": "0+", "default": -1,
      "about": "-1 if it didn't change since the last heartbeat; the maximum time in milliseconds that the coordinator will wait on the member to revoke its tasks otherwise." },

    { "name": "Topology", "type": "Topology", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "The topology metadata of the streams application. Used to initialize the topology of the group and to check if the topology corresponds to the topology initialized for the group. Only sent when memberEpoch = 0, must be non-empty. Null otherwise.",
      "fields": [
        { "name": "Epoch", "type": "int32", "versions": "0+",
          "about": "The epoch of the topology. Used to check if the topology corresponds to the topology initialized on the brokers." },
        { "name": "Subtopologies", "type": "[]Subtopology", "versions": "0+",
          "about": "The sub-topologies of the streams application.",
          "fields": [
            { "name": "SubtopologyId", "type": "string", "versions": "0+",
              "about": "String to uniquely identify the subtopology. Deterministically generated from the topology" },
            { "name": "SourceTopics", "type": "[]string", "versions": "0+",
              "about": "The topics the topology reads from." },
            { "name": "SourceTopicRegex", "type": "[]string", "versions": "0+",
              "about": "The regular expressions identifying topics the subtopology reads from." },
            { "name": "StateChangelogTopics", "type": "[]TopicInfo", "versions": "0+",
              "about": "The set of state changelog topics associated with this subtopology. Created automatically." },
            { "name": "RepartitionSinkTopics", "type": "[]string", "versions": "0+",
              "about": "The repartition topics the subtopology writes to." },
            { "name": "RepartitionSourceTopics", "type": "[]TopicInfo", "versions": "0+",
              "about": "The set of source topics that are internally created repartition topics. Created automatically." },
            { "name": "CopartitionGroups", "type": "[]CopartitionGroup", "versions": "0+",
              "about": "A subset of source topics that must be copartitioned.",
              "fields": [
                { "name": "SourceTopics", "type": "[]int16", "versions": "0+",
                  "about": "The topics the topology reads from. Index into the array on the subtopology level." },
                { "name": "SourceTopicRegex", "type": "[]int16", "versions": "0+",
                  "about": "Regular expressions identifying topics the subtopology reads from. Index into the array on the subtopology level." },
                { "name": "RepartitionSourceTopics", "type": "[]int16", "versions": "0+",
                  "about": "The set of source topics that are internally created repartition topics. Index into the array on the subtopology level." }
              ]}
          ]}
      ]
    },

    { "name": "ActiveTasks", "type": "[]TaskIds", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "Currently owned active tasks for this client. Null if unchanged since last heartbeat." },
    { "name": "StandbyTasks", "type": "[]TaskIds", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "Currently owned standby tasks for this client. Null if unchanged since last heartbeat." },
    { "name": "WarmupTasks", "type": "[]TaskIds", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "Currently owned warm-up tasks for this client. Null if unchanged since last heartbeat." },

    { "name": "ProcessId", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "Identity of the streams instance that may have multiple consumers. Null if unchanged since last heartbeat." },
    { "name": "UserEndpoint", "type": "Endpoint", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "User-defined endpoint for Interactive Queries. Null if unchanged since last heartbeat or if not defined on the client." },
    { "name": "ClientTags", "type": "[]KeyValue", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "Used for rack-aware assignment algorithm. Null if unchanged since last heartbeat." },

    { "name": "TaskOffsets", "type": "[]TaskOffset", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "Cumulative changelog offsets for tasks. Only updated when a warm-up task has caught up, and according to the task offset interval. Null if unchanged since last heartbeat." },
    { "name": "TaskEndOffsets", "type": "[]TaskOffset", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "Cumulative changelog end-offsets for tasks. Only updated when a warm-up task has caught up, and according to the task offset interval. Null if unchanged since last heartbeat." },
    { "name": "ShutdownApplication", "type": "bool", "versions": "0+", "default": false,
      "about": "Whether all Streams clients in the group should shut down." }
  ],

  "commonStructs": [
    { "name": "KeyValue", "versions": "0+", "fields": [
      { "name": "Key", "type": "string", "versions": "0+",
        "about": "key of the config" },
      { "name": "Value", "type": "string", "versions": "0+",
        "about": "value of the config" }
    ]},
    { "name": "TopicInfo", "versions": "0+", "fields": [
      { "name": "Name", "type": "string", "versions": "0+",
        "about": "The name of the topic." },
      { "name": "Partitions", "type": "int32", "versions": "0+",
        "about": "The number of partitions in the topic. Can be 0 if no specific number of partitions is enforced. Always 0 for changelog topics." },
      { "name": "ReplicationFactor", "type": "int16", "versions": "0+",
        "about": "The replication factor of the topic. Can be 0 if the default replication factor should be used." },
      { "name": "TopicConfigs", "type": "[]KeyValue", "versions": "0+",
        "about": "Topic-level configurations as key-value pairs."
      }
    ]},
    { "name": "Endpoint", "versions": "0+", "fields": [
      { "name": "Host", "type": "string", "versions": "0+",
        "about": "host of the endpoint" },
      { "name": "Port", "type": "uint16", "versions": "0+",
        "about": "port of the endpoint" }
    ]},
    { "name": "TaskOffset", "versions": "0+", "fields": [
      { "name": "SubtopologyId", "type": "string", "versions": "0+",
        "about": "The subtopology identifier." },
      { "name": "Partition", "type": "int32", "versions": "0+",
        "about": "The partition." },
      { "name": "Offset", "type": "int64", "versions": "0+",
        "about": "The offset." }
    ]},
    { "name": "TaskIds", "versions": "0+", "fields": [
      { "name": "SubtopologyId", "type": "string", "versions": "0+",
        "about": "The subtopology identifier." },
      { "name": "Partitions", "type": "[]int32", "versions": "0+",
        "about": "The partitions of the input topics processed by this member." }
    ]}
  ]
}

{
  "apiKey": TBD,
  "type": "response",
  "name": "StreamsGroupHeartbeatResponse",
  "validVersions": "0",
  "flexibleVersions": "0+",
  // Supported errors:
  // - GROUP_AUTHORIZATION_FAILED (version 0+)
  // - GROUP_ID_NOT_FOUND (version 0+)
  // - NOT_COORDINATOR (version 0+)
  // - COORDINATOR_NOT_AVAILABLE (version 0+)
  // - COORDINATOR_LOAD_IN_PROGRESS (version 0+)
  // - INVALID_REQUEST (version 0+)
  // - UNKNOWN_MEMBER_ID (version 0+)
  // - FENCED_MEMBER_EPOCH (version 0+)
  // - UNRELEASED_INSTANCE_ID (version 0+)
  // - GROUP_MAX_SIZE_REACHED (version 0+)
  // - TOPIC_AUTHORIZATION_FAILED (version 0+) 
  // - CLUSTER_AUTHORIZATION_FAILED (version 0+)
  // - STREAMS_INVALID_TOPOLOGY (version 0+)
  // - STREAMS_INVALID_TOPOLOGY_EPOCH (version 0+)
  // - STREAMS_TOPOLOGY_FENCED (version 0+)
  "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+",
      "about": "The member id is always generated by the streams consumer."},     
    { "name": "MemberEpoch", "type": "int32", "versions": "0+",
      "about": "The member epoch." },
    { "name": "CurrentAssignmentEpoch", "type": "int32", "versions": "0+",
      "about": "The currentAssignment epoch"},
    { "name": "HeartbeatIntervalMs", "type": "int32", "versions": "0+",
      "about": "The heartbeat interval in milliseconds." },
    { "name": "AcceptableRecoveryLag", "type": "int32", "versions": "0+",
      "about": "The maximal lag a warm-up task can have to be considered caught-up." },
    { "name": "TaskOffsetIntervalMs", "type": "int32", "versions": "0+",
      "about": "The interval in which the task changelog offsets on a client are updated on the broker. The offsets are sent with the next heartbeat after this time has passed." },

    { "name": "Status", "type":  "[]Status", "versions":  "0+",  "nullableVersions": "0+", "default": "null",
      "about": "Indicate zero or more status for the group.  Null if unchanged since last heartbeat." },

    // The streams app knows which partitions to fetch from given this information
    { "name": "ActiveTasks", "type": "[]TaskIds", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "Assigned active tasks for this client. Null if unchanged since last heartbeat." },
    { "name": "StandbyTasks", "type": "[]TaskIds", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "Assigned standby tasks for this client. Null if unchanged since last heartbeat." },
    { "name": "WarmupTasks", "type": "[]TaskIds", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "Assigned warm-up tasks for this client. Null if unchanged since last heartbeat." },

    // IQ-related information
    { "name": "PartitionsByUserEndpoint", "type": "[]EndpointToPartitions", "versions": "0+", "nullableVersions": "0+", "default": "null",
      "about": "Global assignment information used for IQ. Null if unchanged since last heartbeat." ,
      "fields": [
        { "name": "UserEndpoint", "type": "Endpoint", "versions": "0+",
          "about": "User-defined endpoint to connect to the node" },
        { "name": "ActivePartitions", "type": "[]TopicPartition", "versions": "0+",
          "about": "All active partitions available on the node" },
        { "name": "StandbyPartitions", "type": "[]TopicPartition", "versions": "0+",
          "about": "All standby paritions available on the node"}
      ]
    }
  ],
  "commonStructs": [
    { "name": "Status", "versions": "0+", "fields": [
      // Possible status codes
      //  0 - STALE_TOPOLOGY                 - The topology epoch supplied is lower than the topology epoch for this streams group.
      //  1 - MISSING_SOURCE_TOPICS          - One or more source topics are missing or a source topic regex resolves to zero topics.
      //                                       Missing topics are indicated in the StatusDetail.
      //  2 - INCORRECTLY_PARTITIONED_TOPICS - One or more topics are incorrectly partitioned, that is, they are not copartitioned despite being 
      //                                       part of a copartition group, or the number of partitions in a changelog topic does not correspond
      //                                       to the maximal number of source topic partition for that subtopology.
      //                                       Incorrectly partitioned topics are indicated in the StatusDetail.
      //  3 - MISSING_INTERNAL_TOPICS        - One or more internal topics are missing. 
      //                                       Missing topics are indicated in the StatusDetail. 
      //                                       The group coordinator will attempt to create all missing internal topics, if any errors occur during
      //                                       topic creation, this will be indicated in StatusDetail.
      //  4 - SHUTDOWN_APPLICATION           - A client requested the shutdown of the whole application.
      { "name": "StatusCode", "type": "int8", "versions": "0+",
        "about": "A code to indicate that a particular status is active for the group membership" },
      { "name": "StatusDetail", "type": "string", "versions": "0+",
        "about": "A string representation of the status." }
    ]},
    { "name": "TopicPartition", "versions": "0+", "fields": [
      { "name": "Topic", "type": "string", "versions": "0+",
        "about": "topic name" },
      { "name": "Partitions", "type": "[]int32", "versions": "0+",
        "about": "partitions" }
    ]},
    { "name": "TaskIds", "versions": "0+", "fields": [
      { "name": "SubtopologyId", "type": "string", "versions": "0+",
        "about": "The subtopology identifier." },
      { "name": "Partitions", "type": "[]int32", "versions": "0+",
        "about": "The partitions of the input topics processed by this member." }
    ]},
    { "name": "Endpoint", "versions": "0+", "fields": [
      { "name": "Host", "type": "string", "versions": "0+",
        "about": "host of the endpoint" },
      { "name": "Port", "type": "uint16", "versions": "0+",
        "about": "port of the endpoint" }
    ]}
  ]
}

{
  "apiKey": TBD,
  "type": "response",
  "name": "StreamsGroupDescribeResponse",
  "validVersions": "0",
  "flexibleVersions": "0+",
  // Supported errors:
  // - GROUP_AUTHORIZATION_FAILED (version 0+)
  // - NOT_COORDINATOR (version 0+)
  // - COORDINATOR_NOT_AVAILABLE (version 0+)
  // - COORDINATOR_LOAD_IN_PROGRESS (version 0+)
  // - INVALID_REQUEST (version 0+)
  // - INVALID_GROUP_ID (version 0+)
  // - GROUP_ID_NOT_FOUND (version 0+)
  "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": "ErrorMessage", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null",
          "about": "The top-level error message, or null if there was no error." },
        { "name": "GroupId", "type": "string", "versions": "0+", "entityType": "groupId",
          "about": "The group ID string." },
        { "name": "GroupState", "type": "string", "versions": "0+",
          "about": "The group state string, or the empty string." },
        { "name": "GroupEpoch", "type": "int32", "versions": "0+",
          "about": "The group epoch." },
        { "name": "AssignmentEpoch", "type": "int32", "versions": "0+",
          "about": "The assignment epoch." },

        { "name":  "Topology", "type": "Topology", "versions": "0+", "nullableVersions": "0+", "default": "null",
          "about": "The topology metadata currently initialized for the streams application. Can be null in case of a describe error.",
          "fields": [
            { "name": "Epoch", "type": "int32", "versions": "0+",
              "about": "The epoch of the currently initialized topology for this group." },
            { "name": "Subtopologies", "type": "[]Subtopology", "versions": "0+", "nullableVersions": "0+", "default": "null",
              "about": "The subtopologies of the streams application. This contains the configured subtopologies, where the number of partitions are set and any regular expressions are resolved to actual topics. Null if the group is uninitialized, source topics are missing or incorrectly partitioned.",
              "fields": [
                { "name": "SubtopologyId", "type": "string", "versions": "0+",
                  "about": "String to uniquely identify the subtopology." },
                { "name": "SourceTopics", "type": "[]string", "versions": "0+",
                  "about": "The topics the subtopology reads from." },
                { "name": "RepartitionSinkTopics", "type": "[]string", "versions": "0+",
                  "about": "The repartition topics the subtopology writes to." },
                { "name": "StateChangelogTopics", "type": "[]TopicInfo", "versions": "0+",
                  "about": "The set of state changelog topics associated with this subtopology. Created automatically." },
                { "name": "RepartitionSourceTopics", "type": "[]TopicInfo", "versions": "0+",
                  "about": "The set of source topics that are internally created repartition topics. Created automatically." }
              ]}
          ]},
        { "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": "CurrentAssignmentEpoch", "type": "int32", "versions": "0+",
              "about": "The current assignment epoch." },
            { "name": "InstanceId", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null",
              "about": "The member instance ID for static membership." },
            { "name": "RackId", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null",
              "about": "The rack ID." },

            { "name": "ClientId", "type": "string", "versions": "0+",
              "about": "The client ID." },
            { "name": "ClientHost", "type": "string", "versions": "0+",
              "about": "The client host." },

            { "name": "TopologyEpoch", "type": "int32", "versions": "0+",
              "about": "The epoch of the topology on the client." },

            { "name": "ProcessId", "type": "string", "versions": "0+",
              "about": "Identity of the streams instance that may have multiple clients. " },
            { "name": "UserEndpoint", "type": "Endpoint", "versions": "0+", "nullableVersions": "0+", "default": "null",
              "about": "User-defined endpoint for Interactive Queries. Null if not defined for this client." },
            { "name": "ClientTags", "type": "[]KeyValue", "versions": "0+",
              "about": "Used for rack-aware assignment algorithm." },
            { "name": "TaskOffsets", "type": "[]TaskOffset", "versions": "0+",
              "about": "Cumulative changelog offsets for tasks." },
            { "name": "TaskEndOffsets", "type": "[]TaskOffset", "versions": "0+",
              "about": "Cumulative changelog end offsets for tasks." },

            { "name": "Assignment", "type": "Assignment", "versions": "0+",
              "about": "The current assignment." },
            { "name": "TargetAssignment", "type": "Assignment", "versions": "0+",
              "about": "The target assignment." },
            { "name": "IsClassic", "type": "bool", "versions": "0+",
              "about": "True for classic members that have not been upgraded yet." }
        ]},
        { "name": "AuthorizedOperations", "type": "int32", "versions": "0+", "default": "-2147483648",
          "about": "32-bit bitfield to represent authorized operations for this group." }
      ]
    }
  ],
  "commonStructs": [
    { "name": "Endpoint", "versions": "0+", "fields": [
      { "name": "Host", "type": "string", "versions": "0+",
        "about": "host of the endpoint" },
      { "name": "Port", "type": "uint16", "versions": "0+",
        "about": "port of the endpoint" }
    ]},
    { "name": "TaskOffset", "versions": "0+", "fields": [
      { "name": "SubtopologyId", "type": "string", "versions": "0+",
        "about": "The subtopology identifier." },
      { "name": "Partition", "type": "int32", "versions": "0+",
        "about": "The partition." },
      { "name": "Offset", "type": "int64", "versions": "0+",
        "about": "The offset." }
    ]},
    { "name": "TopicPartitions", "versions": "0+", "fields": [
      { "name": "TopicId", "type": "uuid", "versions": "0+",
        "about": "The topic ID." },
      { "name": "TopicName", "type": "string", "versions": "0+", "entityType": "topicName",
        "about": "The topic name." },
      { "name": "Partitions", "type": "[]int32", "versions": "0+",
        "about": "The partitions." }
    ]},
    { "name": "Assignment", "versions": "0+", "fields": [
      { "name": "ActiveTasks", "type": "[]TaskIds", "versions": "0+",
        "about": "Active tasks for this client." },
      { "name": "StandbyTasks", "type": "[]TaskIds", "versions": "0+",
        "about": "Standby tasks for this client." },
      { "name": "WarmupTasks", "type": "[]TaskIds", "versions": "0+",
        "about": "Warm-up tasks for this client. " }
    ]},
    { "name": "TaskIds", "versions": "0+", "fields": [
      { "name": "SubtopologyId", "type": "string", "versions": "0+",
        "about": "The subtopology identifier." },
      { "name": "Partitions", "type": "[]int32", "versions": "0+",
        "about": "The partitions of the input topics processed by this member." }
    ]},
    { "name": "KeyValue", "versions": "0+", "fields": [
      { "name": "Key", "type": "string", "versions": "0+",
        "about": "key of the config" },
      { "name": "Value", "type": "string", "versions": "0+",
        "about": "value of the config" }
    ]},
    { "name": "TopicInfo", "versions": "0+", "fields": [
      { "name": "Name", "type": "string", "versions": "0+",
        "about": "The name of the topic." },
      { "name": "Partitions", "type": "int32", "versions": "0+",
        "about": "The number of partitions in the topic. Can be 0 if no specific number of partitions is enforced. Always 0 for changelog topics." },
      { "name": "ReplicationFactor", "type": "int16", "versions": "0+",
        "about": "The replication factor of the topic. Can be 0 if the default replication factor should be used." },
      { "name": "TopicConfigs", "type": "[]KeyValue", "versions": "0+",
        "about": "Topic-level configurations as key-value pairs."
      }
    ]}
  ]
}

/**
 * A detailed description of a single member in the group.
 */
public class StreamsGroupMemberDescription {

    public StreamsGroupMemberDescription(
        final String memberId,
        final int memberEpoch,
        final int currentAssignmentEpoch,
        final Optional<String> instanceId,
        final Optional<String> rackId,
        final String clientId,
        final String clientHost,
        final int topologyEpoch,
        final String processId,
        final Optional<Endpoint> userEndpoint,
        final Map<String, String> clientTags,
        final List<TaskOffset> taskOffsets,
        final List<TaskOffset> taskEndOffsets,
        final StreamsGroupMemberAssignment assignment,
        final StreamsGroupMemberAssignment targetAssignment,
        final boolean isClassic
    );

    /**
     * The id of the group member.
     */
    public String memberId();

    /**
     * The epoch of the group member.
     */
    public int memberEpoch();

    /**
     * The id of the instance, used for static membership, if available.
     */
    public Optional<String> instanceId(); 
    
    /**
     * The rack ID of the group member.
     */
    public Optional<String> rackId();
    
    /**
     * The client id of the group member.
     */
    public String clientId();

    /**
     * The host of the group member.
     */
    public String clientHost();

    /**
     * The epoch of the group member. 
     */
    public int memberEpoch();

    /**
     * The epoch of the current assignment.
     */
    public int currentAssignmentEpoch();

    /**
     * The epoch of the topology present on the client.
     */
    public int topologyEpoch();
 
    /**
     * Identity of the streams instance that may have multiple clients.
     */
    public String processId(); 

    /**
     * User-defined endpoint for Interactive Queries.
     */
    public Optional<Endpoint> userEndpoint();

    /**
     * Used for rack-aware assignment algorithm.
     */
    public Map<String, String> clientTags();

    /**
     * Cumulative offsets for tasks.
     */
    public List<TaskOffset> taskOffsets();

    /**
     * Cumulative task changelog end offsets for tasks.
     */
    public List<TaskOffset> taskEndOffsets();

    /**
     * The current assignment.
     */
    public StreamsGroupMemberAssignment assignment();

    /**
     * The target assignment.
     */
    public StreamsGroupMemberAssignment targetAssignment();     
    
    /**
     * The flag indicating whether a member is classic.
     */
    public boolean isClassic();

    /**
     * The cumulative offset for one task.
     */
    public static class TaskOffset {

        public TaskOffset(final String subtopologyId, final int partition, final long offset);

        /**
         * The subtopology identifier.
         */
        public String subtopologyId();

        /**
         * The partition of the task.
         */
        public int partition();

        /**
         * The cumulative offset (sum of offsets in all input partitions).
         */
        public long offset();
    }
}