[RW Separation] Assigned Replica Should Be Unassigned When Node Becomes Search dedicated and cluster lacks Sufficient Nodes

[vinaykpud]

Describe the bug

When a node is designated as a search-dedicated node via cluster settings, any non-search replica shards assigned to that node should either be unassigned or moved to other available nodes in the cluster. However, currently, the replicas remain assigned to the search-dedicated node.

Related component

Search:Performance

To Reproduce

1. Create a cluster with two nodes: node-0 and node-1.
2. Create an index with 1 primary (P) and 1 replica (R). The shards will be assigned as follows:
   • P on node-0
   • R on node-1
3. Make node-1 a search-dedicated node by running:

   curl -X PUT "http://localhost:9200/_cluster/settings" \
   -H "Content-Type: application/json" \
   -d '{
     "persistent": {
       "cluster.routing.allocation.search.replica.dedicated.include._name": "node-1"
     }
   }'

4. Observe that the replica shard remains assigned to node-1 instead of being unassigned

Expected behavior

• When a node is designated as a search-dedicated node, any non-search replica shards on that node should:
  • Be unassigned or
  • Be moved to an eligible node in the cluster.

Additional Details

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• Version [e.g. 22]

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