Replicas
By default, Elasticsearch creates five primary shards and a copy of each primary shard when you create an index. These copies are called replicas. So, the replica shard is simply a copy of a primary shard. Replica shards are used to improve the search performance and failover. If a node crashes in a way, Elasticsearch uses one of the available replica shards of the node to avoid any data loss. For this reason, a replica of a primary shard will not be allocated in the same node with the primary shard. Hence, choosing the right amount of shards and replicas is very important. Unlike primary shards, replicas can be added and removed at any time. The number of primary shards must be specified before creating an index.
