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

store / index

The first thing the _routing mapping does is to store the routing value provided (store set to yes) and index it (index set to not_analyzed). The reason why the routing is stored by default is so reindexing data will be possible if the routing value is completely external and not part of the docs.

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 (or derived from the doc). A delete operation will be broadcasted to all shards if no routing value is provided and _routing is required.

path

The routing value can be provided as an external value when indexing (and still stored as part of the document, in much the same way _source is stored). But, it can also be automatically extracted from the index doc based on a path. For example, having the following mapping:

{
    "comment" : {
        "_routing" : {
            "required" : true,
            "path" : "blog.post_id"
        }
    }
}

Will cause the following doc to be routed based on the 111222 value:

{
    "text" : "the comment text"
    "blog" : {
        "post_id" : "111222"
    }
}

Note, using path without explicit routing value provided required an additional (though quite fast) parsing phase.