[[search-request-preference]]
=== Preference

Controls a `preference` of which shard replicas to execute the search
request on. By default, the operation is randomized between the shard
replicas.

The `preference` is a query string parameter which can be set to:

[horizontal]
`_primary`:: 
	The operation will go and be executed only on the primary
	shards.

`_primary_first`:: 
	The operation will go and be executed on the primary
	shard, and if not available (failover), will execute on other shards.

`_replica`::
  The operation will go and be executed only on a replica shard.

`_replica_first`::
  The operation will go and be executed only on a replica shard, and if
  not available (failover), will execute on other shards.

`_local`:: 
	The operation will prefer to be executed on a local
	allocated shard if possible.

`_only_node:xyz`:: 
	Restricts the search to execute only on a node with
	the provided node id (`xyz` in this case).

`_prefer_node:xyz`:: 
	Prefers execution on the node with the provided
	node id (`xyz` in this case) if applicable.

`_shards:2,3`:: 
	Restricts the operation to the specified shards. (`2`
	and `3` in this case). This preference can be combined with other
	preferences but it has to appear first: `_shards:2,3;_primary`

`_only_nodes`::
    Restricts the operation to nodes specified in node specification
    https://www.elastic.co/guide/en/elasticsearch/reference/current/cluster.html

Custom (string) value:: 
	A custom value will be used to guarantee that
	the same shards will be used for the same custom value. This can help
	with "jumping values" when hitting different shards in different refresh
	states. A sample value can be something like the web session id, or the
	user name.

For instance, use the user's session ID to ensure consistent ordering of results
for the user:

[source,js]
------------------------------------------------
curl localhost:9200/_search?preference=xyzabc123 -d '
{
    "query": {
        "match": {
            "title": "elasticsearch"
        }
    }
}
'
------------------------------------------------