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From "Michael Han (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-2919) expired ephemeral node reappears after ZK leader change
Date Fri, 01 Jun 2018 22:00:00 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-2919?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16498625#comment-16498625

Michael Han commented on ZOOKEEPER-2919:

[~junrao] I think this is a bug in ZK. We promised (as [~fpj] also pointed in the [email thread|http://mail-archives.apache.org/mod_mbox/zookeeper-user/201701.mbox/%3CB512F6DE-C0BF-45CE-8102-6F242988268E%40apache.org%3E])
that {quote} Receiving a session expiration means
that a closeSession has been committed {quote}, but this is not true in every case, and sometimes
even when a client receives session expire event it's possible the session close is not committed
across the quorum. This is because we don't differentiate "session closing" with "session
closed" state internally at ZK. Between a session starts expiring and the session expired,
a client will receive session expire for its requests to server but if the quorum is down
at the same time, it's possible the close session is not committed.

Here is a canonical example similar to this Kafka case: 

Client A connects to Server 1. Server 1, 2 and 3 forms an ensemble.
1. For some reasons like network interruption, Server 1 thinks A is dead, so it starts expiring
the session. It starts by setting the "is Closing" property of session object associated with
A to true.
2. A fires a request to Server 1 (and network heals) and gets back session expire event. A
thinks its session is expired but actually the session expiration was not finished yet. A
gave up and starts a new session.
3. Meanwhile, Server 1 proposes to close the old session of A and at the same time, the quorum
is down (1 was leader but got partitioned from 2 and 3). The proposal never landed at Server
2 and Server 3. 
4. 2 and 3 can talk, and they elects 2 as leader, who still has old session of A.
5. Network heals, 1 reconnects to ensemble, and found there is already a leader 2, sync with
2, truncate its log when needed.
6. 2 now happily accepts a new session request from A.
7. Eventually 2 expired old session object because there was not active client associates
with that old session. Now between the new session established and the old session expiration,
the old ephemeral nodes will be visible to clients.

There are two approaches to solve this issue. 

1. ZK users (e.g. Kafka ZK util client) could check the presence of the old ephemeral (either
through polling or register a watcher) before trying recreate another ephemeral with same
old path.
2. ZK fixes the semantic of session expire so when a client sees session expired, the session
is really expired (instead of expiring). 

I'll propose a fix for 2, starting with a test case that can deterministically reproduce this

> expired ephemeral node reappears after ZK leader change
> -------------------------------------------------------
>                 Key: ZOOKEEPER-2919
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2919
>             Project: ZooKeeper
>          Issue Type: Bug
>    Affects Versions: 3.4.9
>            Reporter: Jun Rao
>            Assignee: Michael Han
>            Priority: Major
>         Attachments: zk-myid-4.log, zk-myid-5.log
> 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.
> {code:java}
> [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)
> {code}
> It then established a new session 55d8f690ca20038 at 13:33:33.
> {code:java}
> [2017-07-29 13:33:33,405] INFO Session establishment complete on server rdalnydbbdqs10/,
sessionid = 0x55d8f690ca20038, negotiated timeout = 6000 (org.apache.zookeeper.ClientCnxn)
> {code}
> However, the re-registration of the broker id fails.
> {code:java}
> [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)
> {code}
> 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.
> {code:java}
> [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
> [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)
> {code}
> 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.
> {code:java}
> [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 /
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 / (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
> {code}
> 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?

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