Custom middleware creation in FastAPI - Mini Project: Build & Apply

Custom middleware creation

📖 Scenario: You are building a web API using FastAPI. You want to add a custom middleware that logs the time taken to process each request. This helps you understand how long your API takes to respond.

🎯 Goal: Create a custom middleware in FastAPI that measures and logs the time taken for each request.

📋 What You'll Learn

Create a FastAPI app instance

Define a custom middleware class that measures request processing time

Add the middleware to the FastAPI app

Create a simple GET endpoint to test the middleware

💡 Why This Matters

🌍 Real World

Custom middleware helps monitor and modify requests and responses in web APIs, useful for logging, authentication, and performance tracking.

💼 Career

Understanding middleware creation is essential for backend developers working with FastAPI or similar frameworks to build scalable and maintainable APIs.

1

Create FastAPI app instance

Write code to import FastAPI from fastapi and create an app instance called app.

FastAPI

# Your code here

Need a hint?

Use from fastapi import FastAPI and then app = FastAPI().

2

Define a custom middleware class

Write a class called TimingMiddleware that inherits from BaseHTTPMiddleware imported from starlette.middleware.base. Inside, define an async method dispatch that takes request and call_next as parameters. Use time.perf_counter() to record start and end times around call_next(request). Calculate the duration and print it with the message "Request took {duration} seconds".

FastAPI

from fastapi import FastAPI

app = FastAPI()

# Your code here

Need a hint?

Import BaseHTTPMiddleware and time. Define dispatch method with timing logic.

3

Add the middleware to the FastAPI app

Add the TimingMiddleware to the app using app.add_middleware(TimingMiddleware).

FastAPI

from fastapi import FastAPI
from starlette.middleware.base import BaseHTTPMiddleware
import time

app = FastAPI()

class TimingMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        start_time = time.perf_counter()
        response = await call_next(request)
        end_time = time.perf_counter()
        duration = end_time - start_time
        print(f"Request took {duration} seconds")
        return response

# Your code here

Need a hint?

Use app.add_middleware(TimingMiddleware) to add the middleware.

4

Create a simple GET endpoint

Create a GET endpoint at path "/" using @app.get("/"). Define an async function called read_root that returns a dictionary {"message": "Hello World"}.

FastAPI

from fastapi import FastAPI
from starlette.middleware.base import BaseHTTPMiddleware
import time

app = FastAPI()

class TimingMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        start_time = time.perf_counter()
        response = await call_next(request)
        end_time = time.perf_counter()
        duration = end_time - start_time
        print(f"Request took {duration} seconds")
        return response

app.add_middleware(TimingMiddleware)

# Your code here

Need a hint?

Use @app.get("/") decorator and define async def read_root() returning the dictionary.