Jay S. Bryant 1423480fb6 Make doc/source directory compliant with design in spec

The following spec defines what each project's doc/source
directory is supposed to look like:

https://specs.openstack.org/openstack/docs-specs/specs/pike/os-manuals-migration.html

I had not yet moved existing content to follow this design.
This patch does that, moving the devref to the
'contributor' directory.  It also moves the CLI
related documentation into the 'cli' directory.  I have
updated the autodoc generation to now create the api
documentation in 'doc/source/contributor/api'.

This patch also creates a template for future documentation
contribution.  I have created all of the directories
recommended by the spec and have included documentation
as to what should go in each directory.

The index file is updated to point at the new locations for
existing content.

'doc/.gitignore' is updated so that it won't complain about the
automatically generated 'doc/contributor/api' directory.

Change-Id: I55c50fa0b7c1d06c91e40dbcfd11b1c8e8378aa6

2017-07-19 15:59:02 -05:00

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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.

Routing

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

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

See http://routes.groovie.org/manual.html 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/v2/volumes.py 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.

Serialization

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

Errors

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.

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