Why middleware processes requests globally in FastAPI - Visual Breakdown

Concept Flow - Why middleware processes requests globally

Incoming HTTP Request

↓

Middleware Layer 1

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Middleware Layer 2

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...

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Route Handler

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Response passes back through Middleware Layers

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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.