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Simplified error handling: Much of the complexity in the error handling of the idempotent/transactional producer is a result of the UNKNOWN_PRODUCER_ID case. Since we are proposing to cache producer state for as long as the transactional id expiration time even after removal from the log, this should become a rare error, so we propose to simplify our handling of it. The current handling attempts to reason about the log start offset and whether or not the batch had been previously retried. If we are sure it is safe, then we attempt to adjust the sequence number of the failed request (and any inflight requests which followed). Not only is this behavior complex to implement, but continuing with subsequent batches introduces the potential for reordering. Currently there is no easy way to prevent this from happening.

We propose the following simplifications:

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