Why Custom middleware creation in FastAPI? - Purpose & Use Cases

The Big Idea

What if you could add powerful features everywhere in your app with just one simple piece of code?

The Scenario

Imagine you want to log every request and response in your web app by adding code inside every route handler manually.

The Problem

Manually adding logging or checks in each route is repetitive, easy to forget, and clutters your code with extra details.

The Solution

Custom middleware lets you write one piece of code that runs before and after every request automatically, keeping your routes clean and consistent.

Before vs After

✗ Before

def route():
    log_request()
    process()
    log_response()

✓ After

class CustomMiddleware:
    async def __call__(self, request, call_next):
        log_request()
        response = await call_next(request)
        log_response()
        return response

What It Enables

You can add features like logging, authentication, or error handling globally without repeating code in every route.

Real Life Example

A company wants to track user activity on all pages to improve security and performance without changing each page's code.

Key Takeaways

Manual code repetition is slow and error-prone.

Middleware runs code automatically on every request.

Custom middleware keeps your app clean and consistent.

Practice

1. What is the main purpose of creating custom middleware in FastAPI?

easy

A. To handle user authentication only

B. To define database models

C. To create HTML templates

D. To run code before and after each request is processed

Solution

1. Step 1: Understand middleware role

   Middleware runs code around request processing, before and after the main handler.

2. Step 2: Identify correct purpose

   Custom middleware is not for database or templates but for request/response handling.

3. Final Answer:

   To run code before and after each request is processed -> Option D

4. Quick Check:

   Middleware = pre/post request code [OK]

Hint: Middleware wraps requests to add extra processing [OK]

Common Mistakes:

• Confusing middleware with database or template code
• Thinking middleware only handles authentication
• Believing middleware runs only after requests

2. Which method must be overridden when creating a custom middleware class in FastAPI?

easy

A. dispatch

B. execute

C. process

D. handle_request

Solution

1. Step 1: Recall FastAPI middleware structure

   Custom middleware classes override the dispatch method to handle requests.

2. Step 2: Match method names

   Only dispatch is the correct method name; others are invalid in FastAPI middleware.

3. Final Answer:

   dispatch -> Option A

4. Quick Check:

   dispatch method overrides middleware behavior [OK]

Hint: dispatch handles request and response in middleware [OK]

Common Mistakes:

• Using incorrect method names like handle_request
• Confusing middleware methods with route handlers
• Forgetting to override dispatch method

3. Given this middleware code snippet, what will be printed when a request is processed?

class LogMiddleware:
    async def dispatch(self, request, call_next):
        print('Before request')
        response = await call_next(request)
        print('After request')
        return response

medium

A. Only Before request printed

B. Before request printed, then After request printed after response

C. Only After request printed

D. No output printed

Solution

1. Step 1: Analyze dispatch method flow

   Print 'Before request' runs before calling next handler, 'After request' runs after awaiting response.

2. Step 2: Understand async call_next behavior

   call_next processes the request and returns response; code after await runs after response is ready.

3. Final Answer:

   Before request printed, then After request printed after response -> Option B

4. Quick Check:

   Print before and after call_next = B [OK]

Hint: Code before await runs first, after await runs last [OK]

Common Mistakes:

• Thinking 'After request' prints before response
• Ignoring async await order
• Assuming only one print runs

4. Identify the error in this custom middleware code:

class MyMiddleware:
    async def dispatch(self, request):
        response = await call_next(request)
        return response

medium

A. Response must be created manually, not awaited

B. dispatch method should not be async

C. Missing call_next parameter in dispatch method

D. Middleware class must inherit from BaseHTTPMiddleware

Solution

1. Step 1: Check dispatch method signature

   dispatch must accept both request and call_next parameters to call next handler.

2. Step 2: Identify missing parameter

   Code lacks call_next parameter, causing runtime error when calling call_next(request).

3. Final Answer:

   Missing call_next parameter in dispatch method -> Option C

4. Quick Check:

   dispatch(request, call_next) required [OK]

Hint: dispatch needs call_next argument to forward requests [OK]

Common Mistakes:

• Omitting call_next parameter
• Making dispatch synchronous
• Not inheriting middleware base class (optional but recommended)

5. You want to create a middleware that adds a custom header 'X-Process-Time' showing how long the request took. Which code snippet correctly implements this?

hard

A. class TimerMiddleware:
async def dispatch(self, request, call_next):
start = time.time()
response = await call_next(request)
process_time = time.time() - start
response.headers['X-Process-Time'] = str(process_time)
return response

B. class TimerMiddleware:
async def dispatch(self, request):
start = time.time()
response = await call_next(request)
response.headers['X-Process-Time'] = str(time.time() - start)
return response

C. class TimerMiddleware:
def dispatch(self, request, call_next):
start = time.time()
response = call_next(request)
response.headers['X-Process-Time'] = str(time.time() - start)
return response

D. class TimerMiddleware:
async def dispatch(self, request, call_next):
response = await call_next(request)
response.headers['X-Process-Time'] = time.time()
return response

Solution

1. Step 1: Check dispatch method signature and async usage

   dispatch must be async and accept request and call_next parameters.

2. Step 2: Verify timing and header addition

   Start time before await call_next, calculate duration after, add header as string.

3. Step 3: Identify correct code snippet

   class TimerMiddleware: async def dispatch(self, request, call_next): start = time.time() response = await call_next(request) process_time = time.time() - start response.headers['X-Process-Time'] = str(process_time) return response correctly implements timing, async, and header addition.

4. Final Answer:

   The code snippet that correctly adds X-Process-Time header -> Option A

5. Quick Check:

   Async dispatch with timing and header = A [OK]

Hint: Measure time before and after await call_next, add header [OK]

Common Mistakes:

• Missing call_next parameter
• Not awaiting call_next
• Adding header before timing calculation
• Using synchronous dispatch method