Why middleware processes requests globally in FastAPI - See It in Action

Understanding Global Request Processing with FastAPI Middleware

📖 Scenario: You are building a simple web API using FastAPI. You want to understand how middleware works by processing every request globally before it reaches any specific route.

🎯 Goal: Create a FastAPI app with middleware that logs every incoming request globally, showing how middleware processes requests for all routes.

📋 What You'll Learn

Create a FastAPI app instance called app

Add a middleware function that logs the path of every incoming request

Use the @app.middleware("http") decorator to define the middleware

Create at least one route /hello that returns a greeting message

💡 Why This Matters

🌍 Real World

Middleware is used in real web apps to handle tasks like logging, authentication, and modifying requests or responses globally.

💼 Career

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

1

Create the FastAPI app instance

Create a FastAPI app instance called app by importing FastAPI and initializing it.

FastAPI

from fastapi import FastAPI

# Create the FastAPI app instance called app
# Your code here

Hint

Import FastAPI and then create an instance named app.

2

Add a middleware function to log requests

Add a middleware function using @app.middleware("http") that logs the request path for every incoming request. The function should accept request and call_next parameters and return the response from call_next(request).

FastAPI

from fastapi import FastAPI

app = FastAPI()

# Add middleware to log request paths
# Your code here

Hint

Use @app.middleware("http") decorator and define an async function with request and call_next parameters.

3

Create a route to test the middleware

Create a GET route /hello using @app.get("/hello") that returns a JSON message {"message": "Hello, world!"}.

FastAPI

from fastapi import FastAPI, Request

app = FastAPI()

@app.middleware("http")
async def log_request_path(request: Request, call_next):
    print(f"Request path: {request.url.path}")
    response = await call_next(request)
    return response

# Create a GET route /hello that returns a greeting message
# Your code here

Hint

Use @app.get("/hello") decorator and return the JSON message inside an async function.

4

Complete the app with middleware processing globally

Ensure the middleware processes requests globally by keeping the middleware function and the route in the same app instance app. This setup shows middleware runs for all requests before route handlers.

FastAPI

from fastapi import FastAPI, Request

app = FastAPI()

@app.middleware("http")
async def log_request_path(request: Request, call_next):
    print(f"Request path: {request.url.path}")
    response = await call_next(request)
    return response

@app.get("/hello")
async def hello():
    return {"message": "Hello, world!"}

# Confirm middleware processes all requests globally
# Your code here

Hint

Keep the middleware and route in the same app instance to ensure global processing.

Practice

1. Why does FastAPI middleware process requests globally across all routes?

easy

A. To run only on specific routes chosen by the developer

B. To handle shared tasks like logging or authentication once for all requests

C. To replace route handlers completely

D. To slow down the application for debugging

Solution

1. Step 1: Understand middleware purpose

   Middleware is designed to run code before and after every request to handle common tasks.

2. Step 2: Recognize global effect

   It applies globally to avoid repeating the same code in each route handler.

3. Final Answer:

   To handle shared tasks like logging or authentication once for all requests -> Option B

4. Quick Check:

   Middleware = shared tasks globally [OK]

Hint: Middleware runs for all requests to avoid code repetition [OK]

Common Mistakes:

• Thinking middleware runs only on selected routes
• Believing middleware replaces route handlers
• Assuming middleware slows down app intentionally

2. Which of the following is the correct way to add middleware globally in FastAPI?

easy

A. app.add_middleware(SomeMiddleware)

B. app.middleware(SomeMiddleware)

C. app.route.middleware(SomeMiddleware)

D. SomeMiddleware(app)

Solution

1. Step 1: Recall FastAPI middleware syntax

   FastAPI uses the method add_middleware() on the app instance to add middleware globally.

2. Step 2: Eliminate incorrect options

   Options A, B and C are invalid: A instantiates middleware without adding it to the app, B and C are invalid method calls.

3. Final Answer:

   app.add_middleware(SomeMiddleware) -> Option A

4. Quick Check:

   Use add_middleware() to add middleware globally [OK]

Hint: Use app.add_middleware() to add middleware globally [OK]

Common Mistakes:

• Using app.middleware() which does not exist
• Trying to add middleware on route instead of app
• Instantiating middleware without adding it to app

3. Given this middleware code in FastAPI:

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

class PrintMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        print("Before request")
        response = await call_next(request)
        print("After request")
        return response

app = FastAPI()
app.add_middleware(PrintMiddleware)

@app.get("/hello")
async def hello():
    return {"message": "Hello"}

What will be printed when a client requests /hello?

medium

A. Before request
After request

B. After request
Before request

C. Only Before request

D. No output printed

Solution

1. Step 1: Understand middleware dispatch flow

   The middleware prints "Before request" before calling the next handler, then "After request" after the response is received.

2. Step 2: Trace the request lifecycle

   When /hello is requested, the middleware prints "Before request", then the route runs, then prints "After request".

3. Final Answer:

   Before request After request -> Option A

4. Quick Check:

   Middleware prints before and after request [OK]

Hint: Middleware prints before and after call_next() [OK]

Common Mistakes:

• Assuming prints happen in reverse order
• Thinking only one print runs
• Believing middleware does not print anything

4. You wrote this middleware but it does not run for any requests:

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

app = FastAPI()
app.add_middleware(MyMiddleware)

What is the likely problem?

medium

A. Middleware must be added after route definitions

B. dispatch method should be synchronous

C. MyMiddleware does not inherit from BaseHTTPMiddleware

D. Middleware class must be decorated with @middleware

Solution

1. Step 1: Check middleware class inheritance

   FastAPI middleware classes must inherit from BaseHTTPMiddleware or implement ASGI interface properly.

2. Step 2: Identify missing inheritance

   MyMiddleware lacks inheritance, so FastAPI cannot use it as middleware.

3. Final Answer:

   MyMiddleware does not inherit from BaseHTTPMiddleware -> Option C

4. Quick Check:

   Middleware must inherit BaseHTTPMiddleware [OK]

Hint: Middleware class must inherit BaseHTTPMiddleware [OK]

Common Mistakes:

• Making dispatch synchronous instead of async
• Adding middleware before routes (order usually doesn't block)
• Thinking @middleware decorator is required for class middleware

5. You want to add middleware that logs request time but only for routes under /api. Why does FastAPI middleware still run on all routes, and how can you limit it?

hard

A. FastAPI does not support middleware; use dependencies instead

B. Middleware can be added only to specific routes by passing route list to add_middleware

C. Middleware runs globally but can be disabled per route with a decorator

D. Middleware runs globally by design; to limit, check path inside middleware and skip non-/api requests

Solution

1. Step 1: Understand middleware scope

   FastAPI middleware always runs globally on every request to handle shared tasks.

2. Step 2: Limit middleware effect by path check

   To restrict middleware to /api routes, check the request URL path inside middleware and skip processing for others.

3. Final Answer:

   Middleware runs globally by design; to limit, check path inside middleware and skip non-/api requests -> Option D

4. Quick Check:

   Middleware global; filter inside middleware [OK]

Hint: Middleware always global; filter requests inside middleware [OK]

Common Mistakes:

• Expecting middleware to attach only to some routes automatically
• Trying to pass routes to add_middleware (not supported)
• Thinking middleware can be disabled per route with decorators