databendlabs / openraft / 27400130554

87%

Build Type

push

github

Committed by drmingdrmer

Commit Message

feat: config: Make leader restore upon restart configurable

# Summary

Add `Config::enable_leader_restore` to control whether a node that was a
leader before a restart resumes leadership at startup without an election.
The default (`true`) keeps the current behavior; `false` makes the node
restart as a follower and re-acquire leadership through a normal election,
as standard Raft does.

# Details

Openraft persists leadership in the committed vote, so a leader that
restarts quickly enough resumes serving without an election. This improves
availability, but the state machine may lose un-flushed state across the
restart and is rebuilt only up to the persisted committed index — and
persisting the committed index (`save_committed()`) is optional. The
restored leader can then serve reads from a state machine that is behind
what clients already observed, with no way for the application to detect
it (#1511).

Disabling leader restore closes this window: the restarted node rejoins as
a follower and catches up like any other follower before it can lead
again. This is useful when:

- the application does not persist the committed index, or the state
  machine is transient and rebuilt on startup;
- external readers require consistent reads across a leader restart;
- standard-Raft-compatible restart behavior is required.

Implementation:

- `enable_leader_restore` is `Option<bool>` resolved at runtime with
  `unwrap_or(true)` (same pattern as `allow_log_reversion`), so an unset
  value is distinguishable from an explicit choice.

- When disabled, `Engine::startup()` demotes the committed self-vote to
  non-committed in memory only; the committed vote intentionally stays in
  storage. Persisting the demoted vote would write a smaller value
  (non-committed < committed for the same leader), breaking the
  monotonicity of vote storage and of IO progress tracking. Keeping it is
  safe: votes order by leader id first, then committed flag, so no vote
  orders between ... (continued)

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