arshilkxwork opened a new pull request, #3356:
URL: https://github.com/apache/hugegraph/pull/3356
## Purpose of the PR
- close #3135
PD balances partition leaders only when `/v1/task/balanceLeaders`,
`/v1/balanceLeaders` or the `balanceLeaders` gRPC call is made. When a Store
restarts, raft elects new leaders for its groups on the surviving Stores, and
nothing moves them back. On Kubernetes, where pod restarts are routine,
leader-side work stays concentrated on the Stores that did not restart until
someone calls the endpoint by hand.
## Main Changes
- `PDConfig`: three new keys, `pd.balance-leader-enable` (default `false`),
`pd.balance-leader-interval` (seconds, default `300`) and
`pd.balance-leader-max-skew` (default `2`). Names follow the proposal in the
issue. With the defaults, behavior does not change.
- `TaskScheduleService.init()` schedules `autoBalancePartitionLeader()` with
`scheduleWithFixedDelay`, like the other periodic jobs, when the feature is
enabled. A non-positive interval logs a warning and schedules nothing, so a bad
value cannot fail PD startup with the executor's `IllegalArgumentException`.
- `autoBalancePartitionLeader()` does nothing on a follower PD. On the
leader it counts leaders per Store from the shard groups. A Store that holds
shards but leads none counts as 0. If max minus min is at or below `max-skew`
it returns without touching anything. Otherwise it calls the existing
`balancePartitionLeader(false)`, so the algorithm, the 30 s rate limit and the
refusal during split, move and shard-balance tasks are the same as for a manual
call. The scheduled wrapper logs those refusals (`PDException` code 1001) at
INFO level.
- One guard the manual path does not have: a run is skipped while any Store
that holds a shard is missing from the active-store list.
`balancePartitionLeader` computes its target from shard placement, so with a
Store down it would try to transfer leaders to that Store. The periodic job
waits until every Store holding a replica is heartbeating again.
- Docs: the new keys in `hugegraph-pd/docs/configuration.md` and
`architecture.md`, commented defaults in the shipped `hg-pd-dist`
`application.yml`, and a note in the Store operations guide next to the manual
`balanceLeaders` call.
Not included: the event-driven trigger on a Store returning to `Up`, which
the issue lists as a possible follow-up. The skew check uses max minus min over
the Stores that hold shards, as proposed in the issue. With uneven shard
placement, the best reachable distribution can itself exceed `max-skew`. In
that case each run calls `balancePartitionLeader`, which finds every leader
already on its target and transfers nothing.
## Verifying these changes
- [ ] Trivial rework / code cleanup without any test coverage. (No Need)
- [ ] Already covered by existing tests, such as *(please modify tests
here)*.
- [x] Need tests and can be verified as follows:
- `TaskScheduleServiceTest` (core) gets three cases on six shard groups
over Stores 1-3: leaders spread 2/2/2 makes no transfers; all six leaders on
Store 1 moves four leaders to Stores 2 and 3 and none to Store 1; Store 3
without a heartbeat skips the run.
- `mvn test -pl hugegraph-pd/hg-pd-test -P pd-core-test`: 153 tests, 0
failures, 2 skipped (both skips predate this change).
- Not tested against a running PD and Store cluster.
## Does this PR potentially affect the following parts?
- [ ] Dependencies ([add/update
license](https://hugegraph.apache.org/docs/contribution-guidelines/contribute/#321-check-licenses)
info &
[regenerate_known_dependencies.sh](../install-dist/scripts/dependency/regenerate_known_dependencies.sh))
- [x] Modify configurations
- [ ] The public API
- [ ] Other affects (typed here)
- [ ] Nope
## Documentation Status
- [ ] `Doc - TODO`: required documentation is pending; complete it before
merging.
- [x] `Doc - Done`: documentation is included here or linked below.
- [ ] `Doc - No Need`: no user-visible documentation is affected.
Documentation files in this PR or paired hugegraph-doc PR:
- `hugegraph-pd/docs/configuration.md`
- `hugegraph-pd/docs/architecture.md`
- `hugegraph-pd/hg-pd-dist/src/assembly/static/conf/application.yml`
- `hugegraph-store/docs/operations-guide.md`
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