Explain how HTTP request processing occurs in Django.

The request processing goes through middleware, then reaches the URL router, which determines the appropriate view. The view handles business logic and returns an HTTP response, which also passes through middleware before being sent to the client.

Main stages:

1. Entry into the application: The request is received by a web server (e.g., Nginx) and passed to a WSGI server (Gunicorn, uWSGI), which interacts with the Django application.

2. Middleware: The request passes through a sequence of middleware. Each middleware can modify the request (process_request), perform actions after the view is processed (process_view), or handle exceptions (process_exception).

   # Example custom middleware
   def simple_middleware(get_response):
       def middleware(request):
           # Logic BEFORE view processing
           print(f"Processing request at {request.path}")
   
           response = get_response(request)
   
           # Logic AFTER view processing
           print(f"Response with status {response.status_code} sent for {request.path}")
   
           return response
       return middleware
   

3. URL routing: Django compares the request URL with patterns defined in urls.py. When a match is found, the corresponding view (function or class) is invoked.

   # Example urls.py
   from django.urls import path
   from . import views
   
   urlpatterns = [
       path('articles/<int:article_id>/', views.article_detail),
       path('about/', views.about_page),
   ]
   

4. View: The view receives an HttpRequest object, processes the request (e.g., interacts with the database via ORM, performs business logic), and creates an HttpResponse object.

   # Example views.py
   from django.http import HttpResponse
   from .models import Article
   
   def article_detail(request, article_id):
       try:
           article = Article.objects.get(pk=article_id)
           return HttpResponse(f"Article details: {article.title}")
       except Article.DoesNotExist:
           return HttpResponse("Article not found", status=404)
   
   def about_page(request):
       return HttpResponse("About us")
   

5. Middleware (reverse pass): The HttpResponse object also passes through the middleware sequence in reverse order. Middleware can modify the response (process_response).

6. Sending the response: The WSGI server sends the HTTP response to the client via the web server.

Schematically:

Client <-> Web server <-> WSGI server <-> Middleware (Request) <-> URL router <-> View <-> ORM / Business Logic <-> View <-> Middleware (Response) <-> WSGI server <-> Web server <-> Client