/_aliases remove action ignores must_exist when alias is missing

[lastnico]

Elasticsearch Version

8.6.0

Installed Plugins

No response

Java Version

bundled

OS Version

Linux 5.19.0-35-generic #36-Ubuntu SMP PREEMPT_DYNAMIC Fri Feb 3 18:36:56 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

Problem Description

If I try to perform a single "remove" action with "must_exist": false on the /_aliases endpoint, I still receive an HTTP 404 response with "type": "aliases_not_found_exception".

This seems to contradict documentation of the /aliases API

must_exist
(Optional, Boolean) If true, the alias must exist to perform the action. Defaults to false. Only the remove action supports this parameter.

Steps to Reproduce

[ ] Create index new_index

PUT /new_index
{}

[ ] Try to remove a missing some_alias on new_index index

POST _aliases
{
  "actions": [
    {
      "remove": {
        "index": "new_index",
        "alias": "some_alias",
        "must_exist": false
      }
    }
  ]
}

[ ] This will lead to the following error:

{
  "error": {
    "root_cause": [
      {
        "type": "aliases_not_found_exception",
        "reason": "aliases [some_alias] missing",
        "resource.type": "aliases",
        "resource.id": "some_alias"
      }
    ],
    "type": "aliases_not_found_exception",
    "reason": "aliases [some_alias] missing",
    "resource.type": "aliases",
    "resource.id": "some_alias"
  },
  "status": 404
}

Logs (if relevant)

No response

[stu-elastic]

"must_exist": true stops other actions in alias requests if the alias does not exist.

So other_alias is not created in this case.

POST _aliases
{
  "actions": [
    {
      "remove": {
        "index": "new_index",
        "alias": "some_alias",
        "must_exist": true
      }
    },
    {
      "add": {
        "index": "new_index",
        "alias": "other_alias"
      }
    }
  ]
}

However, the same request with "must_exist": false (or omitted, as it's the default), other_alias is created.

So it seems to be working as intended, but I agree the 404 is confusing when there is only one action and it's a remove with "must_exist": false. Sending over to the Data Management team for their thoughts.

[elasticsearchmachine]

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

[parkertimmins]

After some thought, I think we should continue to return 404 on removal on non-existing aliases if must_exist==false. Rather than making the change in this ticket, the docs should be updated to indicate that the must_exist argument affects the atomicity of applying a list of actions. Additionally, the phrase "single atomic operation" in the docs, should probably be updated to clarify that the operation is only atomic if must_exist==true.

This is my reasoning:

• DELETE /some_index/_alias/non_existing_alias returns 404, as it should.
• The following list of actions with a single remove should be expected to behave the same as the above DELETE . More pragmatically, these share the same removal code, and it would be better to keep them as the same code.

POST _aliases
{
  "actions": [
    {
      "remove": {
        "index": "some_index",
        "alias": "non_existing_alias"
      }
    }
  ]
}

• Though it might make sense to return 200 if must_exist==false, this is the default case, so returning 200 for must_exist==false also requires returning 200 in the above case where the must_exist argument is absent.
• Silently failing in the default case seems undesired.