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OffsetCommitRequest => ConsumerGroup [TopicName [Partition Offset Metadata]]
  ConsumerGroup => string
  TopicName => string
  Partition => int32
  Offset => int64
  Metadata => string

These fields should be mostly self explanatory, except for metadata. This is meant as a way to attach arbitrary metadata that should be committed with this offset commit. It could be the name of a file that contains state information for the processor, or a small piece of state. Basically it is just a generic string field that will be passed back to the client when the offset is fetched. It will likely have a tight size limit to avoid server impact.

Response:

OffsetCommitResponse => [TopicName [Partition ErrorCode]]]
  ErrorCode => int16

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OffsetFetchResponse => [TopicName [Partition Offset Metadata ErrorCode]]

For phase I the implementation would remain the existing zookeeper structure:

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