Using ETag for optimistic locking in a Django REST application

Question

I'm trying to select a REST framework for Django that will allow me to easily use ETags for optimistic locking. I'm planning on examining Django-pistons and the Django Rest Framework libraries, but I'm open to any non-GPL solution (corporate licensing requirements prevent my using those).

My application is vending data from a SQLAlchemy model (not a Django model) in JSON/YAML form, and modulo the ETag issue is working just fine with the Django Rest Framework. However, I can't see an easy way to apply the ETag headers on my views.

For my views, I want to do this:

1. Given a response, easily add an ETag to the response headers I'm sending out on success. This must be calculated by me, since it will be model dependent; it's not enough to hash the response value or anything like that.

2. On POST/PUT, ensure that the ETag I'm receiving matches the one I sent out, or reject the request.

It's step 1 that is giving me a bit of trouble; I'm not sure which REST framework will make this easiest, nor am I sure what the best way to accomplish it is.

Answer 1

Choice of framework does not matter. In any case, you have to maintain an etag in cache(memcache or redis) representing the state of the resource. You can use Generational Caching Algorithm (https://signalvnoise.com/posts/3113-how-key-based-cache-expiration-works). Then you can easily write a mixin that extracts the etag of every resource and sends it. In my personal experience, Django Rest Framework would be ideal for it due to its heavy flexibility and well-written code.

Answer 2

Django supports ETags through decorators (@etag, @last_modified, @condition) or middleware - you can check the docs. If you want to use those decorators with Django Rest Framework, you can use django-rest-framework-condition.