[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.

[float]
[[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].

[float]
[[ccr-overview-beats]]
==== Setting soft deletes on indices created by APM Server or Beats

If you want to replicate indices created by APM Server or Beats, and are
allowing APM Server or Beats to manage index templates, you need to configure
soft deletes on the underlying index templates. To configure soft deletes on the
underlying index templates, incorporate the following changes to the relevant
APM Server or Beats configuration file.

["source","yaml"]
----------------------------------------------------------------------
setup.template.overwrite: true
setup.template.settings:
  index.soft_deletes.retention.operations: 1024
----------------------------------------------------------------------

For additional information on controlling the index templates managed by APM
Server or Beats, see the relevant documentation on loading the Elasticsearch
index template.

[float]
[[ccr-overview-logstash]]
==== Setting soft deletes on indices created by Logstash

If you want to replicate indices created by Logstash, and are using Logstash to
manage index templates, you need to configure soft deletes on a custom Logstash
index template. To configure soft deletes on the underlying index template,
incorporate the following change to a custom Logstash template.

["source","js"]
----------------------------------------------------------------------
{
  "settings" : {
    "index.soft_deletes.retention.operations" : 1024
  }
}
----------------------------------------------------------------------
// NOTCONSOLE

Additionally, you will need to configure the Elasticsearch output plugin to use
this custom template.

["source","ruby"]
----------------------------------------------------------------------
output {
  elasticsearch {
    template => "/path/to/custom/logstash/template.json"
  }
}
----------------------------------------------------------------------

For additional information on controlling the index templates managed by
Logstash, see the relevant documentation on the Elasticsearch output plugin.