What is middleware in Django?

Question

Accepted Answer

**Middleware** is a framework of hooks that process **every request and response** globally, sitting between the web server and your views. Each middleware can inspect/modify requests on the way in and responses on the way out — ideal for cross-cutting concerns like authentication, security headers, sessions, and logging.

## The request/response flow through middleware

```text
Request  → MW1 → MW2 → MW3 → VIEW
Response ← MW1 ← MW2 ← MW3 ← VIEW

Middleware wraps the view like layers of an onion:
  - on the way IN (request): top-to-bottom
  - on the way OUT (response): bottom-to-top (reverse order)
```

## Django's built-in middleware (in settings.py)

```python
MIDDLEWARE = [
    "django.middleware.security.SecurityMiddleware",          # security headers (HSTS, etc.)
    "django.contrib.sessions.middleware.SessionMiddleware",   # enables request.session
    "django.middleware.common.CommonMiddleware",
    "django.middleware.csrf.CsrfViewMiddleware",              # CSRF protection
    "django.contrib.auth.middleware.AuthenticationMiddleware",# enables request.user
    "django.middleware.clickjacking.XFrameOptionsMiddleware", # clickjacking protection
]
# ORDER MATTERS — e.g. SessionMiddleware must come before AuthenticationMiddleware
```

Much of Django's core functionality is middleware: sessions (`request.session`), authentication (`request.user`), CSRF protection, and security headers all come from built-in middleware. **Order matters** — some middleware depends on others running first.

## Writing custom middleware

```python
class TimingMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response          # the next middleware/view

def __call__(self, request):
        # --- code here runs on the way IN (before the view) ---
        import time; start = time.time()

response = self.get_response(request)     # call the next layer (eventually the view)

# --- code here runs on the way OUT (after the view) ---
        response["X-Response-Time"] = f"{time.time() - start:.3f}s"
        return response
```

A middleware is a callable: code before `self.get_response(request)` runs on the request, code after runs on the response. This lets you act both before and after view execution.

## Common middleware uses

```text
✓ Authentication / setting request.user
✓ Security headers (CSP, HSTS, X-Frame-Options)
✓ Request/response logging, timing, metrics
✓ Sessions, locale/i18n detection, CORS
✓ Rate limiting, maintenance mode, custom request preprocessing
```

## Middleware vs views/decorators

```text
Middleware → GLOBAL, cross-cutting concerns affecting EVERY (or most) request
Decorators/view logic → per-view concerns specific to certain endpoints
→ Use middleware for app-wide behavior; decorators for selective behavior.
```

## Why it matters

Middleware is fundamental to how Django processes requests — understanding it explains how much of the framework's core functionality actually works and provides the right mechanism for **global, cross-cutting concerns**.

Recognizing that sessions, authentication (`request.user`), CSRF protection, and security headers all come from built-in middleware demystifies Django's request handling and shows why these features "just work." Understanding the **onion-layer request/response flow** (middleware wrapping the view, processing requests top-to-bottom and responses bottom-to-top) and that **order matters** (some middleware depends on others, like authentication needing sessions first) is important for configuring Django correctly and debugging issues.

The ability to **write custom middleware** is valuable for implementing app-wide behavior cleanly — logging, timing, custom headers, rate limiting, maintenance mode — in one place rather than repeating it across views.

Knowing when to use middleware (global concerns affecting most requests) versus decorators or view logic (per-endpoint concerns) is a key architectural decision.

Middleware is where Django handles request/response processing that applies across the whole application, making it important, practically-relevant knowledge for both understanding the framework's internals and building well-structured applications with proper separation of cross-cutting concerns.