Proposal: Conventions for edX REST APIs

Several groups are currently working on REST APIs for edx-platform (Analytics, Solutions, Mobile), and we need conventions to present a consistent interface to API clients.

Goals

Determine a convention for canonical entity representation (e.g. Course, User, Enrollment)
Enable third party additions to be on equal footing with internally developed edX apps
Discovery (what mix of APIs are live on this particular edx-platform instance?)
Versioning (maybe)

Big Open Questions

Should canonical entities only point to top level sub-api pages like analytics.edx.org/api/courses/my_course instead of the sub-resources? Would simplify.
Should canonical entities even bother pointing to different versions?
How to stitch them together? Explicit Django settings with ("name", "url")?

Example Using a Canoncial Entity

There are certain entities that are such a core concept to edX that they are used everywhere. That being said, it is often difficult to agree on what a canonical representation of these entities even is, and different parts of the system may have conflicting ideas of what constitutes a Course, for instance. This problem is compounded by the need to enable the creation and easy discovery of new APIs, many of which will not be developed by third parties.

The proposal here attempts to sidestep the problem by presenting an extremely minimal base representation, exposing most functionality via a pluggable interface that third parties can register for. As an example:

// GET /api/edx.canonical/courses/edX+Open_DemoX+edx_demo_course
{
    "course_id": "edX+Open_DemoX+edx_demo_course",
    "name": "edX Demonstration Course", 
    "_links": {
        "edx.analytics": {
            "enrollment": {
                "url": "https://analytics-api.edx.org/courses/edX+Open_DemoX+edx_demo_course/enrollment"
                "versions": ["1", "dev"]
            },
            "demographics": {
                "url": "https://analytics-api.edx.org/courses/edX+Open_DemoX+edx_demo_course/demographics"
                "versions": ["dev"]
            }
        },
        "edx.content": {
            "xblock": {
                "url": "https://api.edx.org/api/edx.content/xblocks/edX+Open_DemoX+edx_demo_course"
                "versions": ["2", "3", "dev"]
            }
            "authors": {
                "url": "https://api.edx.org/api/edx.content/courses/edX+Open_DemoX+edx_demo_course/authors"
                "versions": ["2", "dev"]
            }
        },
        "edx.videos": {
            "video_listing": {
                "url": "https://api.edx.org/api/edx.video/video_listing/courses/edX+Open_DemoX+edx_demo_course"
                "versions": ["dev"]
            }
        }
    },
    "_version": "1",
    "_versions": ["1"]
}

The idea would be that installed packages could define any number of resources in different namespaces. The namespaces would represent logical groupings, and may cross app boundaries (say related apps in a repo). The basic guidelines:

Wherever feasible, items linked to in "resources" should include back links to the canonical entity.
These would often be Django apps, but could also be very thin config pieces that do nothing more than point to services living on a different server.
If there are versions specified for a given URL, it means that it will obey the convention of passing a ?v={version} querystring parameter. Without an explicit version, apps should default to the latest. This does require a little more intelligence on the part of the client, but it makes things much more compact. There is the problem of moving the URLs -- this proposal would requires that either a new resource entry is made or that the URL pointed to would itself be responsible for doing the redirection.
These URLs have to be super-cheap to generate. Because there are going to be many things hanging off of it, it should just be a dictionary lookup -- no database or network access to determine whether or not a resource URL will show up. This means that sometimes you might follow a link straight into a 403 error and have to account for that.
The URL for the canonical entities also respect a version ?v={version} param, but that version only applies to the minimal core data exposed. It does not apply to the resource list, which can change at any time when new services are added.
EdX reserves the use of the "edx." prefix for resource groups, but third parties can register their own. The exact mechanism for how we do this hasn't been determined yet.

A Simpler Canonical Entity

Here we omit the sub-resources and versions...

// GET /api/edx.canonical/courses/edX+Open_DemoX+edx_demo_course
{
    "course_id": "edX+Open_DemoX+edx_demo_course",
    "name": "edX Demonstration Course", 
    "_links": {
        "edx.analytics": {
            "url": "https://analytics-api.edx.org/courses/edX+Open_DemoX+edx_demo_course"
        },
        "edx.content": {
            "url": "https://api.edx.org/api/edx.content/courses/edX+Open_DemoX+edx_demo_course/authors"
        },
        "edx.videos": {
            "url": "https://api.edx.org/api/edx.video/video_listings/courses/edX+Open_DemoX+edx_demo_course"
        }
    },
    "_version": "1",
    "_versions": ["1"]
}

High Level Discovery

Each API would also define a top level entry point that would be shown when a request is made to edx-platform on /api

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