Elasticsearch Routing

Opster Team

Last updated: Mar 30, 2021

| 1 min read

In addition to reading this guide, we recommend you run the Elasticsearch Health Check-Up. It will detect issues and improve your Elasticsearch performance by analyzing your shard sizes, threadpools, memory, snapshots, disk watermarks and more.

The Elasticsearch Check-Up is free and requires no installation.


In Elasticsearch, routing refers to document routing. When you index a document, Elasticsearch will determine which shard the document should be routed to for indexing

The shard is selected based on the following formula:

shard = hash(_routing) % number_of_primary_shards

Where the default value of _routing is _id.
It is important to know which shard the document is routed to, because Elasticsearch will need to determine where to find that document later on for document retrieval requests. 


In twitter index with 2 primary shards, the document with _id equal to “440” gets routed to the shard number:   

shard = hash( 440 ) % 2
PUT twitter/_doc/440

Notes and good things to know

  • In order to improve search speed, you can create custom routing. For example, you can enable custom routing that will ensure that only a single shard will be queried (the shard that contains your data).
  • To create custom routing in Elasticsearch, you will need to configure and define that not all routing will be completed by default settings. ( v <= 5.0)
PUT my_index/customer/_mapping
  •  This will ensure that every document in the “customer” type must specify a custom routing. For Elasticsearch version 6 or above you will need to update the same mapping as:
PUT my_index/_mapping

Related log errors to this ES concept

Unexpected failure during ; current state:n
Cluster health status changed from to reason: .
Scheduling reroute for delayed shards in delayed shards
Failed to reroute routing table; current state:n
Unexpected failure during ; current state version
flood stage disk watermark [95%] exceeded on
High disk watermark exceeded on ; shards will be relocated away from this node
After allocating; node would have more than the allowed
replicas will not be assigned to this node
After allocating; node would have less than the required threshold of
Failing shard
Updating number_of_replicas to for indices

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