When you create a follower index, you cannot use it until it is fully initialized. The remote recovery process builds a new copy of a shard on a follower node by copying data from the primary shard in the leader cluster. Elasticsearch uses this remote recovery process to bootstrap a follower index using the data from the leader index. This process provides the follower with a copy of the current state of the leader index, even if a complete history of changes is not available on the leader due to Lucene segment merging.

Remote recovery is a network intensive process that transfers all of the Lucene segment files from the leader cluster to the follower cluster. The follower requests that a recovery session be initiated on the primary shard in the leader cluster. The follower then requests file chunks concurrently from the leader. By default, the process concurrently requests 5 large 1mb file chunks. This default behavior is designed to support leader and follower clusters with high network latency between them.

There are dynamic settings that you can use to rate-limit the transmitted data and manage the resources consumed by remote recoveries. See Cross-cluster replication settings.

You can obtain information about an in-progress remote recovery by using the recovery API on the follower cluster. Remote recoveries are implemented using the snapshot and restore infrastructure. This means that on-going remote recoveries are labelled as type snapshot in the recovery API.