Custom Middleware, CORS & Global Exception Handlers

1Concept

Middleware intercepts every HTTP request and response cycle, allowing request timing, CORS headers (`CORSMiddleware`), request ID tagging, and centralized error logging.

2Architecture Diagram

Request ---> [ Custom Middleware: Timing ] ---> [ CORS Middleware ] ---> Route Handler ---> Response

3Code Example

middleware_code = '''
from fastapi import FastAPI, Request
import time

app = FastAPI()

@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"] = f"{process_time:.4f}s"
    return response
'''
print("=== Custom HTTP Timing Middleware ===")
print(middleware_code.strip())

4Expected Output

=== Custom HTTP Timing Middleware ===
from fastapi import FastAPI, Request
import time

app = FastAPI()

@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"] = f"{process_time:.4f}s"
    return response

5Key Takeaways

• ✓Add CORS support via `from fastapi.middleware.cors import CORSMiddleware`.
• ✓Register custom exception handlers with `@app.exception_handler(CustomException)`.
• ✓Middleware runs in outer-to-inner order on requests and reverse order on responses.