Analysis

The most consistent thing is that all actions use POST.

Nova and Trove use /action while Heat uses /actions

Parameterless actions in Nova are very inconsistent on how no parameters are represented.

There is one case where Server actions (os-instance-actions) have actions as an actual resource.

Current Design

POST /v2/{tenant_id}/servers/{server_id}/action

{
    "some_action": {
        "param1": "value1",
        "param2": "value2"
    }
}

or

{
    "some_action": null
}

or

{
    "some_action": {}
}

or

{
    "some_action": "None"
}

GET /v2/{tenant_id}/servers/{server_id}/os-instance-actions

GET /v2/{tenant_id}/servers/{server_id}/os-instance-actions/{action_id}

POST /v1.0/{accountId}/instances/{instanceId}/action

{
    "some_action": {
        "param1": "value1",
        "param2": "value2"
    }
}

or

{
    "some_action": {}
}

POST /v1/{tenant_id}/stacks/{stack_name}/{stack_id}/actions

{
    "some_action": null
}