[[index-rollover]]
=== Rollover

When indexing time-series data like logs or metrics, you can't write to a single index indefinitely. 
To meet your indexing and search performance requirements and manage resource usage, 
you write to an index until some threshold is met and 
then create a new index and start writing to it instead. 
Using rolling indices enables you to:

* Optimize the active index for high ingest rates on high-performance _hot_ nodes.
* Optimize for search performance on _warm_ nodes.
* Shift older, less frequently accessed data to less expensive _cold_ nodes,
* Delete data according to your retention policies by removing entire indices.

Rollover relies on three things:

* An _index template_ that specifies the settings for each new index in the series.
You optimize this configuration for ingestion, typically using as many shards as you have hot nodes.
* An _index alias_ that references the entire set of indices. 
* A single index designated as the _write index_.
This is the active index that handles all write requests.
On each rollover, the new index becomes the write index.

[discrete]
[role="xpack"]
[testenv="basic"]
[[ilm-automatic-rollover]]
=== Automatic rollover

{ilm-init} 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.

TIP: 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.