Details
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Bug
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Status: Resolved
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Major
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Resolution: Not A Bug
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3.4.14, 3.5.7
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None
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None
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None
Description
When a ZooKeeper client session disappears, the associated ephemeral node that is used for leader election is occasionally not deleted and persists (indefinitely, it seems).
A leader election process may select such a stale node to be the leader. In a scenario where there is a redundant service that takes action when acquiring leadership by means of a ZooKeeper election process, this leads to none of the services being active when the stale ephemeral node is elected.
One of the scenarios where such a stale ephemeral node is created can be triggered by force-killing the ZooKeeper server (kill -9 <pid>) as well as the client, which leads to the session being recreated after restarting the server on its side, even though the actual client session is gone. This node even persists after regular restarts from now on. No pings from its owner-session are received, compared to an active one, yet the session never expires. This scenario involves a single ZooKeeper server, but the problem has also been observed in a cluster of three.
When the ephemeral node is first persisted after restarting (and every restart thereafter), the following is observable in the ZooKeeper server logs. The scenario involves a local ZooKeeper server (version 3.5.7) and a single leader election participant.
Opening datadir:/my/path snapDir:/my/path zookeeper.snapshot.trust.empty : true tickTime set to 2000 minSessionTimeout set to 4000 maxSessionTimeout set to 40000 zookeeper.snapshotSizeFactor = 0.33 Reading snapshot /my/path/version-2/snapshot.71 Created new input stream /my/path/version-2/log.4b Created new input archive /my/path/version-2/log.4b EOF exception java.io.EOFException: Failed to read /my/path/version-2/log.4b Created new input stream /my/path/version-2/log.72 Created new input archive /my/path/version-2/log.72 Ignoring processTxn failure hdr: -1 : error: -110 Ignoring processTxn failure hdr: -1, error: -110, path: null Ignoring processTxn failure hdr: -1 : error: -110 Ignoring processTxn failure hdr: -1, error: -110, path: null Ignoring processTxn failure hdr: -1 : error: -110 Ignoring processTxn failure hdr: -1, error: -110, path: null Ignoring processTxn failure hdr: -1 : error: -110 Ignoring processTxn failure hdr: -1, error: -110, path: null Ignoring processTxn failure hdr: -1 : error: -110 Ignoring processTxn failure hdr: -1, error: -110, path: null Ignoring processTxn failure hdr: -1 : error: -110 Ignoring processTxn failure hdr: -1, error: -110, path: null EOF exception java.io.EOFException: Failed to read /my/path/version-2/log.72 Snapshotting: 0x8b to /my/path/version-2/snapshot.8b ZKShutdownHandler is not registered, so ZooKeeper server won't take any action on ERROR or SHUTDOWN server state changes autopurge.snapRetainCount set to 3 autopurge.purgeInterval set to 3
Could this problem be solved by ZooKeeper checking the sessions for each participating node before starting a leader election?
So far only manual intervention (removing the stale ephemeral node) seems to "fix" the issue temporarily.