Shards

Elasticsearch divides each index into shards, which are standalone Lucene indices. These can be either primary shards or replicas. 

The number of shards and replicas per shard is set when a new index is created. After that, Elasticsearch takes care of evenly distributing the shards across the cluster. For maximum availability and performance, it ensures that replica shards are not on the same node as their correspondent primary shard.