[role="xpack"]
[[data-streams]]
= Data streams
++++
<titleabbrev>Data streams</titleabbrev>
++++

A data stream lets you store append-only time series
data across multiple indices while giving you a single named resource for
requests. Data streams are well-suited for logs, events, metrics, and other
continuously generated data.

You can submit indexing and search requests directly to a data stream. The
stream automatically routes the request to backing indices that store the
stream's data. You can use <<index-lifecycle-management,{ilm} ({ilm-init})>> to
automate the management of these backing indices. For example, you can use
{ilm-init} to automatically move older backing indices to less expensive
hardware and delete unneeded indices. {ilm-init} can help you reduce costs and
overhead as your data grows.

[discrete]
[[backing-indices]]
== Backing indices

A data stream consists of one or more <<index-hidden,hidden>>, auto-generated
backing indices.

image::images/data-streams/data-streams-diagram.svg[align="center"]

Each data stream requires a matching <<indices-templates,index template>>. The
template contains the mappings and settings used to configure the stream's
backing indices.

Every document indexed to a data stream must contain a `@timestamp` field,
mapped as a <<date,`date`>> or <<date_nanos,`date_nanos`>> field type. If the
index template doesn't specify a mapping for the `@timestamp` field, {es} maps
`@timestamp` as a `date` field  with default options.

The same index template can be used for multiple data streams. You cannot
delete an index template in use by a data stream.

[discrete]
[[data-stream-read-requests]]
== Read requests

When you submit a read request to a data stream, the stream routes the request
to all its backing indices.

image::images/data-streams/data-streams-search-request.svg[align="center"]

[discrete]
[[data-stream-write-index]]
== Write index

The most recently created backing index is the data stream’s write index.
The stream adds new documents to this index only.

image::images/data-streams/data-streams-index-request.svg[align="center"]

You cannot add new documents to other backing indices, even by sending requests
directly to the index.

You also cannot perform operations on a write index that may hinder indexing,
such as:

* <<indices-clone-index,Clone>>
* <<indices-close,Close>>
* <<indices-delete-index,Delete>>
* <<freeze-index-api,Freeze>>
* <<indices-shrink-index,Shrink>>
* <<indices-split-index,Split>>

[discrete]
[[data-streams-rollover]]
== Rollover

When you create a data stream, {es} automatically creates a backing index for
the stream. This index also acts as the stream's first write index. A
<<indices-rollover-index,rollover>> creates a new backing index that becomes the
stream's new write index.

We recommend using <<index-lifecycle-management,{ilm-init}>> to automatically
roll over data streams when the write index reaches a specified age or size.
If needed, you can also <<manually-roll-over-a-data-stream,manually roll over>>
a data stream.


[discrete]
[[data-streams-generation]]
== Generation

Each data stream tracks its generation: a six-digit, zero-padded integer that
acts as a cumulative count of the stream's rollovers, starting at `000001`.

When a backing index is created, the index is named using the following
convention:

[source,text]
----
.ds-<data-stream>-<generation>
----

Backing indices with a higher generation contain more recent data. For example,
the `web-server-logs` data stream has a generation of `34`. The stream's most
recent backing index is named `.ds-web-server-logs-000034`.

Some operations, such as a <<indices-shrink-index,shrink>> or
<<snapshots-restore-snapshot,restore>>, can change a backing index's name.
These name changes do not remove a backing index from its data stream.

[discrete]
[[data-streams-append-only]]
== Append-only

Data streams are designed for use cases where existing data is rarely,
if ever, updated. You cannot send update or deletion requests for existing
documents directly to a data stream. Instead, use the
<<update-docs-in-a-data-stream-by-query,update by query>> and
<<delete-docs-in-a-data-stream-by-query,delete by query>> APIs.

If needed, you can <<update-delete-docs-in-a-backing-index,update or delete
documents>> by submitting requests directly to the document's backing index.

TIP: If you frequently update or delete existing documents, use an
<<indices-add-alias,index alias>> and <<indices-templates,index template>>
instead of a data stream. You can still use
<<index-lifecycle-management,{ilm-init}>> to manage indices for the alias.

include::set-up-a-data-stream.asciidoc[]
include::use-a-data-stream.asciidoc[]
include::change-mappings-and-settings.asciidoc[]
