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From "Andrzej Bialecki (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SOLR-12729) SplitShardCmd should lock the parent shard to prevent parallel splitting requests
Date Mon, 03 Sep 2018 12:08:00 GMT

    [ https://issues.apache.org/jira/browse/SOLR-12729?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16602080#comment-16602080
] 

Andrzej Bialecki  commented on SOLR-12729:
------------------------------------------

Shalin suggested that we could use an ephemeral znode to signal that a shard is in the process
of being split. This way the lock would be automatically released on Overseer crash.

We should also check for stale locks, if eg. a sub-shard is permanently stuck in RECOVERY.
This can be implemented in {{InactiveShardPlanAction}}, which is periodically invoked from
the maintenance trigger.

> SplitShardCmd should lock the parent shard to prevent parallel splitting requests
> ---------------------------------------------------------------------------------
>
>                 Key: SOLR-12729
>                 URL: https://issues.apache.org/jira/browse/SOLR-12729
>             Project: Solr
>          Issue Type: Bug
>      Security Level: Public(Default Security Level. Issues are Public) 
>          Components: AutoScaling
>            Reporter: Andrzej Bialecki 
>            Assignee: Andrzej Bialecki 
>            Priority: Major
>             Fix For: 7.5
>
>
> This scenario was discovered by the simulation framework, but it exists also in the non-simulated
code.
> When {{IndexSizeTrigger}} requests SPLITSHARD, which is then successfully started and
“completed” from the point of view of {{ExecutePlanAction}}, the reality is that it still
can take significant amount of time until the moment when the new replicas fully recover and
cause the switch of shard states (parent to INACTIVE, child from RECOVERY to ACTIVE).
> If this time is longer than the trigger's {{waitFor}} the trigger will issue the same
SPLITSHARD request again. {{SplitShardCmd}} doesn't prevent this new request from being processed
because the parent shard is still ACTIVE. However, a section of the code in {{SplitShardCmd}}
will realize that sub-slices with the target names already exist and they are not active,
at which point it will delete the new sub-slices ({{SplitShardCmd:182}}).
> The end result is an infinite loop, where {{IndexSizeTrigger}} will keep generating SPLITSHARD,
and {{SplitShardCmd}} will keep deleting the recovering sub-slices created by the previous
command.
> A simple solution is for the parent shard to be marked to indicate that it’s in a process
of splitting, so that no other split is attempted on the same shard. Furthermore, {{IndexSizeTrigger}}
could temporarily exclude such shards from monitoring.



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