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I would propose that any broker can handle an offset request to make life easy for the client. If the broker happens to be the broker that is the master for the log for the consumer group doing the commit it can just apply the change locally, if not it would invoke a commit request to the appropriate server that is currently the leader for the correct partition. A simple client can just direct their requests anywhere; a client that optimizes a bit can try to hit the right server and save themselves the extra hop.

Some Nuances

All replicas will keep the in-memory lookup structure for offsets. This could just be a simple hashmap. This will contain only committed offsets to ensure we never need to undo updates.

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