Why middleware processes requests globally in FastAPI

Introduction

Middleware runs for every request to handle common tasks like logging or security. It works globally so you don't repeat code in each route.

You want to log every request your app receives.

You need to check user authentication on all routes.

You want to add headers to every response.

You want to measure how long each request takes.

You want to handle errors in a central place.

Syntax

FastAPI

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

app = FastAPI()

class CustomMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        # Code before request
        response = await call_next(request)
        # Code after request
        return response

app.add_middleware(CustomMiddleware)

Middleware classes must implement a dispatch method.

The call_next function calls the next step in the request chain.

Examples

This middleware prints the path of every request before continuing.

FastAPI

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

app = FastAPI()

class LogMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        print(f"Request path: {request.url.path}")
        response = await call_next(request)
        return response

app.add_middleware(LogMiddleware)

This middleware adds a custom header to every response.

FastAPI

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

app = FastAPI()

class AddHeaderMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        response = await call_next(request)
        response.headers["X-Custom-Header"] = "MyValue"
        return response

app.add_middleware(AddHeaderMiddleware)

Sample Program

This example adds middleware that measures and prints how long each request takes. It runs for every request globally.

FastAPI

from fastapi import FastAPI
from starlette.middleware.base import BaseHTTPMiddleware
from starlette.requests import Request
from starlette.responses import Response

app = FastAPI()

class TimerMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        import time
        start = time.time()
        response: Response = await call_next(request)
        duration = time.time() - start
        print(f"Request took {duration:.4f} seconds")
        return response

app.add_middleware(TimerMiddleware)

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

Important Notes

Middleware runs before and after every request automatically.

Use middleware for tasks that apply to all routes to avoid repeating code.

Middleware order matters: it runs in the order added.

Summary

Middleware processes requests globally to handle shared tasks.

This avoids repeating code in each route handler.

Middleware runs before and after every request automatically.

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