[role="xpack"]
[testenv="basic"]
[[using-policies-rollover]]
== Using policies to manage index rollover

The rollover action enables you to automatically roll over to a new index based
on the index size, document count, or age. When a rollover is triggered, a new
index is created, the write alias is updated to point to the new index, and all
subsequent updates are written to the new index.

Rolling over to a new index based on size, document count, or age is preferable
to time-based rollovers. Rolling over at an arbitrary time often results in
many small indices, which can have a negative impact on performance and
resource usage.

You control when the rollover action is triggered by specifying one or more
rollover parameters. The rollover is performed once any of the criteria are
met. Because the criteria are checked periodically, the index might grow
slightly beyond the specified threshold. To control how often the criteria are
checked, specify the `indices.lifecycle.poll_interval` cluster setting.

IMPORTANT: New indices created via rollover will not automatically inherit the
policy used by the old index, and will not use any policy by default. Therefore,
it is highly recommended to apply the policy via
<<applying-policy-to-template,index template>>, including a Rollover alias
setting, for your indices which specifies the policy you wish to use for each
new index.

The rollover action takes the following parameters:

[[rollover-action-params]]
.`rollover` Action Parameters
[options="header"]
|===
|Name |Description
|max_size |The maximum estimated size the primary shard of the index is allowed
to grow to. Defaults to `null`. Optional.
|max_docs |The maximum number of document the index should
contain. Defaults to `null`. Optional.
|max_age |The maximum age of the index. Defaults to `null`. Optional.
|===

These parameters are used to determine when the index is considered "full" and
a rollover should be performed. Where multiple criteria are defined the
rollover operation will be performed once any of the criteria are met.

The following request defines a policy with a rollover action that triggers
when the index size reaches 25GB. The old index is subsequently deleted after
30 days.

NOTE: Once an index rolls over, {ilm} uses the timestamp of the rollover
operation rather than the index creation time to evaluate when to move the
index to the next phase. For indices that have rolled over, the `min_age`
criteria specified for a phase is relative to the rollover time for indices. In
this example, that means the index will be deleted 30 days after rollover, not
30 days from when the index was created.

[source,js]
--------------------------------------------------
PUT /_ilm/policy/my_policy
{
  "policy": {
    "phases": {
      "hot": {
        "actions": {
          "rollover": {
            "max_size": "25GB"
          }
        }
      },
      "delete": {
        "min_age": "30d",
        "actions": {
          "delete": {}
        }
      }
    }
  }
}
--------------------------------------------------
// CONSOLE

To use an {ilm} policy, you need to specify it in the index template used to
create the indices. For example, the following template associates `my_policy`
with indices created from the template `my_template`.

[source,js]
-----------------------
PUT _template/my_template
{
  "index_patterns": ["test-*"], <1>
  "settings": {
    "number_of_shards": 1,
    "number_of_replicas": 1,
    "index.lifecycle.name": "my_policy", <2>
    "index.lifecycle.rollover_alias": "test-alias" <3>
  }
}
-----------------------
// CONSOLE
<1> Template applies to all indices with the prefix test-
<2> Associates my_policy with all indices created with this template
<3> Rolls over the write alias test when the rollover action is triggered

To be able to start using the policy for these `test-*` indexes we need to
bootstrap the process by creating the first index.

[source,js]
-----------------------
PUT test-000001 <1>
{
  "aliases": {
    "test-alias":{
      "is_write_index": true <2>
    }
  }
}
-----------------------
// CONSOLE
<1> Creates the index called test-000001. The rollover action increments the
suffix number for each subsequent index.
<2> Designates this index as the write index for this alias.

When the rollover is performed, the newly-created index is set as the write
index for the rolled over alias. Documents sent to the alias are indexed into
the new index, enabling indexing to continue uninterrupted.

[[skipping-rollover]]
=== Skipping Rollover

The `index.lifecycle.indexing_complete` setting indicates to {ilm} whether this
index has already been rolled over. If it is set to `true`, that indicates that
this index has already been rolled over and does not need to be rolled over
again. Therefore, {ilm} will skip any Rollover Action configured in the
associated lifecycle policy for this index. This is useful if you need to make
an exception to your normal Lifecycle Policy and switching the alias to a
different index by hand, but do not want to remove the index from {ilm}
completely.

This setting is set to `true` automatically by ILM upon the successful
completion of a Rollover Action. However, it will be removed if
<<ilm-remove-policy,the policy is removed>> from the index.

IMPORTANT: If `index.lifecycle.indexing_complete` is set to `true` on an index,
it will not be rolled over by {ilm}, but {ilm} will verify that this index is no
longer the write index for the alias specified by
`index.lifecycle.rollover_alias`. If that setting is missing, or if the index is
still the write index for that alias, this index will be moved to the
<<index-lifecycle-error-handling,error step>>.

For example, if you wish to change the name of new indices while retaining
previous data in accordance with your configured policy, you can create the
template for the new index name pattern and the first index with the new name
manually, change the write index of the alias using the <<indices-aliases, Index
Aliases API>>, and set `index.lifecycle.indexing_complete` to `true` on the old
index to indicate that it does not need to be rolled over. This way, {ilm} will
continue to manage the old index in accordance with its existing policy, as well
as the new one, with no interruption.
