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Preference

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Preference

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Controls a preference of which shard copies on which to execute the search. By default, Elasticsearch selects from the available shard copies in an unspecified order, taking the allocation awareness and adaptive replica selection configuration into account. However, it may sometimes be desirable to try and route certain searches to certain sets of shard copies, for instance to make better use of per-copy caches.

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

`_primary`	The operation will be executed only on primary shards. [6.1.0] Deprecated in 6.1.0. will be removed in 7.0. See the warning below for more information.
`_primary_first`	The operation will be executed on primary shards if possible, but will fall back to other shards if not. [6.1.0] Deprecated in 6.1.0. will be removed in 7.0. See the warning below for more information.
`_replica`	The operation will be executed only on replica shards. If there are multiple replicas then the order of preference between them is unspecified. [6.1.0] Deprecated in 6.1.0. will be removed in 7.0. See the warning below for more information.
`_replica_first`	The operation will be executed on replica shards if possible, but will fall back to other shards if not. If there are multiple replicas then the order of preference between them is unspecified. [6.1.0] Deprecated in 6.1.0. will be removed in 7.0. See the warning below for more information.
`_only_local`	The operation will be executed only on shards allocated to the local node.
`_local`	The operation will be executed on shards allocated to the local node if possible, and will fall back to other shards if not.
`_prefer_nodes:abc,xyz`	The operation will be executed on nodes with one of the provided node ids (`abc` or `xyz` in this case) if possible. If suitable shard copies exist on more than one of the selected nodes then the order of preference between these copies is unspecified.
`_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\|_local`
`_only_nodes:abc,xyz,...`	Restricts the operation to nodes specified according to the node specification. If suitable shard copies exist on more than one of the selected nodes then the order of preference between these copies is unspecified.
Custom (string) value	Any value that does not start with `_`. If two searches both give the same custom string value for their preference and the underlying cluster state does not change then the same ordering of shards will be used for the searches. This does not guarantee that the exact same shards will be used each time: the cluster state, and therefore the selected shards, may change for a number of reasons including shard relocations and shard failures, and nodes may sometimes reject searches causing fallbacks to alternative nodes. However, in practice the ordering of shards tends to remain stable for long periods of time. A good candidate for a custom preference value is something like the web session id or the user name.

For instance, use the user’s session ID xyzabc123 as follows:

GET /_search?preference=xyzabc123
{
    "query": {
        "match": {
            "title": "elasticsearch"
        }
    }
}

The _only_local preference guarantees only to use shard copies on the local node, which is sometimes useful for troubleshooting. All other options do not fully guarantee that any particular shard copies are used in a search, and on a changing index this may mean that repeated searches may yield different results if they are executed on different shard copies which are in different refresh states.

The _primary, _primary_first, _replica and _replica_first are deprecated as their use is not recommended. They do not help to avoid inconsistent results that arise from the use of shards that have different refresh states, and Elasticsearch uses synchronous replication so the primary does not in general hold fresher data than its replicas. The _primary_first and _replica_first preferences silently fall back to non-preferred copies if it is not possible to search the preferred copies. The _primary and _replica preferences will silently change their preferred shards if a replica is promoted to primary, which can happen at any time. The _primary preference can also put undesirable extra load on the primary shards. The cache-related benefits of these options can also be obtained using _only_nodes, _prefer_nodes, or a custom string value instead.

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