[[indices-shadow-replicas]]
== Shadow replica indices

experimental[]

If you would like to use a shared filesystem, you can use the shadow replicas
settings to choose where on disk the data for an index should be kept, as well
as how Elasticsearch should replay operations on all the replica shards of an
index.

In order to fully utilize the `index.data_path` and `index.shadow_replicas`
settings, you need to enable using it in elasticsearch.yml:

[source,yaml]
--------------------------------------------------
node.enable_custom_paths: true
--------------------------------------------------

You can then create an index with a custom data path, where each node will use
this path for the data:

[WARNING]
========================
Because shadow replicas do not index the document on replica shards, it's
possible for the replica's known mapping to be behind the index's known mapping
if the latest cluster state has not yet been processed on the node containing
the replica. Because of this, it is highly recommended to use pre-defined
mappings when using shadow replicas.
========================

[source,js]
--------------------------------------------------
curl -XPUT 'localhost:9200/my_index' -d '
{
    "index" : {
        "number_of_shards" : 1,
        "number_of_replicas" : 4,
        "data_path": "/var/data/my_index",
        "shadow_replicas": true
    } 
}'
--------------------------------------------------

[WARNING]
========================
In the above example, the "/var/data/my_index" path is a shared filesystem that
must be available on every node in the Elasticsearch cluster. You must also
ensure that the Elasticsearch process has the correct permissions to read from
and write to the directory used in the `index.data_path` setting.
========================

An index that has been created with the `index.shadow_replicas` setting set to
"true" will not replicate document operations to any of the replica shards,
instead, it will only continually refresh. Once segments are available on the
filesystem where the shadow replica resides (after an Elasticsearch "flush"), a
regular refresh (governed by the `index.refresh_interval`) can be used to make
the new data searchable.

NOTE: Since documents are only indexed on the primary shard, realtime GET
requests could fail to return a document if executed on the replica shard,
therefore, GET API requests automatically have the `?preference=_primary` flag
set if there is no preference flag already set.

In order to ensure the data is being synchronized in a fast enough manner, you
may need to tune the flush threshold for the index to a desired number. A flush
is needed to fsync segment files to disk, so they will be visible to all other
replica nodes. Users should test what flush threshold levels they are
comfortable with, as increased flushing can impact indexing performance.

The Elasticsearch cluster will still detect the loss of a primary shard, and
transform the replica into a primary in this situation. This transformation will
take slightly longer, since no `IndexWriter` is maintained for each shadow
replica.

Below is the list of settings that can be changed using the update
settings API:

`index.data_path` (string)::
    Path to use for the index's data. Note that by default Elasticsearch will
    append the node ordinal by default to the path to ensure multiple instances
    of Elasticsearch on the same machine do not share a data directory.

`index.shadow_replicas`::
    Boolean value indicating this index should use shadow replicas. Defaults to
    `false`.

`index.shared_filesystem`::
    Boolean value indicating this index uses a shared filesystem. Defaults to
    the `true` if `index.shadow_replicas` is set to true, `false` otherwise.

=== Node level settings related to shadow replicas

These are non-dynamic settings that need to be configured in `elasticsearch.yml`

`node.add_id_to_custom_path`::
    Boolean setting indicating whether Elasticsearch should append the node's
    ordinal to the custom data path. For example, if this is enabled and a path
    of "/tmp/foo" is used, the first locally-running node will use "/tmp/foo/0",
    the second will use "/tmp/foo/1", the third "/tmp/foo/2", etc. Defaults to
    `true`.

`node.enable_custom_paths`::
    Boolean value that must be set to `true` in order to use the
    `index.data_path` setting. Defaults to `false`.

