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  1. Solr
  2. SOLR-14347

Autoscaling placement wrong when concurrent replica placements are calculated

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Details

    • Bug
    • Status: Closed
    • Critical
    • Resolution: Fixed
    • 8.5
    • 8.6
    • AutoScaling
    • None

    Description

      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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            ab Andrzej Bialecki
            ab Andrzej Bialecki
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