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https://issues.apache.org/jira/browse/SOLR-6095?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14032868#comment-14032868
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Jessica Cheng commented on SOLR-6095:
-------------------------------------
The problem is I don't think getChildren returns nodes in the same order. Its
javadoc states
{quote}
The list of children returned is not sorted and no guarantee is provided as to
its natural or lexical order.
{quote}
If somehow getChildren doesn't return nodes in the same order (unless we can
verify otherwise, and add this as a regression test against each zk upgrade
since the API doesn't guarantee it), the sort can possibly get different
ordering of nodeB and nodeD so that they both believe they're the top item in
their own invocation, and we're back to the temporary two-Overseer case (for
one loop iteration).
> SolrCloud cluster can end up without an overseer
> ------------------------------------------------
>
> Key: SOLR-6095
> URL: https://issues.apache.org/jira/browse/SOLR-6095
> Project: Solr
> Issue Type: Bug
> Components: SolrCloud
> Affects Versions: 4.8
> Reporter: Shalin Shekhar Mangar
> Assignee: Noble Paul
> Fix For: 4.9, 5.0
>
> Attachments: SOLR-6095.patch, SOLR-6095.patch, SOLR-6095.patch
>
>
> We have a large cluster running on ec2 which occasionally ends up without an
> overseer after a rolling restart. We always restart our overseer nodes at the
> very last otherwise we end up with a large number of shards that can't
> recover properly.
> This cluster is running a custom branch forked from 4.8 and has SOLR-5473,
> SOLR-5495 and SOLR-5468 applied. We have a large number of small collections
> (120 collections each with approx 5M docs) on 16 Solr nodes. We are also
> using the overseer roles feature to designate two specified nodes as
> overseers. However, I think the problem that we're seeing is not specific to
> the overseer roles feature.
> As soon as the overseer was shutdown, we saw the following on the node which
> was next in line to become the overseer:
> {code}
> 2014-05-20 09:55:39,261 [main-EventThread] INFO solr.cloud.ElectionContext
> - I am going to be the leader ec2-xxxxxxxxxx.compute-1.amazonaws.com:8987_solr
> 2014-05-20 09:55:39,265 [main-EventThread] WARN solr.cloud.LeaderElector -
> org.apache.zookeeper.KeeperException$NodeExistsException: KeeperErrorCode =
> NodeExists for /overseer_elect/leader
> at org.apache.zookeeper.KeeperException.create(KeeperException.java:119)
> at org.apache.zookeeper.KeeperException.create(KeeperException.java:51)
> at org.apache.zookeeper.ZooKeeper.create(ZooKeeper.java:783)
> at
> org.apache.solr.common.cloud.SolrZkClient$10.execute(SolrZkClient.java:432)
> at
> org.apache.solr.common.cloud.ZkCmdExecutor.retryOperation(ZkCmdExecutor.java:73)
> at
> org.apache.solr.common.cloud.SolrZkClient.makePath(SolrZkClient.java:429)
> at
> org.apache.solr.common.cloud.SolrZkClient.makePath(SolrZkClient.java:386)
> at
> org.apache.solr.common.cloud.SolrZkClient.makePath(SolrZkClient.java:373)
> at
> org.apache.solr.cloud.OverseerElectionContext.runLeaderProcess(ElectionContext.java:551)
> at
> org.apache.solr.cloud.LeaderElector.runIamLeaderProcess(LeaderElector.java:142)
> at
> org.apache.solr.cloud.LeaderElector.checkIfIamLeader(LeaderElector.java:110)
> at org.apache.solr.cloud.LeaderElector.access$200(LeaderElector.java:55)
> at
> org.apache.solr.cloud.LeaderElector$ElectionWatcher.process(LeaderElector.java:303)
> at
> org.apache.zookeeper.ClientCnxn$EventThread.processEvent(ClientCnxn.java:522)
> at org.apache.zookeeper.ClientCnxn$EventThread.run(ClientCnxn.java:498)
> {code}
> When the overseer leader node is gracefully shutdown, we get the following in
> the logs:
> {code}
> 2014-05-20 09:55:39,254 [Thread-63] ERROR solr.cloud.Overseer - Exception in
> Overseer main queue loop
> org.apache.solr.common.SolrException: Could not load collection from ZK:sm12
> at
> org.apache.solr.common.cloud.ZkStateReader.getExternCollectionFresh(ZkStateReader.java:778)
> at
> org.apache.solr.common.cloud.ZkStateReader.updateClusterState(ZkStateReader.java:553)
> at
> org.apache.solr.common.cloud.ZkStateReader.updateClusterState(ZkStateReader.java:246)
> at
> org.apache.solr.cloud.Overseer$ClusterStateUpdater.run(Overseer.java:237)
> at java.lang.Thread.run(Thread.java:745)
> Caused by: java.lang.InterruptedException
> at java.lang.Object.wait(Native Method)
> at java.lang.Object.wait(Object.java:503)
> at org.apache.zookeeper.ClientCnxn.submitRequest(ClientCnxn.java:1342)
> at org.apache.zookeeper.ZooKeeper.exists(ZooKeeper.java:1040)
> at
> org.apache.solr.common.cloud.SolrZkClient$4.execute(SolrZkClient.java:226)
> at
> org.apache.solr.common.cloud.SolrZkClient$4.execute(SolrZkClient.java:223)
> at
> org.apache.solr.common.cloud.ZkCmdExecutor.retryOperation(ZkCmdExecutor.java:73)
> at
> org.apache.solr.common.cloud.SolrZkClient.exists(SolrZkClient.java:223)
> at
> org.apache.solr.common.cloud.ZkStateReader.getExternCollectionFresh(ZkStateReader.java:767)
> ... 4 more
> 2014-05-20 09:55:39,254 [Thread-63] INFO solr.cloud.Overseer - Overseer
> Loop exiting : ec2-xxxxxxxxxx.compute-1.amazonaws.com:8986_solr
> 2014-05-20 09:55:39,256 [main-EventThread] WARN common.cloud.ZkStateReader
> - ZooKeeper watch triggered, but Solr cannot talk to ZK
> 2014-05-20 09:55:39,259 [ShutdownMonitor] INFO server.handler.ContextHandler
> - stopped
> o.e.j.w.WebAppContext{/solr,file:/vol0/cloud86/solr-webapp/webapp/},/vol0/cloud86/webapps/solr.war
> {code}
> Notice how the overseer kept on running almost till the last point i.e. until
> the jetty context stopped. On some runs, we got the following on the overseer
> leader node on graceful shutdown:
> {code}
> 2014-05-19 21:33:43,657 [Thread-75] ERROR solr.cloud.Overseer - Exception in
> Overseer main queue loop
> org.apache.solr.common.SolrException: Could not load collection from ZK:sm71
> at
> org.apache.solr.common.cloud.ZkStateReader.getExternCollectionFresh(ZkStateReader.java:778)
> at
> org.apache.solr.common.cloud.ZkStateReader.updateClusterState(ZkStateReader.java:553)
> at
> org.apache.solr.common.cloud.ZkStateReader.updateClusterState(ZkStateReader.java:246)
> at
> org.apache.solr.cloud.Overseer$ClusterStateUpdater.run(Overseer.java:237)
> at java.lang.Thread.run(Thread.java:745)
> Caused by: java.lang.InterruptedException
> at java.lang.Object.wait(Native Method)
> at java.lang.Object.wait(Object.java:503)
> at org.apache.zookeeper.ClientCnxn.submitRequest(ClientCnxn.java:1342)
> at org.apache.zookeeper.ZooKeeper.getData(ZooKeeper.java:1153)
> at
> org.apache.solr.common.cloud.SolrZkClient$7.execute(SolrZkClient.java:277)
> at
> org.apache.solr.common.cloud.SolrZkClient$7.execute(SolrZkClient.java:274)
> at
> org.apache.solr.common.cloud.ZkCmdExecutor.retryOperation(ZkCmdExecutor.java:73)
> at
> org.apache.solr.common.cloud.SolrZkClient.getData(SolrZkClient.java:274)
> at
> org.apache.solr.common.cloud.ZkStateReader.getExternCollectionFresh(ZkStateReader.java:769)
> ... 4 more
> 2014-05-19 21:33:43,662 [main-EventThread] WARN common.cloud.ZkStateReader
> - ZooKeeper watch triggered, but Solr cannot talk to ZK
> 2014-05-19 21:33:43,663 [Thread-75] INFO solr.cloud.Overseer - Overseer
> Loop exiting : ec2-xxxxxxxxxxxx.compute-1.amazonaws.com:8987_solr
> 2014-05-19 21:33:43,664 [OverseerExitThread] ERROR solr.cloud.Overseer -
> could not read the data
> org.apache.zookeeper.KeeperException$SessionExpiredException: KeeperErrorCode
> = Session expired for /overseer_elect/leader
> at org.apache.zookeeper.KeeperException.create(KeeperException.java:127)
> at org.apache.zookeeper.KeeperException.create(KeeperException.java:51)
> at org.apache.zookeeper.ZooKeeper.getData(ZooKeeper.java:1155)
> at
> org.apache.solr.common.cloud.SolrZkClient$7.execute(SolrZkClient.java:277)
> at
> org.apache.solr.common.cloud.SolrZkClient$7.execute(SolrZkClient.java:274)
> at
> org.apache.solr.common.cloud.ZkCmdExecutor.retryOperation(ZkCmdExecutor.java:73)
> at
> org.apache.solr.common.cloud.SolrZkClient.getData(SolrZkClient.java:274)
> at
> org.apache.solr.cloud.Overseer$ClusterStateUpdater.checkIfIamStillLeader(Overseer.java:329)
> at
> org.apache.solr.cloud.Overseer$ClusterStateUpdater.access$300(Overseer.java:85)
> at
> org.apache.solr.cloud.Overseer$ClusterStateUpdater$1.run(Overseer.java:293)
> 2014-05-19 21:33:43,665 [ShutdownMonitor] INFO server.handler.ContextHandler
> - stopped
> o.e.j.w.WebAppContext{/solr,file:/vol0/cloud87/solr-webapp/webapp/},/vol0/cloud87/webapps/solr.war
> {code}
> Again the overseer was clinging on till the last moment and by the time it
> exited the ZK session expired and it couldn't delete the
> /overseer_elect/leader node. The exception on the next-in-line node was the
> same i.e. NodeExists for /overseer_elect/leader.
> In both cases, we are left with no overseers after restart. I can easily
> reproduce this problem by just restarting overseer leader nodes repeatedly.
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