Fix bugs with index.number_of_routing_shards setting. (#109656)

[Rassyan]

Description

This PR addresses the issues outlined in issue #109656 regarding the index.number_of_routing_shards setting. The key fixes include:

1. Default Value Alignment: Ensures that the default value of index.number_of_routing_shards matches the computed value of routing_num_shards.
2. Immutability Enforcement: Prevents modification of index.number_of_routing_shards after index creation.
3. API Consistency: Ensures that the _settings API correctly returns the value of index.number_of_routing_shards.

Self-Tests After Fix

Test 1: Default Value Alignment

DELETE test

PUT test
{
  "settings": {
    "index.number_of_shards": 3
  }
}

GET test/_settings?include_defaults&filter_path=**.*routing*shards
---
{
  "test": {
    "defaults": {
      "index": {
        "number_of_routing_shards": "768"
      }
    }
  }
}

GET /_cluster/state/metadata/test?filter_path=**.*routing*shards
---
{
  "metadata": {
    "indices": {
      "test": {
        "routing_num_shards": 768
      }
    }
  }
}

Test 2: Immutability Enforcement

POST test/_close

PUT test/_settings
{
  "settings": {
    "index.number_of_routing_shards": 6
  }
}
---
{
  "error": {
    "root_cause": [
      {
        "type": "illegal_argument_exception",
        "reason": "final test setting [index.number_of_routing_shards], not updateable"
      }
    ],
    "type": "illegal_argument_exception",
    "reason": "final test setting [index.number_of_routing_shards], not updateable"
  },
  "status": 400
}

POST test/_open

GET test/_settings?include_defaults&filter_path=**.*routing*shards
---
{
  "test": {
    "defaults": {
      "index": {
        "number_of_routing_shards": "768"
      }
    }
  }
}

GET /_cluster/state/metadata/test?filter_path=**.*routing*shards
---
{
  "metadata": {
    "indices": {
      "test": {
        "routing_num_shards": 768
      }
    }
  }
}

Test 3: Explicit Setting Retrieval

DELETE test1

PUT test1
{
  "settings": {
    "index.number_of_shards": 3,
    "index.number_of_routing_shards": 6
  }
}

GET test1/_settings?include_defaults&filter_path=**.*routing*shards
---
{
  "test1": {
    "settings": {
      "index": {
        "number_of_routing_shards": "6"
      }
    }
  }
}

GET /_cluster/state/metadata/test1?filter_path=**.*routing*shards
---
{
  "metadata": {
    "indices": {
      "test1": {
        "routing_num_shards": 6,
        "settings": {
          "index": {
            "number_of_routing_shards": "6"
          }
        }
      }
    }
  }
}

Summary

This fix ensures that the index.number_of_routing_shards setting is correctly set and retrieved, and enforces its immutability post-creation. The routing_num_shards remains an internal setting, as originally designed, and does not require modification.

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[Rassyan]

Hi team 👋 @jakelandis @ywangd @mark-vieira
Just a friendly ping regarding this PR. It's been 9 months since submission and I wanted to check if someone could help review or share feedback when you get a chance.

Happy to make any adjustments needed. Thanks for your time! 🙌

[ywangd]

@elastic/es-data-management Could you please help provide a response here? Thanks!