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https://issues.apache.org/jira/browse/SOLR-6095?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14014535#comment-14014535
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Shalin Shekhar Mangar commented on SOLR-6095:
---------------------------------------------
Except we don't do our rolling restarts like that. Our restart scripts iterates
through hosts looked up using EC2 APIs (and it almost always returns the node
names in the same order) and restarting them one by one and after each restart,
waits for 60 seconds, verifies that node is up again and continues with the
next host.
Since the script originally created the nodes too in the same order, the
election nodes are also approximately in the same order. This causes each host
restart to displace the overseer to the next host in line which is again
displaced and so on.
> 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
>
>
> 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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