[FIXED] Stuck consumer after leader change #6469
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Signed-off-by: Maurice van Veen <[email protected]>
neilalexander
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neilalexander
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Related to #6469, about the following code: ```go // Update delivered first. o.updateDelivered(dseq, seq, dc, ts) // Send message. o.outq.send(pmsg) ``` `o.updateDelivered` requires proposing delivered state through Raft, and even if proposing fails, we immediately sent the message to the client. This is great for performance, but really bad for properly replicating this piece of data. Before the before-mentioned PR there would be a bunch of nasty side-effects of stuck consumer, perceived data loss through missed redeliveries, etc. Because clients could get messages that a new leader wouldn't know about if proposals failed. The core issue is that we should only send the message AFTER we had quorum on updating delivered state. Otherwise the following could happen: message gets sent to the client, `updateDelivered` proposal fails, leader changes, `AckSync` will now timeout indefinitely even with retries because the new leader doesn't know this message was even delivered. Signed-off-by: Maurice van Veen <[email protected]>
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When a client requests for messages, in
o.deliverMsgthe following is done:For
o.updateDeliveredwe need to establish quorum to have all servers know the message was delivered, and this can fail. So we would have sent the client messages that a new leader will not know have ever been delivered.Once a new leader gets elected and receives an ACK for a message it doesn't know was delivered it would move
o.sseqahead. This is incorrect, since the message is not ino.pendingit results in ack floors not being updated. And if any messages before the one that was acked were not acknowledged/NAK-ed, that would mean these messages would never be redelivered resulting in the stuck consumer symptom.Signed-off-by: Maurice van Veen [email protected]