ZOOKEEPER-4925: Fix data loss due to propagation of discontinuous committedLog #2254
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…mittedLog There are two variants of `ZooKeeperServer::processTxn`. Those two variants diverge significantly since ZOOKEEPER-3484. `processTxn(Request request)` pops outstanding change from `outstandingChanges` and adds txn to `committedLog` for follower to sync in addition to what `processTxn(TxnHeader hdr, Record txn)` does. The `Learner` uses `processTxn(TxnHeader hdr, Record txn)` to commit txn to memory after ZOOKEEPER-4394, which means it leaves `committedLog` untouched in `SYNCHRONIZATION` phase. This way, a stale follower will have hole in its `committedLog` after joining cluster. The stale follower will propagate the in memory hole to other stale nodes after becoming leader. This causes data loss. The test case fails on master and 3.9.3, and passes on 3.9.2. So only 3.9.3 is affected. This commit drops `processTxn(TxnHeader hdr, Record txn)` as `processTxn(Request request)` is capable in `SYNCHRONIZATION` phase too. Also, this commit rejects discontinuous proposals in `syncWithLeader` and `committedLog`, so to avoid possible data loss. Refs: ZOOKEEPER-4925, ZOOKEEPER-4394, ZOOKEEPER-3484
li4wang
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zookeeper-server/src/test/java/org/apache/zookeeper/server/quorum/QuorumSyncTest.java
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The changes for fixing the data loss issue looks good. It's a very critical issue. Thanks for fixing it. The changes for blocking proposal and commit requests being out of order looks good in general. We need to make sure it doesn't introduce any unexpected issue. |
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Hi @li4wang, thank for for reviewing this! I have pushed a new commit. Please take another to look and let me know whether it solves your concerns. |
li4wang
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Jun 4, 2025
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@cnauroth @anmolnar @eolivelli @tisonkun Would you like to take another look ? I plan to merge this in days to not block 3.9.4. |
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There are two variants of
ZooKeeperServer::processTxn. Those two variants diverge significantly since ZOOKEEPER-3484.processTxn(Request request)pops outstanding change fromoutstandingChangesand adds txn tocommittedLogfor follower to sync in addition to whatprocessTxn(TxnHeader hdr, Record txn)does. TheLearnerusesprocessTxn(TxnHeader hdr, Record txn)to commit txn to memory after ZOOKEEPER-4394, which means it leavescommittedLoguntouched inSYNCHRONIZATIONphase.This way, a stale follower will have hole in its
committedLogafter joining cluster. The stale follower will propagate the in memory hole to other stale nodes after becoming leader. This causes data loss.The test case fails on master and 3.9.3, and passes on 3.9.2. So only 3.9.3 is affected.
This commit drops
processTxn(TxnHeader hdr, Record txn)asprocessTxn(Request request)is capable inSYNCHRONIZATIONphase too.Also, this commit rejects discontinuous proposals in
syncWithLeaderandcommittedLog, so to avoid possible data loss.Refs: ZOOKEEPER-4925, ZOOKEEPER-4394, ZOOKEEPER-3484