Why middleware processes requests globally in FastAPI - Visual Breakdown

Concept Flow - Why middleware processes requests globally

Incoming HTTP Request

↓

Middleware Layer 1

↓

Middleware Layer 2

↓

...

↓

Route Handler

↓

Response passes back through Middleware Layers

↓

Client receives Response

Middleware acts like a global checkpoint that every request passes through before reaching route handlers, and responses pass back through it before reaching the client.

Execution Sample

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.middleware("http")
async def simple_middleware(request, call_next):
    response = await call_next(request)
    return response

This middleware intercepts every HTTP request globally before it reaches any route handler.

Execution Table

Step	Action	Request State	Middleware Called	Next Called	Response State
1	Request arrives	Raw HTTP request	No	No	No
2	Middleware intercepts	Raw HTTP request	Yes	No	No
3	Middleware calls next	Passed to next layer or route	Yes	Yes	No
4	Route handler processes	Request data available	No	No	Response created
5	Response returns through middleware	Response ready	Yes	No	Response modified or passed
6	Response sent to client	Response ready	No	No	Response sent

💡 Request fully processed after passing through middleware and route handler; response sent back to client.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
request	Raw HTTP request	Intercepted by middleware	Passed to next	Received by route	Returning response	Response sent
response	None	None	None	Created by route	Passed through middleware	Sent to client

Key Moments - 2 Insights

Why does middleware run for every request instead of just some?

Middleware is designed to process all requests globally, as shown in execution_table steps 2 and 3, so it can apply common logic like logging or security before any route handles the request.

Does middleware run before or after the route handler?

Middleware runs both before and after the route handler. It intercepts the request first (step 2), calls the next handler (step 3), then processes the response on the way back (step 5).

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the middleware call the next handler?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Check the 'Next Called' column in execution_table row for step 3.

According to variable_tracker, what is the state of 'response' after step 4?

A None

B Passed through middleware

C Created by route

D Sent to client

💡 Hint

Look at the 'response' row and the 'After Step 4' column in variable_tracker.

If middleware did not call 'call_next', what would happen to the request?

A Request would be processed normally

B Request would never reach route handler

C Request would skip middleware

D Response would be sent twice

💡 Hint

Refer to execution_table steps 2 and 3 where middleware calls next to pass request forward.

Concept Snapshot

Middleware in FastAPI runs globally for every request.
It intercepts requests before route handlers.
It must call 'call_next' to pass control.
It can modify requests and responses.
Acts like a global checkpoint for all HTTP traffic.

Full Transcript

In FastAPI, middleware processes every incoming HTTP request globally before it reaches any route handler. The request first arrives and is intercepted by middleware, which can perform actions like logging or security checks. Middleware then calls the next handler in line, which could be another middleware or the route handler itself. After the route handler creates a response, it passes back through the middleware, which can modify it before sending it to the client. This global processing ensures consistent behavior for all requests. The key is that middleware must call 'call_next' to continue the request flow; otherwise, the request stops there and never reaches the route handler.

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