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1. On startup or on co-ordinator failover, the consumer sends a ClusterMetadataRequest ConsumerMetadataRequest to any of the brokers in the "bootstrap.brokers" list. In the ClusterMetadataResponseConsumerMetadataResponse, it receives the location of the co-ordinator for it's group.  
2. The consumer sends a RegisterConsumer request to it's co-ordinator broker. In the RegisterConsumerResponse, it receives the list of topic partitions that it should own.
3. At this time, group management is done and the consumer starts fetching data and (optionally) committing offsets for the list of partitions it owns.

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{
  Version                => int16
  CorrelationId          => int64
  GroupId                => String
  ConsumerHost           => String
  SessionTimeout         => int64
  Topics                 => [String]
}

{
  Version                => int16
  CorrelationId          => int64
  ErrorCode              => int16
  GenerationId           => int16
  GroupId                => String 
  ConsumerId             => String
  PartitionsToOwn        => [TopicAndPartition]
}

HeartbeatRequest

{
  Version                => int16
  CorrelationId          => int64
  GroupId                => String
  ConsumerId             => String
}

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