[Feature Request] Support index templates to configure search replicas

[vinaykpud]

Is your feature request related to a problem? Please describe

Currently index templates don't support adding search replicas for an index.
existing doc: https://opensearch.org/docs/latest/im-plugin/index-templates/

This is related to : #15306

Describe the solution you'd like

Index templates should support format like this we can configure number_of_search_only_replicas

PUT _index_template/daily_logs
{
  "index_patterns": [
    "logs-2020-01-*"
  ],
  "template": {
    "aliases": {
      "my_logs": {}
    },
    "settings": {
      "number_of_shards": 2,
      "number_of_replicas": 1,
      "number_of_search_only_replicas": 1
    },
    "mappings": {
      "properties": {
        "timestamp": {
          "type": "date",
          "format": "yyyy-MM-dd HH:mm:ss||yyyy-MM-dd||epoch_millis"
        },
        "value": {
          "type": "double"
        }
      }
    }
  }
}

Related component

Search:Performance

Describe alternatives you've considered

No response

Additional context

No response

[mgodwan]

What are the challenges you see with this? Given this is a setting, index template should already be supporting this.

[vinaykpud]

Hi @mgodwan,
We want to add support to define number_of_search_only_replicas, its not currently supported.

Created template:

curl -X PUT "localhost:9200/_index_template/daily_logs" \
-H "Content-Type: application/json" \
-d '{
  "index_patterns": [
    "logs-2020-01-*"
  ],
  "template": {
    "aliases": {
      "my_logs": {}
    },
    "settings": {
      "number_of_shards": 2,
      "number_of_replicas": 1,
      "number_of_search_only_replicas": 1
    },
    "mappings": {
      "properties": {
        "timestamp": {
          "type": "date",
          "format": "yyyy-MM-dd HH:mm:ss||yyyy-MM-dd||epoch_millis"
        },
        "value": {
          "type": "double"
        }
      }
    }
  }
}'

Create index using above template:

curl -X PUT "localhost:9200/logs-2020-01-01" \
-H "Content-Type: application/json" \
-d '{}'

List shards:

 curl -X GET "localhost:9200/_cat/shards?v"
index           shard prirep state   docs store ip        node
logs-2020-01-01 0     p      STARTED    0  230b 127.0.0.1 runTask-1
logs-2020-01-01 0     r      STARTED    0    0b 127.0.0.1 runTask-2
logs-2020-01-01 1     p      STARTED    0  230b 127.0.0.1 runTask-0
logs-2020-01-01 1     r      STARTED    0  230b 127.0.0.1 runTask-2

Here we expect search shards to be created as we declared "number_of_search_only_replicas": 1 in the template.