_routing field
edit_routing
field
editA 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
.
Custom routing patterns can be implemented by specifying a custom routing
value per document. For instance:
PUT my-index-000001/_doc/1?routing=user1&refresh=true { "title": "This is a document" } GET my-index-000001/_doc/1?routing=user1
This document uses |
|
The same |
The value of the _routing
field is accessible in queries:
Querying on the |
Data streams do not support custom routing. Instead, target the appropriate backing index for the stream.
Searching with custom routing
editCustom 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):
Making a routing value required
editWhen 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:
Unique IDs with custom routing
editWhen 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.
Routing to an index partition
editAn index can be configured such that custom routing values will go to a subset of the shards rather than a single shard. This helps mitigate the risk of ending up with an imbalanced cluster while still reducing the impact of searches.
This is done by providing the index level setting index.routing_partition_size
at index creation.
As the partition size increases, the more evenly distributed the data will become at the
expense of having to search more shards per request.
When this setting is present, the formula for calculating the shard becomes:
shard_num = (hash(_routing) + hash(_id) % routing_partition_size) % num_primary_shards
That is, the _routing
field is used to calculate a set of shards within the index and then the
_id
is used to pick a shard within that set.
To enable this feature, the index.routing_partition_size
should have a value greater than 1 and
less than index.number_of_shards
.
Once enabled, the partitioned index will have the following limitations:
-
Mappings with
join
field relationships cannot be created within it. -
All mappings within the index must have the
_routing
field marked as required.