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Preference

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Preference

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Controls a preference of the 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:

`_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.

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