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https://issues.apache.org/jira/browse/SOLR-14347?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17064940#comment-17064940
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Andrzej Bialecki commented on SOLR-14347:
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It turns out that the bug was caused by the fact that per-collection policies
are applied during calculations and cause side-effects that later affect
calculations for other collections.
Setting the LockLevel.CLUSTER fixed this because all computations became
sequential, but at a relatively high cost of blocking all other CLUSTER level
operations. It appears that re-creating a {{Policy.Session}} in
{{PolicyHelper.getReplicaLocations(...)}} fixes this behavior too, because the
new Session doesn't carry over the side-effects from previous per-collection
policies. There is a slight performance impact of this approach, because
re-creating a Session is costly for large clusters, but it's less intrusive
than locking out all other CLUSTER level ops.
We may re-visit this issue at some point to reduce this cost, but I think this
fix at least protects us from the current completely wrong behavior.
> Autoscaling placement wrong when concurrent replica placements are calculated
> -----------------------------------------------------------------------------
>
> 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
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> 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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