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Updated on 2022-09-15 GMT+08:00
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Why Do ZooKeeper Servers Fail to Start After Many znodes Are Created?

Question

After a large number of znodes are created, ZooKeeper servers in the ZooKeeper cluster become faulty and cannot be automatically recovered or restarted.

Logs of followers:

2016-06-23 08:00:18,763 | WARN  | QuorumPeer[myid=26](plain=/10.16.9.138:2181)(secure=disabled) | Exception when following the leader | org.apache.zookeeper.server.quorum.Follower.followLeader(Follower.java:93)
java.net.SocketTimeoutException: Read timed out
    at java.net.SocketInputStream.socketRead0(Native Method)
    at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
    at java.net.SocketInputStream.read(SocketInputStream.java:170)
    at java.net.SocketInputStream.read(SocketInputStream.java:141)
    at java.io.BufferedInputStream.fill(BufferedInputStream.java:246)
    at java.io.BufferedInputStream.read(BufferedInputStream.java:265)
    at java.io.DataInputStream.readInt(DataInputStream.java:387)
    at org.apache.jute.BinaryInputArchive.readInt(BinaryInputArchive.java:63)
    at org.apache.zookeeper.server.quorum.QuorumPacket.deserialize(QuorumPacket.java:83)
    at org.apache.jute.BinaryInputArchive.readRecord(BinaryInputArchive.java:99)
    at org.apache.zookeeper.server.quorum.Learner.readPacket(Learner.java:156)
    at org.apache.zookeeper.server.quorum.Learner.registerWithLeader(Learner.java:276)
    at org.apache.zookeeper.server.quorum.Follower.followLeader(Follower.java:75)
    at org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:1094)
2016-06-23 08:00:18,764 | INFO  | QuorumPeer[myid=26](plain=/10.16.9.138:2181)(secure=disabled) | shutdown called | org.apache.zookeeper.server.quorum.Follower.shutdown(Follower.java:198)
java.lang.Exception: shutdown Follower
    at org.apache.zookeeper.server.quorum.Follower.shutdown(Follower.java:198)
    at org.apache.zookeeper.server.quorum.QuorumPeer.stopFollower(QuorumPeer.java:1141)
    at org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:1098)

Logs of the leader:

2016-06-23 07:30:57,481 | WARN  | QuorumPeer[myid=25](plain=/10.16.9.136:2181)(secure=disabled) | Unexpected exception | org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:1108)
java.lang.InterruptedException: Timeout while waiting for epoch to be acked by quorum
    at org.apache.zookeeper.server.quorum.Leader.waitForEpochAck(Leader.java:1221)
    at org.apache.zookeeper.server.quorum.Leader.lead(Leader.java:487)
    at org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:1105)
2016-06-23 07:30:57,482 | INFO  | QuorumPeer[myid=25](plain=/10.16.9.136:2181)(secure=disabled) | Shutdown called | org.apache.zookeeper.server.quorum.Leader.shutdown(Leader.java:623)
java.lang.Exception: shutdown Leader! reason: Forcing shutdown
    at org.apache.zookeeper.server.quorum.Leader.shutdown(Leader.java:623)
    at org.apache.zookeeper.server.quorum.QuorumPeer.stopLeader(QuorumPeer.java:1149)
    at org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:1110)

Answer

After a large number of znodes are created, a large volume of data needs to be synchronized between the follower and leader. If the data synchronization is not complete within the specified time, all ZooKeeper servers fail to start.

Go to the All Configurations page of the ZooKeeper service by referring to Modifying Cluster Service Configuration Parameters. To recover ZooKeeper servers, increase the values of syncLimit and initLimit in the ZooKeeper configuration file zoo.cfg until ZooKeeper servers are successfully started.

Table 1 Parameters

Parameter

Description

Default Value

syncLimit

Interval (unit: tick) at which data is synchronized between the follower and the leader. If the leader does not respond to the follower within the specified time, the connection between the leader and follower cannot be set up.

15

initLimit

Interval (unit: tick) within which the connection and synchronization between the follower and leader must be completed.

15

If ZooKeeper servers do not recover even after initLimit and syncLimit are set to 300 ticks, check that no other application is killing the ZooKeeper. For example, if the parameter value is 300 and the ticket duration is 2000 ms, the maximum synchronization duration is 600s (300 x 2000 ms).

There may exist the situation where an overwhelming amount of data is created in ZooKeeper and it takes long to synchronize data between the follower and the leader and to save data to the hard disk. This means that ZooKeeper needs to run for a long time. Ensure that no other monitoring application kills the ZooKeeper while ZooKeeper is running.