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• ConsumerCoordinator contains the following members and metadata structures (just following Section 4):
  • A ZKClient that is different from the one that KafkaServer used.
  • ZkElection: a Zookeeper based election module, which will trigger the startup call back procedure of the ConsumerCoordinator upon success election. Hence at the same time period only one ConsumerCoordinator of the servers will be "active".
  • Wiki MarkupArray\[RebalanceRequestHandler\]: a list of rebalance handler threads which is used for processing the rebalancing tasks for groups, each has a BlockingQueue\[String\] storing assigned rebalance tasks.
  • A KafkaScheduler heartbeat request scheduling thread which periodically sends heartbeat request to all consumers (frequency based on consumer's session timeout value) that is registered to this coordinator.
  • A KafkaScheduler offset committing thread which periodically writes the current consumed offset for each group to Zookeeper using the coordinator's ZKClient.
  • A HeartbeatRequestPurgatory storing expiration watchers for sent HeartbeatRequest (in implementation we rename "ping" to "heartbeat")
  • A StopFetcherRequestPurgatory and a StartFetcherRequestPurgatory storing expiration watchers for Stop/StartFetcherRequestPurgatory.

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