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Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

, we optimize the beginQuorumEpochRequest timeout, 

The controller quorum fetch loop relies on a timing invariant for correctness and liveness: the  controller.quorum.fetch.timeout.ms must be significantly larger than Raft maximum fetch wait time(currently current hardcode to 500 ms) must be significantly smaller than half of the controller.quorum.fetch.timeout.ms ) after KAFKA-16926 Optimize BeginQuorumEpoch heartbeat.

In practice, the algorithm makes progress because:

raft_max_fetch_wait_time (500 ms) * 2  <=  controller.quorum.fetch.timeout.ms  * 2ms 

If operators set an unusually small controller.quorum.fetch.timeout.ms raft_max_fetch_wait_time * 2, this invariant can be violated, risking premature timeouts, spurious retries, and degraded stability. To preserve the invariant across configurations and upgrades, we propose to enforce a lower bound of 1000 ms for controller.quorum.fetch.timeout.ms.

For example:

We assume network latency is 100ms

T=0ms:     The Leader send beginQurum resquest to the follower.

T=100ms:     The Follower start fetching and sends fetch request and starts 500ms *fetchTimer*
T=600ms: Leader does not receive fetch request (RAFT_MAX_FETCH_WAIT_MS e.g. 500)
                     do two things:
                     1) The Leader send BeginQuorum request to the follower again.
                     2) The follower sends fetch request to leader and start *fetchTimer*
T=700ms: four cases
                    1) the follower receives BeginQuorum and response it (100 network latency) -> leader still works
                    2) the follower does not receive beginQuorum request -> leader MAYBE not work
                    1) the leader receives fetch and response it (100 network latency) -> follower still works
                   2) the leader does not receive fetch Request -> follower MAYBE not work

T=800ms: Follower's fetch timer is expired.

then the follower transitions to the *Prospective state*, and it triggers election... (wasted resources even the leader is working fine).

From the above flow, we give the leader and follower two changes to check each state, one is beginQuorum, another is fetch but their start point is different.

Public Interfaces

Configuration change (validation only):

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