Requirements for leader indices

Cross-cluster replication works by replaying the history of individual write operations that were performed on the shards of the leader index. This means that the history of these operations needs to be retained on the leader shards so that they can be pulled by the follower shard tasks. The underlying mechanism used to retain these operations is soft deletes. A soft delete occurs whenever an existing document is deleted or updated. By retaining these soft deletes up to configurable limits, the history of operations can be retained on the leader shards and made available to the follower shard tasks as it replays the history of operations.

Soft deletes must be enabled for indices that you want to use as leader indices. Enabling soft deletes requires the addition of some index settings at index creation time. You must add these settings to your create index requests or to the index templates that you use to manage the creation of new indices.