Middleware Basics

🎯 What You'll Learn

• Understand what middleware is and how it works
• Create custom middleware using the @app.middleware decorator
• Add processing before and after request handling
• Understand middleware execution order

📚 Theory

Middleware is a function that works with every request before it is processed by any specific path operation, and also with every response before returning it.

How Middleware Works

1. Request Processing: Takes each request that comes to your application
2. Pre-processing: Can modify the request or run any needed code
3. Pass Through: Passes the request to be processed by the rest of the application
4. Response Processing: Takes the response generated by the application
5. Post-processing: Can modify the response or run any needed code
6. Return: Returns the final response

Creating Middleware

To create middleware, use the @app.middleware("http") decorator on top of a function. The middleware function receives:

• The request object
• A call_next function that passes the request to the corresponding path operation

@app.middleware("http")
async def add_process_time_header(request: Request, call_next):
    start_time = time.perf_counter()
    response = await call_next(request)
    process_time = time.perf_counter() - start_time
    response.headers["X-Process-Time"] = str(process_time)
    return response

🔧 Key Concepts

• @app.middleware("http"): Decorator to create HTTP middleware
• call_next: Function that calls the next middleware or path operation
• Request Processing: Code before call_next() runs before the request is handled
• Response Processing: Code after call_next() runs after the response is generated
• Custom Headers: Can add custom headers with 'X-' prefix for proprietary headers

💡 Best Practices

• Use time.perf_counter() instead of time.time() for precise timing measurements
• Add custom headers with 'X-' prefix for proprietary headers
• Keep middleware lightweight to avoid performance impacts
• Consider middleware execution order when adding multiple middlewares

🔍 Testing Your Middleware

To see your middleware in action:

1. Check Response Headers: Look for the X-Process-Time header in the response headers section
2. View Headers in UI: Response headers are displayed in the Test Endpoints tab
3. Compare Endpoints: Notice how different endpoints have different processing times
4. Test Different Methods: Try GET, POST, and other HTTP methods to see consistent timing

🔗 Additional Resources

• FastAPI Middleware Documentation
• Starlette Middleware