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Briefly, this error occurs when OpenSearch tries to replicate segments from a primary shard to a replica shard. However, the shard routing is marked as primary, which prevents it from acting as a replica. To resolve this issue, you can try the following: 1) Check your shard allocation settings and adjust them if necessary to ensure proper distribution of primary and replica shards. 2) You may need to manually reassign the primary status to another shard. 3) Ensure that your cluster has enough nodes to host both primary and replica shards.
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This guide will help you check for common problems that cause the log ” Shard routing is marked primary thus cannot perform segment replication as replica ” to appear. To understand the issues related to this log, read the explanation below about the following OpenSearch concepts: routing, index, replication, shard.
Overview
In OpenSearch, routing refers to document routing. When you index a document, OpenSearch 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 OpenSearch will need to determine where to find that document later on for document retrieval requests.
Examples
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 OpenSearch, 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
{
"order":{
"_routing":{
"required":true
}
}
}- This will ensure that every document in the “customer” type must specify a custom routing.
Quick links
Overview
In OpenSearch, an index (plural: indices) contains a schema and can have one or more shards and replicas. An OpenSearch index is divided into shards and each shard is an instance of a Lucene index.
Indices are used to store the documents in dedicated data structures corresponding to the data type of fields. For example, text fields are stored inside an inverted index whereas numeric and geo fields are stored inside BKD trees.
Examples
Create index
The following example is based on OpenSearch version 5.x onwards. An index with two shards, each having one replica will be created with the name test_index1
PUT /test_index1?pretty
{
"settings" : {
"number_of_shards" : 2,
"number_of_replicas" : 1
},
"mappings" : {
"properties" : {
"tags" : { "type" : "keyword" },
"updated_at" : { "type" : "date" }
}
}
}List indices
All the index names and their basic information can be retrieved using the following command:
GET _cat/indices?v
Index a document
Let’s add a document in the index with the command below:
PUT test_index1/_doc/1
{
"tags": [
"opster",
"OpenSearch"
],
"date": "01-01-2020"
}Query an index
GET test_index1/_search
{
"query": {
"match_all": {}
}
}Query multiple indices
It is possible to search multiple indices with a single request. If it is a raw HTTP request, index names should be sent in comma-separated format, as shown in the example below, and in the case of a query via a programming language client such as python or Java, index names are to be sent in a list format.
GET test_index1,test_index2/_search
Delete indices
DELETE test_index1
Common problems
- It is good practice to define the settings and mapping of an Index wherever possible because if this is not done, OpenSearch tries to automatically guess the data type of fields at the time of indexing. This automatic process may have disadvantages, such as mapping conflicts, duplicate data and incorrect data types being set in the index. If the fields are not known in advance, it’s better to use dynamic index templates.
- OpenSearch supports wildcard patterns in Index names, which sometimes aids with querying multiple indices, but can also be very destructive too. For example, It is possible to delete all the indices in a single command using the following commands:
DELETE /*
To disable this, you can add the following lines in the OpenSearch.yml:
action.destructive_requires_name: true
Log Context
Log “Shard routing is marked primary thus cannot perform segment replication as replica” classname is IndexShard.java.
We extracted the following from OpenSearch source code for those seeking an in-depth context :
if (getReplicationTracker().isPrimaryMode()) {
logger.warn("Shard is in primary mode and cannot perform segment replication as a replica.");
return false;
}
if (this.routingEntry().primary()) {
logger.warn("Shard routing is marked primary thus cannot perform segment replication as replica");
return false;
}
if (state().equals(IndexShardState.STARTED) == false
&& (state() == IndexShardState.POST_RECOVERY && shardRouting.state() == ShardRoutingState.INITIALIZING) == false) {
logger.warn(
