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Dave Latham commented on ZOOKEEPER-2845: ---------------------------------------- Any updates here? We were considering upgrading our zookeeper, but don't want to go to a release with a known data inconsistency problem. > Data inconsistency issue due to retain database in leader election > ------------------------------------------------------------------ > > Key: ZOOKEEPER-2845 > URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2845 > Project: ZooKeeper > Issue Type: Bug > Components: quorum > Affects Versions: 3.4.10, 3.5.3, 3.6.0 > Reporter: Fangmin Lv > Assignee: Fangmin Lv > Priority: Critical > > In ZOOKEEPER-2678, the ZKDatabase is retained to reduce the unavailable time > during leader election. In ZooKeeper ensemble, it's possible that the > snapshot is ahead of txn file (due to slow disk on the server, etc), or the > txn file is ahead of snapshot due to no commit message being received yet. > If snapshot is ahead of txn file, since the SyncRequestProcessor queue will > be drained during shutdown, the snapshot and txn file will keep consistent > before leader election happening, so this is not an issue. > But if txn is ahead of snapshot, it's possible that the ensemble will have > data inconsistent issue, here is the simplified scenario to show the issue: > Let's say we have a 3 servers in the ensemble, server A and B are followers, > and C is leader, and all the snapshot and txn are up to T0: > 1. A new request reached to leader C to create Node N, and it's converted to > txn T1 > 2. Txn T1 was synced to disk in C, but just before the proposal reaching out > to the followers, A and B restarted, so the T1 didn't exist in A and B > 3. A and B formed a new quorum after restart, let's say B is the leader > 4. C changed to looking state due to no enough followers, it will sync with > leader B with last Zxid T0, which will have an empty diff sync > 5. Before C take snapshot it restarted, it replayed the txns on disk which > includes T1, now it will have Node N, but A and B doesn't have it. > Also I included the a test case to reproduce this issue consistently. > We have a totally different RetainDB version which will avoid this issue by > doing consensus between snapshot and txn files before leader election, will > submit for review. -- This message was sent by Atlassian JIRA (v6.4.14#64029)