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_routing field

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`_routing` field

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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

The default value used for _routing is the document’s _id or the document’s _parent ID, if present.

Custom routing patterns can be implemented by specifying a custom routing value per document. For instance:

PUT my_index/my_type/1?routing=user1&refresh=true 
{
  "title": "This is a document"
}

GET my_index/my_type/1?routing=user1

	This document uses `user1` as its routing value, instead of its ID.
	The same `routing` value needs to be provided when getting, deleting, or updating the document.

The value of the _routing field is accessible in queries:

GET my_index/_search
{
  "query": {
    "terms": {
      "_routing": [ "user1" ] 
    }
  }
}

Querying on the _routing field (also see the ids query)

Searching with custom routing

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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):

GET my_index/_search?routing=user1,user2 
{
  "query": {
    "match": {
      "title": "document"
    }
  }
}

This search request will only be executed on the shards associated with the user1 and user2 routing values.

Making a routing value required

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When using custom routing, it is important to provide the routing value whenever indexing, getting, deleting, or updating a document.

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 custom routing value required for all CRUD operations:

PUT my_index2
{
  "mappings": {
    "my_type": {
      "_routing": {
        "required": true 
      }
    }
  }
}

PUT my_index2/my_type/1 
{
  "text": "No routing value provided"
}

	Routing is required for `my_type` documents.
	This index request throws a `routing_missing_exception`.

Unique IDs with custom routing

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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 different _routing values.

It is up to the user to ensure that IDs are unique across the index.

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