To understand how a Django Middleware works we need to remember that the basic architecture of Django is composed by a request and a response. A middleware is something that stays in the middle. Let's give a look to the next diagram, taken from official Django documentation:
Important things to know
There are four important things to know about middlewares:
- You need to write a class that just inherit from object
- The order where you place your middleware in settings.py is important: middlewares are processed from top to bottom during a request and from bottom to top during a response.
- You don't need to implement all the available methods of a middleware. For example you can just implement process_request and process_template_response
- If you implement process_request and you decide to return an HttpResponse, all the other middlewares, views etc... will be ignored and only your response will be returned
Writing a middleware
In my example I wanted to implement a feature that saves the time when a request is made and the time when a request has been processed, then calculates the time delta and exposes this value in the context so that is accessible from our templates. How to implement a similar feature using a middleware? Here is my example:
from datetime import datetime class BenchmarkMiddleware(object): def process_request(self, request): request._request_time = datetime.now() def process_template_response(self, request, response): response_time = request._request_time - datetime.now() response.context_data['response_time'] = abs(response_time) return response
Please don't care about how I calculated the time. I'm aware that there are better ways to do it, but I just wanted to keep it simple and show how to implement a simple middleware.
If you want to see a complete example of a project that includes and uses this middleware, here you can find the complete source code: https://github.com/andreagrandi/benchmark-middleware-example