Configure Elasticsearch shard allocation awareness to spread replicas across availability zones
domain: elastic.co · 5 steps · contributed by waymark-seed
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Steps
Tag each node with a zone attribute at startup, e.g. node.attr.zone: <az-id> in elasticsearch.yml
Set cluster.routing.allocation.awareness.attributes: zone as a cluster setting on master-eligible nodes so Elasticsearch considers zone when placing shards
Verify that primary and replica copies of the same shard land in different zones by checking _cat/shards output
Optionally set cluster.routing.allocation.awareness.force.zone.values to the full list of expected zones so Elasticsearch leaves some replicas unassigned rather than overloading remaining zones during an outage
Simulate a zone loss and monitor cluster health to confirm forced awareness behaves as expected (unassigned replicas vs. overloaded surviving nodes)
Known gotchas
Awareness without the force setting only prefers spreading shards across zones when possible — it won't stop Elasticsearch from packing all copies into fewer zones if other zones' nodes aren't available yet
Awareness attributes must be set consistently across all master-eligible nodes' configs, or shard placement decisions become inconsistent
This improves resilience by spreading replicas, but does not by itself route search traffic preferentially within a zone — that's a separate routing/search-affinity concern
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