1. What is Middleware?#

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

A perfect example is adding a “Process Time” header to every response so you can monitor performance:

import time
from fastapi import Request

@app.middleware("http")
async def add_process_time_header(request: Request, call_next):
    start_time = time.time()
    
    # The request goes "down" into the path operation
    response = await call_next(request) 
    
    # The response comes "up" back through the middleware
    process_time = time.time() - start_time
    response.headers["X-Process-Time"] = f"{process_time:.4f}s"
    
    return response

2. The CORS Headache (and the Fix)#

CORS is a browser security mechanism. If your frontend is at localhost:3000 and your API is at localhost:8000, the browser will block the request unless you explicitly permit it.

FastAPI provides a built-in CORSMiddleware to handle this gracefully:

from fastapi.middleware.cors import CORSMiddleware

origins = [
    "http://localhost:3000",
    "https://my-production-app.com",
]

app.add_middleware(
    CORSMiddleware,
    allow_origins=origins,
    allow_credentials=True,
    allow_methods=["*"], # Allow GET, POST, etc.
    allow_headers=["*"], # Allow all headers
)

3. Why This Matters#

• Centralization: Instead of checking for an X-API-Key in every route, you can do it once in a middleware.
• Standardization: Ensure every response from your server has the same security headers.
• Performance: Track and log slow requests globally without cluttering your business logic.

🛠️ Implementation Checklist#

• Created a custom timing middleware.
• Configured CORSMiddleware with specific allowed origins.
• Verified the X-Process-Time header appears in the browser’s Network tab.
• Confirmed that the global exception handler from Day 8 still works alongside the middleware.

📚 Resources#

1. Official Docs: FastAPI Middleware
2. Official Docs: CORS (Cross-Origin Resource Sharing)
3. Book: FastAPI: Modern Python Web Development (Chapter 5: Middleware & Advanced Config).