Adding a Method to the OpenStack API

The interface is a mostly RESTful API. REST stands for Representational State Transfer and provides an architecture “style” for distributed systems using HTTP for transport. Figure out a way to express your request and response in terms of resources that are being created, modified, read, or destroyed.


To map URLs to controllers+actions, OpenStack uses the Routes package, a clone of Rails routes for Python implementations. See for more information.

URLs are mapped to “action” methods on “controller” classes in cinder/api/openstack/__init__/ApiRouter.__init__ .

See for all syntax, but you’ll probably just need these two:

  • mapper.connect() lets you map a single URL to a single action on a controller.

  • mapper.resource() connects many standard URLs to actions on a controller.

Controllers and actions

Controllers live in cinder/api/openstack, and inherit from cinder.wsgi.Controller.

See cinder/api/v3/ for an example.

Action methods take parameters that are sucked out of the URL by mapper.connect() or .resource(). The first two parameters are self and the WebOb request, from which you can get the req.environ, req.body, req.headers, etc.


Actions return a dictionary, and wsgi.Controller serializes that to JSON or XML based on the request’s content-type.


There will be occasions when you will want to return a REST error response to the caller and there are multiple valid ways to do this:

  • If you are at the controller level you can use a faults.Fault instance to indicate the error. You can either return the Fault instance as the result of the action, or raise it, depending on what’s more convenient: raise faults.Fault(webob.exc.HTTPBadRequest(explanation=msg)).

  • If you are raising an exception our WSGI middleware exception handler is smart enough to recognize webob exceptions as well, so you don’t really need to wrap the exceptions in a Fault class and you can just let the middleware add it for you: raise webob.exc.HTTPBadRequest(explanation=msg).

  • While most errors require an explicit webob exception there are some Cinder exceptions (NotFound and Invalid) that are so common that they are directly handled by the middleware and don’t need us to convert them, we can just raise them at any point in the API service and they will return the appropriate REST error to the caller. So any NotFound exception, or child class, will return a 404 error, and any Invalid exception, or child class, will return a 400 error.