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

beta::[]

{search-snaps-cap} let you reduce your operating costs by using
<<snapshot-restore, snapshots>> for resiliency rather than maintaining
<<scalability,replica shards>> within a cluster. When you mount an index from a
snapshot as a {search-snap}, {es} copies the index shards to local storage
within the cluster. This ensures that search performance is comparable to
searching any other index, and minimizes the need to access the snapshot
repository. Should a node fail, shards of a {search-snap} index are
automatically recovered from the snapshot repository.

This can result in significant cost savings. With {search-snaps}, you may be
able to halve your cluster size without increasing the risk of data loss or
reducing the amount of data you can search. Because {search-snaps} rely on the
same snapshot mechanism you use for backups, they have a 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.
Search performance is comparable to regular indices because the shard data is
copied onto nodes in the cluster when the {search-snap} is mounted.

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 restored from the snapshot,
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
will fail or return partial results until they are reallocated.

You typically manage {search-snaps} through {ilm-init}. The
<<ilm-searchable-snapshot, searchable snapshots>> action automatically converts
an index to a {search-snap} when it reaches the `cold` phase. You can also make
indices in existing snapshots searchable by manually mounting them as
{search-snaps} with 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 cannot 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}.

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.

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.

[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}.
====

[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 restore the shard
data from the repository onto local storage. Once the restore process
completes, these shards respond to searches using the data held in local
storage and do not need to access the repository. This avoids incurring the
cost or performance penalty associated with reading data from the repository.

If a node holding one of these shards fails, {es} automatically allocates it to
another node, and that node restores the shard data from the repository. No
replicas are needed, and no complicated monitoring or orchestration is
necessary to restore lost shards.

{es} restores {search-snap} shards in the background and you can search them
even if they have not been fully restored. If a search hits a {search-snap}
shard before it has been fully restored, {es} eagerly retrieves the data needed
for the search. If a shard is freshly allocated to a node and still warming up,
some searches will be slower. However, searches typically access a very small
fraction of the total shard data so the performance penalty is typically small.

Replicas of {search-snaps} shards are restored by copying data from the
snapshot repository. In contrast, replicas of regular indices are restored by
copying data from the primary.
