[ZOOKEEPER-2919] expired ephemeral node reappears after ZK leader change

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 3.4.9
• Assignee: Michael Han
• Reporter: Jun Rao
• Created: 16/Oct/17 22:58
• Updated: 03/Sep/19 18:37

Description

We found the following issue when using ZK. A client (a Kafka broker) registered an ephemeral node in ZK. The client then received a session expiration event and created the new session. The client tried to create the same ephemeral node in ZK in the new session but received a NodeExistException. The following are the details.

From Kafka broker 1:
Broker 1 received the expiration of session 55bcff0f02d0002 at 13:33:26.

[2017-07-29 13:33:26,706] INFO Unable to reconnect to ZooKeeper service, session 0x55bcff0f02d0002 has expired, closing socket connection (org.apache.zookeeper.ClientCnxn)

It then established a new session 55d8f690ca20038 at 13:33:33.

[2017-07-29 13:33:33,405] INFO Session establishment complete on server rdalnydbbdqs10/10.122.104.12:2181, sessionid = 0x55d8f690ca20038, negotiated timeout = 6000 (org.apache.zookeeper.ClientCnxn)

However, the re-registration of the broker id fails.

[2017-07-29 13:33:33,408] INFO Result of znode creation is: NODEEXISTS (kafka.utils.ZKCheckedEphemeral)
[2017-07-29 13:33:33,408] ERROR Error handling event ZkEvent[New session event sent to kafka.server.KafkaHealthcheck$SessionExpireListener@74ad6d14] (org.I0Itec.zkclient.ZkEvent
Thread)
java.lang.RuntimeException: A broker is already registered on the path /brokers/ids/1. This probably indicates that you either have configured a brokerid that is already in use, or else you have shutdown this broker and restarted it faster than the zookeeper timeout so it appears to be re-registering.
        at kafka.utils.ZkUtils.registerBrokerInZk(ZkUtils.scala:375)
        at kafka.utils.ZkUtils.registerBrokerInZk(ZkUtils.scala:361)
        at kafka.server.KafkaHealthcheck.register(KafkaHealthcheck.scala:71)
        at kafka.server.KafkaHealthcheck$SessionExpireListener.handleNewSession(KafkaHealthcheck.scala:105)
        at org.I0Itec.zkclient.ZkClient$6.run(ZkClient.java:736)
        at org.I0Itec.zkclient.ZkEventThread.run(ZkEventThread.java:72)

From ZK server (my id 4) :
It expired the old session 55bcff0f02d0002 correctly before broker received the session expiration. It then went to ZK leader election soon after.

[2017-07-29 13:33:26,000] INFO Expiring session 0x55bcff0f02d0002, timeout of 6000ms exceeded (org.apache.zookeeper.server.ZooKeeperServer)
[2017-07-29 13:33:26,019] INFO Processed session termination for sessionid: 0x55bcff0f02d0002 (org.apache.zookeeper.server.PrepRequestProcessor)
[2017-07-29 13:33:33,582] INFO Shutting down (org.apache.zookeeper.server.quorum.CommitProcessor)
[2017-07-29 13:33:34,344] INFO New election. My id =  4, proposed zxid=0x5830d1163b (org.apache.zookeeper.server.quorum.FastLeaderElection)
[2017-07-29 13:34:22,499] INFO FOLLOWING - LEADER ELECTION TOOK - 48915 (org.apache.zookeeper.server.quorum.Learner)

From ZK server (my id 5):
It lost the connection to the old session 55bcff0f02d0002 before the session got expired. It then went into ZK leader election and became the leader. However, it didn't think the old session 55bcff0f02d0002 was expired after becoming the leader. Therefore, the new session 55d8f690ca20038 failed to create /brokers/ids/1. Only after that, it eventually expired the old session 55bcff0f02d0002.

[2017-07-29 13:33:24,216] WARN caught end of stream exception (org.apache.zookeeper.server.NIOServerCnxn)
EndOfStreamException: Unable to read additional data from client sessionid 0x55bcff0f02d0002, likely client has closed socket
        at org.apache.zookeeper.server.NIOServerCnxn.doIO(NIOServerCnxn.java:228)
        at org.apache.zookeeper.server.NIOServerCnxnFactory.run(NIOServerCnxnFactory.java:203)
        at java.lang.Thread.run(Thread.java:745)
2017-07-29 13:33:24,216] INFO Closed socket connection for client /10.122.73.147:59615 which had sessionid 0x55bcff0f02d0002 (org.apache.zookeeper.server.NIOServerCnxn)
[2017-07-29 13:33:30,921] INFO New election. My id =  5, proposed zxid=0x5830d1113f (org.apache.zookeeper.server.quorum.FastLeaderElection)
[2017-07-29 13:33:31,126] INFO LEADING - LEADER ELECTION TOOK - 1122 (org.apache.zookeeper.server.quorum.Leader)
[2017-07-29 13:33:33,405] INFO Established session 0x55d8f690ca20038 with negotiated timeout 6000 for client /10.122.73.147:47106 (org.apache.zookeeper.server.ZooKeeperServer)
[2017-07-29 13:33:33,407] INFO Got user-level KeeperException when processing sessionid:0x55d8f690ca20038 type:create cxid:0x5 zxid:0x5900000352 txntype:-1 reqpath:n/a Error Path:/brokers/ids/1 Error:KeeperErrorCode = NodeExists for /brokers/ids/1 (org.apache.zookeeper.server.PrepRequestProcessor)
[2017-07-29 13:33:40,002] INFO Expiring session 0x55bcff0f02d0002, timeout of 6000ms exceeded (org.apache.zookeeper.server.ZooKeeperServer)
[2017-07-29 13:33:40,074] INFO Processed session termination for sessionid: 0x55bcff0f02d0002 (org.apache.zookeeper.server.PrepRequestProcessor)

According to http://mail-archives.apache.org/mod_mbox/zookeeper-user/201701.mbox/%3CB512F6DE-C0BF-45CE-8102-6F242988268E%40apache.org%3E from fpj, a ZK client in a new session shouldn't see the ephemeral node created in its previous session. So, could this be a potential bug in ZK during ZK leader transition?

Issue Links

• relates to: ZOOKEEPER-3018 Ephemeral node not deleted after session is gone