index.number_of_replicas cannot be reset to a default value

[Ingramz]

Elasticsearch version (bin/elasticsearch --version): 7.6.2

Plugins installed: []

JVM version (java -version): Bundled

OS version (uname -a if on a Unix-like system): Any

Description of the problem including expected versus actual behavior:

Per documentation, one should be able to reset an index setting to its default value by using null as value. This seems to work for a setting such as refresh_interval, but not for number_of_replicas, resulting in following response:

{
    "error": {
        "root_cause": [
            {
                "type": "illegal_argument_exception",
                "reason": "must specify numberOfReplicas for index [twitter]"
            }
        ],
        "type": "illegal_argument_exception",
        "reason": "must specify numberOfReplicas for index [twitter]"
    },
    "status": 400
}

It is uncertain whether there are more settings that exhibit this behavior.

Steps to reproduce:

Create Index:

PUT /twitter

Check defaults (see that number_of_replicas is 1):

GET /twitter

Set number_of_replicas temporarily to 0:

PUT /twitter/_settings
{
    "index" : {
        "number_of_replicas" : 0
    }
}

Verify that number_of_replicas is 0:

GET /twitter

Try to reset the value per documentation:

PUT /twitter/_settings
{
    "index" : {
        "number_of_replicas" : null
    }
}

Provide logs (if relevant): N/A

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Settings)

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[cbuescher]

@Ingramz thanks for raising this, I'm not sure its a bug or the intended behaviour that should be documented better. Labeling accordingly so the team can weigh in.

[jasontedor]

This is a bug, I opened #56656.

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Indices APIs)

[jasontedor]

I'm relabeling this since this isn't a general problem with the settings infrastructure, but a case for this specific API (the update index settings API).