[role="xpack"]
[testenv="platinum"]
[[ccr-requirements]]
=== Requirements for leader indices

{ccr-cap} 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 {es} 7.0.0.

IMPORTANT: This means that {ccr} can not be used on existing indices. If you have
existing data that you want to replicate from another cluster, you must
{ref}/docs-reindex.html[reindex] your data into a new index with soft deletes 
enabled.


[[ccr-overview-soft-deletes]]
==== 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 {ref}/index-modules.html[Index modules].