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From James Bucher <jbuc...@expedia.com>
Subject Re: Flink 1.2.1 JobManager Election Deadlock
Date Wed, 06 Sep 2017 20:58:16 GMT
It seems I forgot to attach the full logs. They should be attached now.

From: James Bucher <jbucher@expedia.com<mailto:jbucher@expedia.com>>
Date: Wednesday, September 6, 2017 at 10:43 AM
To: "user@flink.apache.org<mailto:user@flink.apache.org>" <user@flink.apache.org<mailto:user@flink.apache.org>>
Subject: Flink 1.2.1 JobManager Election Deadlock

Hey all,

Just wanted to report this for posterity in case someone else sees something similar. We were
running Flink 1.2.1 in Kubernetes. We use an HA setup with an external Zookeeper and S3 for
checkpointing. We recently noticed a job that appears to have deadlocked on JobManager Leader
election. We think the issue happened something like the following:

  1.  Job was up and running normally
  2.  Some cluster event caused the JobManager Pod (process) to get restarted.
  3.  JobManager came up again but got stuck on LeaderElection. At this time the JobManager
UI sent back a response with "Service temporarily unavailable due to an ongoing leader election.
Please refresh."
  4.  JobManager never exited this state.
  5.  This state persisted across Pod restarts/deletes.

In order to try to pin down this problem we brought up the Job in another Flink Cluster and
started debugging this issue. As a first step I upped the logging level on the JobManager
and applied the change. This resulted in the following log (Full logs attached to this email):


2017-09-05 18:50:55,072 TRACE org.apache.flink.shaded.org.apache.curator.utils.DefaultTracerDriver
 - Trace: GetDataBuilderImpl-Background - 4 ms

2017-09-05 18:50:55,075 DEBUG org.apache.flink.runtime.jobmanager.ZooKeeperSubmittedJobGraphStore
 - Received CHILD_ADDED event (path: /4e4cdc8fb8c1437c620cd4063bd265e1)

2017-09-05 18:50:55,088 DEBUG org.apache.flink.runtime.jobmanager.ZooKeeperSubmittedJobGraphStore
 - Received CHILD_ADDED event notification for job 4e4cdc8fb8c1437c620cd4063bd265e1

2017-09-05 18:50:56,072 DEBUG org.apache.zookeeper.ClientCnxn                            
  - Reading reply sessionid:0x15e3df8b5a57b4b, packet:: clientPath:null serverPath:null finished:false
header:: 1,3  replyHeader:: 1,21476396435,0  request:: '/traveler-profile.us-west-2c.test.expedia.com,F
 response:: s{4305881524,4305881524,1497456679255,1497456679255,0,3,0,0,0,3,21475572278}

2017-09-05 18:50:56,078 DEBUG org.apache.zookeeper.ClientCnxn                            
  - Reading reply sessionid:0x15e3df8b5a57b4b, packet:: clientPath:null serverPath:null finished:false
header:: 2,3  replyHeader:: 2,21476396435,0  request:: '/traveler-profile.us-west-2c.test.expedia.com/krazyglue,F
 response:: s{4305881525,4305881525,1497456679260,1497456679260,0,1,0,0,0,1,4305881526}

2017-09-05 18:50:56,079 DEBUG org.apache.zookeeper.ClientCnxn                            
  - Reading reply sessionid:0x15e3df8b5a57b4b, packet:: clientPath:null serverPath:null finished:false
header:: 3,3  replyHeader:: 3,21476396435,0  request:: '/traveler-profile.us-west-2c.test.expedia.com/krazyglue/mip,F
 response:: s{4305881526,4305881526,1497456679263,1497456679263,0,1,0,0,0,1,4305881527}

2017-09-05 18:50:56,080 DEBUG org.apache.zookeeper.ClientCnxn                            
  - Reading reply sessionid:0x15e3df8b5a57b4b, packet:: clientPath:null serverPath:null finished:false
header:: 4,3  replyHeader:: 4,21476396435,0  request:: '/traveler-profile.us-west-2c.test.expedia.com/krazyglue/mip/flink,F
 response:: s{4305881527,4305881527,1497456679267,1497456679267,0,1,0,0,0,1,4305881528}

2017-09-05 18:50:56,081 DEBUG org.apache.zookeeper.ClientCnxn                            
  - Reading reply sessionid:0x15e3df8b5a57b4b, packet:: clientPath:null serverPath:null finished:false
header:: 5,3  replyHeader:: 5,21476396435,0  request:: '/traveler-profile.us-west-2c.test.expedia.com/krazyglue/mip/flink/default,F
 response:: s{4305881528,4305881528,1497456679270,1497456679270,0,27,0,0,0,5,21475449005}

2017-09-05 18:50:56,085 INFO  org.apache.flink.runtime.leaderretrieval.ZooKeeperLeaderRetrievalService
 - Starting ZooKeeperLeaderRetrievalService.

2017-09-05 18:50:56,087 DEBUG org.apache.zookeeper.ClientCnxn                            
  - Reading reply sessionid:0x15e3df8b5a57b4b, packet:: clientPath:null serverPath:null finished:false
header:: 6,3  replyHeader:: 6,21476396435,0  request:: '/traveler-profile.us-west-2c.test.expedia.com,F
 response:: s{4305881524,4305881524,1497456679255,1497456679255,0,3,0,0,0,3,21475572278}

2017-09-05 18:50:56,087 DEBUG org.apache.zookeeper.ClientCnxn                            
  - Reading reply sessionid:0x15e3df8b5a57b4b, packet:: clientPath:null serverPath:null finished:false
header:: 7,3  replyHeader:: 7,21476396435,0  request:: '/traveler-profile.us-west-2c.test.expedia.com/krazyglue,F
 response:: s{4305881525,4305881525,1497456679260,1497456679260,0,1,0,0,0,1,4305881526}

2017-09-05 18:50:56,088 DEBUG org.apache.zookeeper.ClientCnxn                            
  - Reading reply sessionid:0x15e3df8b5a57b4b, packet:: clientPath:null serverPath:null finished:false
header:: 8,3  replyHeader:: 8,21476396435,0  request:: '/traveler-profile.us-west-2c.test.expedia.com/krazyglue/mip,F
 response:: s{4305881526,4305881526,1497456679263,1497456679263,0,1,0,0,0,1,4305881527}

2017-09-05 18:50:56,090 DEBUG org.apache.zookeeper.ClientCnxn                            
  - Reading reply sessionid:0x15e3df8b5a57b4b, packet:: clientPath:null serverPath:null finished:false
header:: 9,3  replyHeader:: 9,21476396435,0  request:: '/traveler-profile.us-west-2c.test.expedia.com/krazyglue/mip/flink,F
 response:: s{4305881527,4305881527,1497456679267,1497456679267,0,1,0,0,0,1,4305881528}

2017-09-05 18:50:56,091 DEBUG org.apache.zookeeper.ClientCnxn                            
  - Reading reply sessionid:0x15e3df8b5a57b4b, packet:: clientPath:null serverPath:null finished:false
header:: 10,3  replyHeader:: 10,21476396435,0  request:: '/traveler-profile.us-west-2c.test.expedia.com/krazyglue/mip/flink/default,F
 response:: s{4305881528,4305881528,1497456679270,1497456679270,0,27,0,0,0,5,21475449005}

2017-09-05 18:50:56,092 DEBUG org.apache.zookeeper.ClientCnxn                            
  - Reading reply sessionid:0x15e3df8b5a57b4b, packet:: clientPath:/traveler-profile.us-west-2c.test.expedia.com/krazyglue/mip/flink/default/leader
serverPath:/traveler-profile.us-west-2c.test.expedia.com/krazyglue/mip/flink/default/leader
finished:false header:: 11,3  replyHeader:: 11,21476396435,0  request:: '/traveler-profile.us-west-2c.test.expedia.com/krazyglue/mip/flink/default/leader,T
 response:: s{21475449005,21475449005,1503519415753,1503519415753,0,1,0,0,0,1,21475449006}

2017-09-05 18:50:56,092 TRACE org.apache.flink.shaded.org.apache.curator.utils.DefaultTracerDriver
 - Trace: ExistsBuilderImpl-Background - 1 ms

2017-09-05 18:50:56,093 DEBUG org.apache.zookeeper.ClientCnxn                            
  - Reading reply sessionid:0x15e3df8b5a57b4b, packet:: clientPath:/traveler-profile.us-west-2c.test.expedia.com/krazyglue/mip/flink/default/leader
serverPath:/traveler-profile.us-west-2c.test.expedia.com/krazyglue/mip/flink/default/leader
finished:false header:: 12,4  replyHeader:: 12,21476396435,0  request:: '/traveler-profile.us-west-2c.test.expedia.com/krazyglue/mip/flink/default/leader,T
 response:: ,s{21475449005,21475449005,1503519415753,1503519415753,0,1,0,0,0,1,21475449006}

2017-09-05 18:50:56,093 TRACE org.apache.flink.shaded.org.apache.curator.utils.DefaultTracerDriver
 - Trace: GetDataBuilderImpl-Background - 1 ms

2017-09-05 18:50:56,093 DEBUG org.apache.flink.runtime.leaderretrieval.ZooKeeperLeaderRetrievalService
 - Leader node has changed.

At this point I dug into the logs to try and see what was going on. It quickly became apparent
that this probably had something to do with Apache Curator which Flink relies upon to do the
leader election. At this point I grabbed a dump of zookeeper using: https://github.com/ctapmex/zkTreeUtil:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<root path="/traveler-profile.us-west-2c.test.expedia.com/krazyglue/mip/flink">
  <zknode name="default">
    <zknode name="leaderlatch">
      <zknode name="00000000000000000000000000000000">
        <zknode name="job_manager_lock"/>
      </zknode>
      <zknode ephemeral="true" name="_c_9cd3f122-65de-40e7-8e2b-aa34e6a85b83-latch-0000000020"/>
    </zknode>
    <zknode name="checkpoint-counter">
      <zknode name="4e4cdc8fb8c1437c620cd4063bd265e1" value="&#0;&#0;&#0;&#1;"/>
    </zknode>
    <zknode name="checkpoints">
      <zknode name="4e4cdc8fb8c1437c620cd4063bd265e1"/>
    </zknode>
    <zknode name="jobgraphs">
      <zknode name="4e4cdc8fb8c1437c620cd4063bd265e1" value="��&#0;&#5;sr&#0;;org.apache.flink.runtime.state.RetrievableStreamStateHandle&#0;&#1;&#30;&#24;�U�+&#2;&#0;&#1;L&#0;&#24;wrappedStreamStateHandlet&#0;2Lorg/apache/flink/runtime/state/StreamStateHandle;xpsr&#0;9org.apache.flink.runtime.state.filesystem.FileStateHandle&#4;�u�b�&#27;�&#2;&#0;&#2;J&#0;&#9;stateSizeL&#0;&#8;filePatht&#0;&#31;Lorg/apache/flink/core/fs/Path;xp&#0;&#0;&#0;&#0;&#0;&#15;h�sr&#0;&#29;org.apache.flink.core.fs.Path&#0;&#0;&#0;&#0;&#0;&#0;&#0;&#1;&#2;&#0;&#1;L&#0;&#3;urit&#0;&#14;Ljava/net/URI;xpsr&#0;&#12;java.net.URI�&#1;x.C�I�&#3;&#0;&#1;L&#0;&#6;stringt&#0;&#18;Ljava/lang/String;xpt&#0;�s3a://ewetest-traveler-profile/client-state/krazyglue/traveler-profile.us-west-2c.test.expedia.com/krazyglue/mip/flink/zookeeperCheckpoints/submittedJobGraph6507f982fe76x"/>
    </zknode>
    <zknode name="leader">
      <zknode name="00000000000000000000000000000000"/>
    </zknode>
  </zknode>
</root>

In a healthy cluster there is generally a node under “job_manager_lock” which represents
the elected leader. This pointed to a problem with the Apache Curator framework. I opened
up both Flink 1.2. And 1.3.2 to see what version of Curator Flink 1.2.1 and 1.3.2 used. I
noticed that Flink 1.3.2 uses Curator 2.12.0 whereas Flink 1.2.1 uses 2.8.0. Looking at the
release notes for curator there were quite a few bugs might be the cause of this (https://issues.apache.org/jira/browse/CURATOR-264
for example). I was curious to see if leader election would become unstuck if the cluster
was upgraded to Flink 1.3.2. I upgraded the JobManager to 1.3.2 and sure enough leader election
proceeded correctly. Upon Upgrading the TaskManagers the job restored from its last checkpoint
correctly and continued.

Other interesting things of note:

  1.  Restarting/Killing the JobManager didn’t resolve the issue. I suspect this is due
to bad state in zookeeper
  2.  Zookeeper was running throughout this time. We didn’t see issues in our kafka cluster
and Zookeeper has a leader elected correctly and the Zookeeper command line utility worked
without issue.
  3.  Restarts of the JobManager appeared to correctly create ephemeral nodes in zookeeper
to represent possible leaders (which also appear to get cleaned up correctly).

As far as I can tell this issue is resolved with the newer Apache Curator version in 1.3.2.
It may be useful to keep an eye out for this issue however.

Thanks,
James Bucher

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