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https://issues.apache.org/jira/browse/SOLR-14347?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17062007#comment-17062007
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David Smiley commented on SOLR-14347:
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I wonder if we should not take this approach and instead introduce a separate
mechanism that is purely used for replica placement/balancing? Such a
mechanism might not be a lock; maybe it'd be some data structure that
represents the target cluster shape we want to get to. If it were held in
ZooKeeper and atomically updated, we wouldn't even need the Overseer to be the
single point of management of it. I'm just spitballing here.
> Autoscaling placement wrong due to incorrect LockLevel for collection
> modifications
> -----------------------------------------------------------------------------------
>
> Key: SOLR-14347
> URL: https://issues.apache.org/jira/browse/SOLR-14347
> Project: Solr
> Issue Type: Bug
> Security Level: Public(Default Security Level. Issues are Public)
> Components: AutoScaling
> Affects Versions: 8.5
> Reporter: Andrzej Bialecki
> Assignee: Andrzej Bialecki
> Priority: Major
> Attachments: SOLR-14347.patch
>
>
> Steps to reproduce:
> * create a cluster of a few nodes (tested with 7 nodes)
> * define per-collection policies that distribute replicas exclusively on
> different nodes per policy
> * concurrently create a few collections, each using a different policy
> * resulting replica placement will be seriously wrong, causing many policy
> violations
> Running the same scenario but instead creating collections sequentially
> results in no violations.
> I suspect this is caused by incorrect locking level for all collection
> operations (as defined in {{CollectionParams.CollectionAction}}) that create
> new replica placements - i.e. CREATE, ADDREPLICA, MOVEREPLICA, DELETENODE,
> REPLACENODE, SPLITSHARD, RESTORE, REINDEXCOLLECTION. All of these operations
> use the policy engine to create new replica placements, and as a result they
> change the cluster state. However, currently these operations are locked (in
> {{OverseerCollectionMessageHandler.lockTask}} ) using
> {{LockLevel.COLLECTION}}. In practice this means that the lock is held only
> for the particular collection that is being modified.
> A straightforward fix for this issue is to change the locking level to
> CLUSTER (and I confirm this fixes the scenario described above). However,
> this effectively serializes all collection operations listed above, which
> will result in general slow-down of all collection operations.
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