How to Query number of Routing Shards?

Elastic Stack Elasticsearch
1

How can I find out the number of routing shards an index was created with? I tried using the Get Index Settings API, but it doesn't show up there:

# Create the Index
PUT twitter
{
  "settings": {
    "index": {
      "number_of_shards": 5,
      "number_of_routing_shards": 20
    }
  }
}

# Get the settings
GET twitter/_settings

gives me this result:

{
  "twitter" : {
    "settings" : {
      "index" : {
        "creation_date" : "1578512730838",
        "number_of_shards" : "5",
        "number_of_replicas" : "1",
        "uuid" : "ZQ5h-HjdS3mFojrrpc_3GA",
        "version" : {
          "created" : "7050199"
        },
        "provided_name" : "twitter"
      }
    }
  }
}

which doesn't tell me the number of routing shards. I even tried querying the setting directly, but GET twitter/_settings/index.number_of_routing_shards just returns { }

I'm using elasticearch 7.5.1

2

try using include_defaults=true as a parameter

3

That gave me a response, but curiously it returns 1 as the answer.

GET twitter/_settings/index.number_of_routing_shards?include_defaults=true

{
  "twitter" : {
    "settings" : { },
    "defaults" : {
      "index" : {
        "number_of_routing_shards" : "1"
      }
    }
  }
}
4

Hm, indeed. This always seems to be "1", which is a fallback, but it should actually work with a multipler of the number of shards. Let me check that tomorrow and come back to you.

5

I found https://github.com/elastic/elasticsearch/issues/33036 telling that the setting is not exposed in the index settings. The issue however is still open.

6

Fair enough, thanks @spinscale!

Looks like a possible workaround mentioned in that issue is to call the _split API with an invalid number of target shards, and the error response will include the number of routing shards.
So in this example you could try

POST twitter/_split/target-index
{
  "settings": {
    "index.number_of_shards": 15
  }
}

{
  "error": {
    "root_cause": [
      {
        "type": "illegal_state_exception",
        "reason": "the number of routing shards [20] must be a multiple of the target shards [15]"
      }
    ],
    "type": "illegal_state_exception",
    "reason": "the number of routing shards [20] must be a multiple of the target shards [15]"
  },
  "status": 500
}

The problem is you could potentially pick a valid number and split the index. To be safer, you could try setting index.number_of_shards to a very high number, making it unlikely that the number of routing shards is a multiple. Probably a better solution is to just use templates and trust the setting in the template is correct.