[[mapping-routing-field]]
=== `_routing`

The routing field allows to control the `_routing` aspect when indexing
data and explicit routing control is required. It is stored and indexed.

[float]
==== required

Another aspect of the `_routing` mapping is the ability to define it as
required by setting `required` to `true`. This is very important to set
when using routing features, as it allows different APIs to make use of
it. For example, an index operation will be rejected if no routing value
has been provided.

[float]
==== id uniqueness

When indexing documents specifying a custom `_routing`, the uniqueness
of the `_id` is not guaranteed throughout all the shards that the index
is composed of. In fact, documents with the same `_id` might end up in
different shards if indexed with different `_routing` values.
