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https://issues.apache.org/jira/browse/HBASE-3914?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13146330#comment-13146330
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stack commented on HBASE-3914:
------------------------------
Can we do this discussion in a new issue Mingjie? Mind opening one?
Is the above log from a 0.90 or a 0.92/trunk? Thanks.
> ROOT region appeared in two regionserver's onlineRegions at the same time
> -------------------------------------------------------------------------
>
> Key: HBASE-3914
> URL: https://issues.apache.org/jira/browse/HBASE-3914
> Project: HBase
> Issue Type: Bug
> Components: master
> Affects Versions: 0.90.3
> Reporter: Jieshan Bean
> Assignee: Jieshan Bean
> Fix For: 0.90.4
>
> Attachments: HBASE-3914-V2.patch, HBASE-3914.patch
>
>
> This could be happen under the following steps with little probability:
> (I suppose the cluster nodes names are RS1/RS2/HM, and there's more than
> 10,000 regions in the cluster)
> 1.Root region was opened in RS1.
> 2.Due to some reason(Maybe the hdfs process was got abnormal),RS1 aborted.
> 3.ServerShutdownHandler process start.
> 4.HMaster was restarted, during the finishInitialization's handling, ROOT
> region was unsetted, and assigned to RS2.
> 5.Root region was opened successfully in RS2.
> 6.But after while, ROOT region was unsetted again by RS1's
> ServerShutdownHandler. Then it was reassigned. Before that, the RS1 was
> restarted. So there's two possibilities:
> Case a:
> ROOT region was assigned to RS1.
> It seemed nothing would be affected. But the root region was still online
> in RS2.
>
> Case b:
> ROOT region was assigned to RS2.
> The ROOT Region couldn't be opened until it would be reassigned to other
> regionserver, because it was showed online in this regionserver.
> This could be proved from the logs:
> 1. ROOT region was opened with two times:
> 2011-05-17 10:32:59,188 DEBUG
> org.apache.hadoop.hbase.master.handler.OpenedRegionHandler: Opened region
> -ROOT-,,0.70236052 on 162-2-77-0,20020,1305598359031
> 2011-05-17 10:33:01,536 DEBUG
> org.apache.hadoop.hbase.master.handler.OpenedRegionHandler: Opened region
> -ROOT-,,0.70236052 on 162-2-16-6,20020,1305597548212
> 2.Regionserver 162-2-16-6 was aborted, so it was reassigned to 162-2-77-0,
> but already online on this server:
> 10:49:30,920 INFO org.apache.hadoop.hbase.regionserver.HRegionServer:
> Received request to open region: -ROOT-,,0.70236052 10:49:30,920 DEBUG
> org.apache.hadoop.hbase.regionserver.handler.OpenRegionHandler: Processing
> open of -ROOT-,,0.70236052 10:49:30,920 WARN
> org.apache.hadoop.hbase.regionserver.handler.OpenRegionHandler: Attempted
> open of -ROOT-,,0.70236052 but already online on this server
> This could be cause a long break of ROOT region offline, though it happened
> under a special scenario. And I have checked the code, it seems a tiny bug
> here.
> There's 2 references about assignRoot():
> 1.
> HMaster# assignRootAndMeta:
> if (!catalogTracker.verifyRootRegionLocation(timeout)) {
> this.assignmentManager.assignRoot();
> this.catalogTracker.waitForRoot();
> assigned++;
> }
> 2.
> ServerShutdownHandler# process:
>
> if (isCarryingRoot()) { // -ROOT-
> try {
> this.services.getAssignmentManager().assignRoot();
> } catch (KeeperException e) {
> this.server.abort("In server shutdown processing, assigning root",
> e);
> throw new IOException("Aborting", e);
> }
> }
> I think each time call the method of assignRoot(), we should verify Root
> Region's Location first. Because before the assigning, the ROOT region could
> have been assigned by another place.
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