This is the scenario I have in mind 1. Client A gets assigned partitions P1 and P2. 2. Client A polls a message with offset X from P1, opens a transaction and produces to some output topic. 3. Client B joins the group and gets assigned P2 4. Client A tries to sendOffsets with group metadata but is fenced (due to the rebalance that bumped the generation.id). At this moment I can't use that producer anymore so I will just "ignore" that message, since the broker will abort the transaction. 5. Client A polls again and will get the message with offset X + 1, since that partition wasn't revoked due to the cooperative rebalance.
In an eager rebalance this can't happen since the rebalance that bumped the generation.id and fenced my producer will also revoke the partitions and force my consumer to fetch the offset again and poll the message with offset X again. I couldn't test this locally yet since isn't trivial, but could this happen in theory or am I missing something? Thanks.
