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Status

Current state: Draft

Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread]

JIRA: here [Change the link from KAFKA-1 to your own ticket]

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

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

In practice, the algorithm makes progress because:

raft_max_fetch_wait_time (

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500 ms)

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* 2  <=  controller.quorum.fetch.timeout.

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ms 

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):

• controller.quorum.fetch.timeout.ms

  • Type: int (ms)

  • Lower bound: 1000 ms (enforced)

  • Default: unchanged (2000ms)

  • Behavior: Values < 1000 ms are invalid and will be rejected with a clear error message.

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• Config validation:

  • Update the config definition for controller.quorum.fetch.timeout.ms to use atLeast(1000) (or equivalent validation hook).

  • Apply the same validation path for both static startup configs and any dynamic/config-provider paths that may set this value.

• Error messaging:

  • On invalid values, throw a ConfigException with actionable guidance, e.g.:

    “controller.quorum.fetch.timeout.ms must be ≥ 1000 ms to ensure the controller fetch loop makes progress without violating timing assumptions.”

• Documentation:

  • Update the config reference to document the lower bound and the rationale (ties to fetch-wait timing and controller liveness).

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• Backward compatibility:

  • Deployments already using values ≥ 1000 ms are unaffected.

  • Deployments with < 1000 ms will fail fast at startup (or on reconfiguration) with a clear error.

• No deprecations; this is a validation tightening to ensure correctness.

• Migration:

  • Operators must raise any sub-1000 ms values to ≥ 1000 ms before/with the upgrade.

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• Unit tests for config validation (values below/at/above 1000).

• Integration test: start controller with invalid value → expect ConfigException and clear message.

• (Optional) Property-based test ensuring no accepted value violates the max_fetch_wait << timeout / 2 invariant under defaults.

Rejected Alternatives

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Silent clamping to 1000 ms:

• Rejected to avoid non-obvious behavior and hidden config drift. Validation with an explicit error is clearer and safer.

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Derive timeout automatically (e.g., k * max_fetch_wait):

• Reduces operator control; we prefer a simple invariant and explicit validation.

1. Do not change the limitation:

   • Maybe cause the system unavailable if users' configuration is wrong.

2. Make raft

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1. max fetch wait configurable instead:

   • Out of scope here. Even if exposed, we would still need a minimum for the timeout to maintain a robust ratio.