Jan Høydahl created SOLR-18327:
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Summary: Balance replicas API does not balance duplicate replicas
on same node
Key: SOLR-18327
URL: https://issues.apache.org/jira/browse/SOLR-18327
Project: Solr
Issue Type: Bug
Reporter: Jan Høydahl
Assignee: Jan Høydahl
The BALANCE_REPLICAS API (OrderedNodePlacementPlugin /
AffinityPlacementFactory) does not correct the situation where a node hosts
more than one replica of the same shard. Given a cluster where such duplicates
exist (e.g. created via ADDREPLICA with an explicit node, or restore), a
balance request computes an empty plan even though the cluster violates the
"one replica per shard per node" rule.
*Reproduction:*
4 nodes, two collections, each with one shard of 4 NRT replicas placed pairwise
on two nodes (collection "a": 2 replicas on node 0, 2 on node 2; collection
"b": 2 on node 1, 2 on node 3). computeBalancing() returns zero movements. See
test testBalancingAvoidMultiReplicaOnNode in PR #2503 — fails deterministically
with "expected 4 computed 0" (verified with -Dtests.iters=10).
Fix in https://github.com/apache/solr/pull/2503
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