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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-6060) Regions's in OPENING state from failed regionservers takes a long time to recover
Date Thu, 07 Jun 2012 23:33:23 GMT

    [ https://issues.apache.org/jira/browse/HBASE-6060?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13291416#comment-13291416
] 

stack commented on HBASE-6060:
------------------------------

I think I understand this patch now.

In AM, we introduce a new Map, a Map of dead servers to their current set of region plans
and regions-in-transition.

This Map is populated in AM when SSH calls AM to say process a server that just died (processServerShutdown).

If we try to assign a region in another thread at this time, a check by AM into this new Map
will cause us to return a 'DON'T DO ANYTHING' flag because its being done elsewhere (in SSH
eventually -- and we return a flag rather than null because null triggers a different kind
of special handling).

If SSH is running, it is what is meant to prevail assigning regions rather than any balance
or user assignment that may have been made concurrently.

When it finishes, SSH clears the dead server from the AM Map.

Does this sound right?

I have some feedback on details of this patch but I would first like to get high-level comments
out of the way.

1. Is this patch even handling the root cause of this JIRA: i.e. dealing w/ OPENING znodes
that were made against the dead server?  In AM#processServerShutdown, we iterate the list
of regions we get back from AM#this.servers and from this set we will remove RIT.  But won't
this original set of regions be missing regions that are OPENING or even OPENED but not yet
handled because its only after the OPENED region has updated its znode and the znode has been
removed by the OPEN handler do we add a region to AM#this.servers?  Or is the OPENING region
handled elsewhere?

2. This patch distributes state and updating of the state across AM and SSH.  It makes it
tough to follow what is going on (least, it makes it tough for lesser mortals like myself
to figure it).  The new Map is populated as a side-effect of the call to AM#processServerShutdown.
 The dead server is cleared explicitly from the AM Map on the tail of SSH on its way out (Reading
SSH only, you'd be clueless why this was being done).  During the processing of SSH, we are
updating a data structure that is held by this Map over in AM.  Is there anything we can do
to better encapsulate this new state management?  What is we changed the method in AM from
processServerShutdown to startProcessingServerShutdown and closed it off with a finishProcessingServerShutdown?
 Do we have to keep around a list of regionplans?  Can't we just keep list of dead servers
and any attempt at getting a region plan related to the dead server or server being processed
as dead is put off because SSH will get t
 o it?

Here are some minor items while they are on my mind:

1. On REGION_PLAN_ALREADY_INUSE, why not return null and if we get a null back from getRegionPlan
in assign, check  serverManager.createDestinationServersList(serverToExclude) is empty; if
it is, update timeoutmonitor.  This'd be cleaner than adding this override of RegionState
data structure to do more than just carry a plan?
2. On the tail of SSH, we do this.services.getAssignmentManager().removeDeadServerRegions(this.serverName);
 Just before that we clear regionsFromRegionPlansForServer and regionsInTransition.  The clearings
are unnecessary?

Good stuff Ram and Rajeshbabu.  I think this patch can work after a little cleanup.  I'll
have some more feedback this evening.  Thought I'd send this in meantime so you have something
to chew on when you get up this morning.

                
> Regions's in OPENING state from failed regionservers takes a long time to recover
> ---------------------------------------------------------------------------------
>
>                 Key: HBASE-6060
>                 URL: https://issues.apache.org/jira/browse/HBASE-6060
>             Project: HBase
>          Issue Type: Bug
>          Components: master, regionserver
>            Reporter: Enis Soztutar
>            Assignee: rajeshbabu
>             Fix For: 0.96.0, 0.94.1, 0.92.3
>
>         Attachments: 6060-94-v3.patch, 6060-94-v4.patch, 6060-94-v4_1.patch, 6060-94-v4_1.patch,
6060-trunk.patch, 6060-trunk.patch, 6060-trunk_2.patch, 6060-trunk_3.patch, HBASE-6060-92.patch,
HBASE-6060-94.patch
>
>
> we have seen a pattern in tests, that the regions are stuck in OPENING state for a very
long time when the region server who is opening the region fails. My understanding of the
process: 
>  
>  - master calls rs to open the region. If rs is offline, a new plan is generated (a new
rs is chosen). RegionState is set to PENDING_OPEN (only in master memory, zk still shows OFFLINE).
See HRegionServer.openRegion(), HMaster.assign()
>  - RegionServer, starts opening a region, changes the state in znode. But that znode
is not ephemeral. (see ZkAssign)
>  - Rs transitions zk node from OFFLINE to OPENING. See OpenRegionHandler.process()
>  - rs then opens the region, and changes znode from OPENING to OPENED
>  - when rs is killed between OPENING and OPENED states, then zk shows OPENING state,
and the master just waits for rs to change the region state, but since rs is down, that wont
happen. 
>  - There is a AssignmentManager.TimeoutMonitor, which does exactly guard against these
kind of conditions. It periodically checks (every 10 sec by default) the regions in transition
to see whether they timedout (hbase.master.assignment.timeoutmonitor.timeout). Default timeout
is 30 min, which explains what you and I are seeing. 
>  - ServerShutdownHandler in Master does not reassign regions in OPENING state, although
it handles other states. 
> Lowering that threshold from the configuration is one option, but still I think we can
do better. 
> Will investigate more. 

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