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A document is routed to a particular shard in an index using the following formula:
shard_num = hash(_routing) % num_primary_shards
Custom routing patterns can be implemented by specifying a custom
value per document. For instance:
This document uses
The value of the
_routing field is accessible in queries, aggregations, scripts,
and when sorting:
Querying on the
Aggregating on the
Sorting on the
Custom routing can reduce the impact of searches. Instead of having to fan out a search request to all the shards in an index, the request can be sent to just the shard that matches the specific routing value (or values):
Forgetting the routing value can lead to a document being indexed on more than
one shard. As a safeguard, the
_routing field can be configured to make a
routing value required for all CRUD operations:
When indexing documents specifying a custom
_routing, the uniqueness of the
_id is not guaranteed across all of the shards in the index. In fact,
documents with the same
_id might end up on different shards if indexed with
It is up to the user to ensure that IDs are unique across the index.