Re: ROOT region appeared in two regionserver's onlineRegions at the same time

[bijieshan]

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.
Expecting for anyone's reply.

Thanks!

Regards,
Jieshan Bean


-----邮件原件-----
发件人: bijieshan [mailto:bijies...@huawei.com] 
发送时间: 2011年5月20日 15:34
收件人: user@hbase.apache.org
抄送: Chenjian
主题: ROOT region appeared in two regionserver's onlineRegions at the same time

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.

Thanks!

Regards,
Jieshan Bean