Re: [PR] [server] Add listRebalances API and expose rebalance timestamps [fluss]

["morazow (via GitHub)" <gi...@apache.org>]

morazow commented on PR #3985:
URL: https://github.com/apache/fluss/pull/3985#issuecomment-5340158735

   **Scope note for reviewers:** per-bucket detail for historical rebalances
   
   This PR keeps `listRebalances()` deliberately summary-only (id, status, timestamps). The per-bucket plan of a retained historical rebalance is already persisted, each history znode stores the full `RebalanceTask`, plan included.
   
   But `listRebalanceProgress(id)` still throws `NoRebalanceInProgressException` for non-current ids, so that plan is not yet reachable through the API.
   
   It is easier to enable this API: `listRebalanceProgress(id)` fall back to the ZooKeeper history when the id is not current, and build the RebalanceProgress from the stored plan. No proto or serde change is needed; the existing response message already carries per-bucket plans. 
   
   Keeping it out of this PR keeps the review surface small and leaves one design point open for discussion:
   
   - History stores plans plus the overall final status, not per-bucket outcomes (those live only in coordinator memory). A historical RebalanceProgress would stamp every bucket with the overall status — exact for COMPLETED, approximate for FAILED/CANCELED. If per-bucket outcomes matter, the follow-up would instead need a serde bump to store RebalanceResultForBucket per bucket. Opinions welcome.
   
   The follow-up also changes one contract: `NoRebalanceInProgressException` would no longer be thrown for ids still in Zookeeper, which I read as the intent of #3965.


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