[[searchable-snapshots]]
== {search-snaps-cap}

{search-snaps-cap} let you use <<snapshot-restore,snapshots>> to search
infrequently accessed and read-only data in a very cost-effective fashion. The
<<cold-tier,cold>> and <<frozen-tier,frozen>> data tiers use {search-snaps} to
reduce your storage and operating costs.

{search-snaps-cap} eliminate the need for <<scalability,replica shards>>,
potentially halving the local storage needed to search your data.
{search-snaps-cap} rely on the same snapshot mechanism you already use for
backups and have minimal impact on your snapshot repository storage costs.

[discrete]
[[using-searchable-snapshots]]
=== Using {search-snaps}

Searching a {search-snap} index is the same as searching any other index.

By default, {search-snap} indices have no replicas. The underlying snapshot
provides resilience and the query volume is expected to be low enough that a
single shard copy will be sufficient. However, if you need to support a higher
query volume, you can add replicas by adjusting the `index.number_of_replicas`
index setting.

If a node fails and {search-snap} shards need to be recovered elsewhere, there
is a brief window of time while {es} allocates the shards to other nodes where
the cluster health will not be `green`. Searches that hit these shards may fail
or return partial results until the shards are reallocated to healthy nodes.

You typically manage {search-snaps} through {ilm-init}. The
<<ilm-searchable-snapshot, searchable snapshots>> action automatically converts
a regular index into a {search-snap} index when it reaches the `cold` or
`frozen` phase. You can also make indices in existing snapshots searchable by
manually mounting them using the <<searchable-snapshots-api-mount-snapshot,
mount snapshot>> API.

To mount an index from a snapshot that contains multiple indices, we recommend
creating a <<clone-snapshot-api, clone>> of the snapshot that contains only the
index you want to search, and mounting the clone. You should not delete a
snapshot if it has any mounted indices, so creating a clone enables you to
manage the lifecycle of the backup snapshot independently of any
{search-snaps}. If you use {ilm-init} to manage your {search-snaps} then it
will automatically look after cloning the snapshot as needed.

You can control the allocation of the shards of {search-snap} indices using the
same mechanisms as for regular indices. For example, you could use
<<shard-allocation-filtering>> to restrict {search-snap} shards to a subset of
your nodes.

The speed of recovery of a {search-snap} index is limited by the repository
setting `max_restore_bytes_per_sec` and the node setting
`indices.recovery.max_bytes_per_sec` just like a normal restore operation. By
default `max_restore_bytes_per_sec` is unlimited, but the default for
`indices.recovery.max_bytes_per_sec` depends on the configuration of the node.
See <<recovery-settings>>.

We recommend that you <<indices-forcemerge, force-merge>> indices to a single
segment per shard before taking a snapshot that will be mounted as a
{search-snap} index. Each read from a snapshot repository takes time and costs
money, and the fewer segments there are the fewer reads are needed to restore
the snapshot or to respond to a search.

[TIP]
====
{search-snaps-cap} are ideal for managing a large archive of historical data.
Historical information is typically searched less frequently than recent data
and therefore may not need replicas for their performance benefits.

For more complex or time-consuming searches, you can use <<async-search>> with
{search-snaps}.
====

[[searchable-snapshots-repository-types]]
// tag::searchable-snapshot-repo-types[]
Use any of the following repository types with searchable snapshots:

* {plugins}/repository-s3.html[AWS S3]
* {plugins}/repository-gcs.html[Google Cloud Storage]
* {plugins}/repository-azure.html[Azure Blob Storage]
* {plugins}/repository-hdfs.html[Hadoop Distributed File Store (HDFS)]
* <<snapshots-filesystem-repository,Shared filesystems>> such as NFS
* <<snapshots-read-only-repository,URL repositories>>

You can also use alternative implementations of these repository types, for
instance
{plugins}/repository-s3-client.html#repository-s3-compatible-services[Minio],
as long as they are fully compatible. Use the <<repo-analysis-api>> API
to analyze your repository's suitability for use with searchable snapshots.
// end::searchable-snapshot-repo-types[]

[discrete]
[[how-searchable-snapshots-work]]
=== How {search-snaps} work

When an index is mounted from a snapshot, {es} allocates its shards to data
nodes within the cluster. The data nodes then automatically retrieve the
relevant shard data from the repository onto local storage, based on the
<<searchable-snapshot-mount-storage-options,mount options>> specified. If
possible, searches use data from local storage. If the data is not available
locally, {es} downloads the data that it needs from the snapshot repository.

If a node holding one of these shards fails, {es} automatically allocates the
affected shards on another node, and that node restores the relevant shard data
from the repository. No replicas are needed, and no complicated monitoring or
orchestration is necessary to restore lost shards. Although searchable snapshot
indices have no replicas by default, you may add replicas to these indices by
adjusting `index.number_of_replicas`. Replicas of {search-snap} shards are
recovered by copying data from the snapshot repository, just like primaries of
{search-snap} shards. In contrast, replicas of regular indices are restored by
copying data from the primary.

[discrete]
[[searchable-snapshot-mount-storage-options]]
==== Mount options

To search a snapshot, you must first mount it locally as an index. Usually
{ilm-init} will do this automatically, but you can also call the
<<searchable-snapshots-api-mount-snapshot,mount snapshot>> API yourself. There
are two options for mounting an index from a snapshot, each with different
performance characteristics and local storage footprints:

[[fully-mounted]]
Fully mounted index::
Loads a full copy of the snapshotted index's shards onto node-local storage
within the cluster. {ilm-init} uses this option in the `hot` and `cold` phases.
+
Search performance for a fully mounted index is normally
comparable to a regular index, since there is minimal need to access the
snapshot repository. While recovery is ongoing, search performance may be
slower than with a regular index because a search may need some data that has
not yet been retrieved into the local copy. If that happens, {es} will eagerly
retrieve the data needed to complete the search in parallel with the ongoing
recovery.

[[partially-mounted]]
Partially mounted index::
Uses a local cache containing only recently searched parts of the snapshotted
index's data. This cache has a fixed size and is shared across nodes in the
frozen tier. {ilm-init} uses this option in the `frozen` phase.
+
If a search requires data that is not in the cache, {es} fetches the missing
data from the snapshot repository. Searches that require these fetches are
slower, but the fetched data is stored in the cache so that similar searches
can be served more quickly in future. {es} will evict infrequently used data
from the cache to free up space.
+
Although slower than a fully mounted index or a regular index, a
partially mounted index still returns search results quickly, even for
large data sets, because the layout of data in the repository is heavily
optimized for search. Many searches will need to retrieve only a small subset of
the total shard data before returning results.

To partially mount an index, you must have one or more nodes with a shared cache
available. By default, dedicated frozen data tier nodes (nodes with the
`data_frozen` role and no other data roles) have a shared cache configured using
the greater of 90% of total disk space and total disk space subtracted a
headroom of 100GB.

Using a dedicated frozen tier is highly recommended for production use. If you
do not have a dedicated frozen tier, you must configure the
`xpack.searchable.snapshot.shared_cache.size` setting to reserve space for the
cache on one or more nodes. Partially mounted indices
are only allocated to nodes that have a shared cache.

[[searchable-snapshots-shared-cache]]
`xpack.searchable.snapshot.shared_cache.size`::
(<<static-cluster-setting,Static>>)
Disk space reserved for the shared cache of partially mounted indices.
Accepts a percentage of total disk space or an absolute <<byte-units,byte
value>>. Defaults to `90%` of total disk space for dedicated frozen data tier
nodes. Otherwise defaults to `0b`.

`xpack.searchable.snapshot.shared_cache.size.max_headroom`::
(<<static-cluster-setting,Static>>, <<byte-units,byte value>>)
For dedicated frozen tier nodes, the max headroom to maintain. If
`xpack.searchable.snapshot.shared_cache.size` is not explicitly set, this
setting defaults to `100GB`. Otherwise it defaults to `-1` (not set). You can
only configure this setting if `xpack.searchable.snapshot.shared_cache.size` is
set as a percentage.

To illustrate how these settings work in concert let us look at two examples
when using the default values of the settings on a dedicated frozen node:

* A 4000 GB disk will result in a shared cache sized at 3900 GB. 90% of 4000 GB
is 3600 GB, leaving 400 GB headroom. The default `max_headroom` of 100 GB
takes effect, and the result is therefore 3900 GB.
* A 400 GB disk will result in a shared cache sized at 360 GB.

You can configure the settings in `elasticsearch.yml`:

[source,yaml]
----
xpack.searchable.snapshot.shared_cache.size: 4TB
----

IMPORTANT: Currently, you can configure
`xpack.searchable.snapshot.shared_cache.size` on any node. However, if the cache size is set on any
node that does not have the <<data-frozen-node,`data_frozen`>> role, it will be treated as though it
is set to `0b`. Additionally, nodes with a shared  cache can only have a single
<<path-settings,data path>>.

[discrete]
[[back-up-restore-searchable-snapshots]]
=== Back up and restore {search-snaps}

You can use <<snapshot-lifecycle-management,regular snapshots>> to back up a
cluster containing {search-snap} indices. When you restore a snapshot
containing {search-snap} indices, these indices are restored as {search-snap}
indices again.

Before you restore a snapshot containing a {search-snap} index, you must first
<<snapshots-register-repository,register the repository>> containing the
original index snapshot. When restored, the {search-snap} index mounts the
original index snapshot from its original repository. If wanted, you
can use separate repositories for regular snapshots and {search-snaps}.

A snapshot of a {search-snap} index contains only a small amount of metadata
which identifies its original index snapshot. It does not contain any data from
the original index. The restore of a backup will fail to restore any
{search-snap} indices whose original index snapshot is unavailable.

[discrete]
[[searchable-snapshots-reliability]]
=== Reliability of {search-snaps}

The sole copy of the data in a {search-snap} index is the underlying snapshot,
stored in the repository. If the repository fails or corrupts the contents of
the snapshot then the data is lost. Although {es} may have made copies of the
data onto local storage, these copies may be incomplete and cannot be used to
recover any data after a repository failure. You must make sure that your
repository is reliable and protects against corruption of your data while it is
at rest in the repository.

The blob storage offered by all major public cloud providers typically offers
very good protection against data loss or corruption. If you manage your own
repository storage then you are responsible for its reliability.
