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I have read that when a new indexing request is sent to ES cluster. ES will specify which shard should that document be stored in depending on routing. Then that node which hosts that primary shard (aka coordinating node) will broadcast the indexing request to each node containing a replica for that shard and it will respond to the client that the document has been indexed successfully if the primary shard and it's replicas stored/indexed that document.

Does that mean that ES supports high availability(node tolerant) for reading requests and not for writing request or it's the default behavior and can be changed?

Simple Code
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1 Answers1

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The main purpose of replicas is for failover, if the node holding a primary shard dies, a replica is promoted to the role of primary. Also, replica shards can serve read requests thus improving search performance.

For write requests though, indexing will be affected if one of your nodes (which has the primary shard for a live index) in the cluster suddenly runs out of disk space because if a node disk usage hits configured watermark levels then ES throws a cluster block exception preventing any writes to the node. If ALL nodes are down/ unreachable indexing will stop however if only one or some nodes go down, indexing shouldn't completely stop as replica shards on other nodes are promoted to primary if the node holding the original primary is offline. Ideally, it goes without saying that some analysis and effort should go to right size an ES cluster and have monitoring in place to prevent any issues.

ben5556
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