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. Soft deletes are enabled by default on new indices created on or after Elasticsearch 7.0.0.

This means that cross-cluster replication can not be used on existing indices. If you have existing data that you want to replicate from another cluster, you must reindex your data into a new index with soft deletes enabled.

Soft delete settings

index.soft_deletes.enabled
Whether or not soft deletes are enabled on the index. Soft deletes can only be configured at index creation and only on indices created on or after 6.5.0. The default value is true.
index.soft_deletes.retention_lease.period
The maximum period to retain a shard history retention lease before it is considered expired. Shard history retention leases ensure that soft deletes are retained during merges on the Lucene index. If a soft delete is merged away before it can be replicated to a follower the following process will fail due to incomplete history on the leader. The default value is 12h.

For more information about index settings, see Index modules.