[[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 critera are
checked, specify the `indices.lifecycle.poll_interval` cluster setting.

The rollover action takes the following parameters:

.`rollover` Action Parameters
|===
|Name |Description
|max_size |The maximum estimated size the index is allowed to grow
to. Defaults tonull. Optional.
|max_docs |The maximum number of document the index should
contain. Defaults tonull. 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 `after`
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/my_policy
{
  "policy": {
    "phases": {
      "hot": {
        "actions": {
          "rollover": {
            "max_size": "25GB"
          }
        }
      },
      "delete": {
        "after": "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.