Chapter 4: Middleware and the ASGI Pipeline

ASP.NET Core middleware wraps the request delegate. ASGI middleware does the same conceptual job: inspect or modify requests and responses around downstream handling.

Common Python middleware:

• CORS middleware.
• Trusted host middleware.
• HTTPS redirect middleware, depending on deployment.
• Session middleware for cookie-backed browser sessions.
• Custom correlation ID or request logging middleware.

Example:

from fastapi import FastAPI, Request
from starlette.middleware.cors import CORSMiddleware

app = FastAPI()

app.add_middleware(
    CORSMiddleware,
    allow_origins=["https://recipes.example.com"],
    allow_credentials=True,
    allow_methods=["GET", "POST", "PUT", "DELETE"],
    allow_headers=["Authorization", "Content-Type"],
)


@app.middleware("http")
async def add_request_id(request: Request, call_next):
    request_id = request.headers.get("x-request-id", "generated-later")
    response = await call_next(request)
    response.headers["x-request-id"] = request_id
    return response

Best practice:

• Keep middleware small.
• Use dependencies for endpoint-specific behavior.
• Use middleware for truly cross-cutting behavior.
• Be careful with request body reads inside middleware; body streams can be consumed.
• Put proxy, HTTPS, host, and CORS decisions in deployment-aware configuration.