Elasticsearch Routing

By Opster Team

Updated: Mar 21, 2023

| 1 min read


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

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Related log errors to this ES concept

Failed to schedule execute reroute post unassigned shard
Failed to reroute routing table current state n
Updating number of replicas to for indices
Marking unavailable shards as stale
Skipping monitor as a check is already in progress
Unexpected failure during current state n
Unexpected failure during current state version
Scheduling reroute for delayed shards in delayed shards
Failing shard
Cluster health status changed from to reason
Flood stage disk watermark exceeded on all indices on this node will be marked read-only
High disk watermark exceeded on shards will be relocated away from this node

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