Idempotent PUT resource class

The current method for creating or updating a custom resource class in the placement API has two flaws which can be resolved by deprecating the existing POST /resource_classes and changing PUT /resource_classes/{name} to be an idempotent create or validate.

Problem description

Microversion 1.2 of the placement API added support for managing custom resource classes in the usual POST to create, PUT to update style. It turns out, however, that the most common interaction is to want to create a custom resource class if it doesn’t already exist. The current process for this is:

GET /resource_classes/CUSTOM_FOOBAR

If the response is 404 then:

POST /resource_classes

{"name": "CUSTOM_FOOBAR"}

If the response is successful then it was created. If the response is 409 then the resource class already exists and some other process created it in the time since the GET. Once could also do the POST without the GET and accept the 409 as a form of success but that’s not a normal form of interaction in HTTP APIs.

Meanwhile, PUT to update has the form:

PUT /resource_classes/CUSTOM_FOOBAR

{"name": "CUSTOM_NEWBAR"}

This is not actually something we want to allow. We do not want existing references to a resource class to be renamed as those updates will not be reflected anywhere outside the placement service.

Use Cases

As a developer of systems that manage custom resource classes I want to manage them simply, efficiently and correctly.

Proposed change

Since the meaning of a single custom resource class is present in just the URL we can adjust PUT /resources_class/{name} to be an idempotent creator and validator of a single resource class. To create a new custom resource class:

PUT /resource_class/CUSTOM_FOOBAR
<empty body>

Status: 201 Created

If it might already exist, that’s okay:

PUT /resource_class/CUSTOM_FOOBAR
<empty body>

Status: 204 No Content

No GET or POST is required, the PUT requires no body, and the undesirable rename behavior with the previous mechanics of the PUT request (described in the problem statement above) will be removed.

This functionality will be implemented in a new microversion that will provide new handler code for the PUT method. Support for the POST method (which currently accepts a body with a name attribute) will be kept around as this allows the new microversion to continue accepting creation requests in the old style and this is good for stability.

If, sometime in the future, we realize we need to add additional fields to a resource class, such as description, then we should again bump the microversion to allow a PUT with a body that includes those new fields, but does not include the resource class name. This will allow us to continue having the desired idempotent behavior with PUT requests and will still prevent the rename confusion (described above). Until such a time that we need those additional fields, including a body on the PUT request is redundant so we may as well not allow it.


We could do nothing, but that leaves us with the potentially dangerous rename behavior.

Data model impact


REST API impact

The main change is to add a microversion which adjusts the handling of PUT /resource_classes/{name} so it no longer takes a body and either creates or verifies the existence of the custom resource class identified by {name}:

PUT /resource_classes/CUSTOM_FOOBAR
<empty body>

Successful responses include no body and have one of the following status codes:

  • 201 Created: if the custom resource class is newly created

  • 204 No Content: if the custom resource class already exists

Possible error response codes are:

  • 400 Bad Request: if the format of the proposed resource class is not valid

Security impact

The rename problem described above is a data integrity issue that this change resolves. The surface area of that problem is small because currently the placement API is admin only.

Notifications impact


Other end user impact


Performance Impact

There’s a very minor performance impact via the current scheduler report client because we are now doing a maximum of one instead of two requests when handling custom resource classes.

Other deployer impact

Because this change is being done on a microversion, older versions of the scheduler report client will continue to work against newer placement APIs.

Developer impact




Primary assignee:


Other contributors:


Work Items

  • Create new handler code for PUT /resource_classes/{name} in a new microversion.

  • Add gabbi tests that exercise the new microversion.

  • Update microversion history.

  • Update placement-api-ref.

  • Update the scheduler report client to use the new interface.




New gabbi and existing scheduler functional tests and tempest tests will exercise this change.

Documentation Impact

The placement-api-ref will need to be updated to reflect this change, but the change has no impact on installation or configuration, so those docs should be fine.




Release Name