[jira] [Resolved] (HBASE-8281) Unassigned regions: dropped messages from Master to RS

["Andrew Kyle Purtell (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HBASE-8281?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Andrew Kyle Purtell resolved HBASE-8281.
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    Resolution: Incomplete

> Unassigned regions: dropped messages from Master to RS
> ------------------------------------------------------
>
>                 Key: HBASE-8281
>                 URL: https://issues.apache.org/jira/browse/HBASE-8281
>             Project: HBase
>          Issue Type: Bug
>    Affects Versions: 0.89-fb
>            Reporter: Amitanand Aiyer
>            Priority: Major
>
> We have seen a couple of scenarios where transcient network issue between the RS and Master results in regions being unassigned (and staying unassigned) until someone intervenes manually with hbck -fix.
> The events occur as follows. 
> RS checks in for a regionServerReport.
>   Master wants to assign a region to the RS. Hence adds a MSG_REGION_OPEN msg to the return results, and marks the region as PENDING_OPEN.
>   The messages from the master to the RS is not delivered due to network error. Master does not do anything to revert the state changes.
> Network heals, and the RS is able to do regionServerReports in future; it is in good standing with the master. But, RS does not know that it has to open the region. Master thinks that the RS is going to open the region.
> Region remains unassigned until we intervene with hbck.
> Possible fix:
>   I think it may be a mistake to unilaterally change the RegionState to pendingOpen once the master decides that it wants to send the message. Perhaps, we should create an intermediate state, where the master will keep sending the OPEN message to the RS until it acks. And, update the RegionState to PendingOpen only after the RS has acked.
> While this would fix the particular scenario in which the unassigned regions were caused. We might want to update all the Master-RS communication (even region closes?)to expect message failures, and wait for an ack before it updates the state in master.



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