What is FastAPI middleware and when to use it?

Question

Accepted Answer

**Middleware** runs **before and after** every request for cross-cutting concerns.

```python
from fastapi import FastAPI, Request
from fastapi.middleware.cors import CORSMiddleware
import time

app = FastAPI()

# Built-in CORS middleware
app.add_middleware(
    CORSMiddleware,
    allow_origins=["http://localhost:3000"],
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

# Custom middleware
@app.middleware("http")
async def timing_middleware(request: Request, call_next):
    start_time = time.time()

# Before request
    print(f"Request: {request.method} {request.url.path}")

response = await call_next(request)

# After request
    process_time = time.time() - start_time
    response.headers["X-Process-Time"] = str(process_time)
    return response

# Class-based middleware
class LogMiddleware:
    def __init__(self, app):
        self.app = app

async def __call__(self, scope, receive, send):
        print(f"Request: {scope['path']}")
        await self.app(scope, receive, send)

app.add_middleware(LogMiddleware)
```

**Common uses:**
- CORS.
- Request logging.
- Authentication.
- Rate limiting.
- Response time tracking.

Python: What is FastAPI middleware and when to use it?