Custom middleware creation in FastAPI - Step-by-Step Execution

Concept Flow - Custom middleware creation

Request received by FastAPI

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Middleware intercepts request

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Middleware processes or modifies request

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Pass request to next handler (route or next middleware)

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Response generated by route

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Middleware intercepts response

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Middleware processes or modifies response

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Response sent back to client

Middleware sits between the client and route handlers, intercepting requests and responses to process or modify them.

Execution Sample

FastAPI

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

app = FastAPI()

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

app.add_middleware(CustomMiddleware)

This code creates a middleware that prints messages before and after processing each request.

Execution Table

Step	Action	Request State	Response State	Output/Effect
1	Request received by FastAPI	Request object created	No response yet	Incoming request starts processing
2	Middleware dispatch starts	Request passed to middleware	No response yet	Prints 'Before request'
3	Middleware calls next handler	Request forwarded to route handler	No response yet	Route processes request
4	Route generates response	Request handled	Response object created	Response ready to return
5	Middleware resumes after call_next	Request handled	Response received	Prints 'After response'
6	Middleware returns response	Request handled	Response returned	Response sent to client
7	Request cycle ends	No active request	Response sent	Cycle complete

💡 Response sent back to client, middleware cycle complete

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	Final
request	None	Request object	Request object	Request object	Request handled
response	None	None	None	Response object	Response sent

Key Moments - 3 Insights

Why do we call 'await call_next(request)' inside the middleware?

Because 'call_next' passes the request to the next handler (like the route). Without calling it, the request would stop and no response would be generated. See execution_table step 3.

What happens if we modify the response after 'call_next'?

The middleware can change the response before sending it back to the client. This happens after step 5 in the execution_table, where the middleware can add headers or change content.

Can middleware stop the request from reaching the route?

Yes, if middleware returns a response directly without calling 'call_next', the route is skipped. This is not shown in the example but is important to know.

Visual Quiz - 3 Questions

Look at the execution_table, what is printed first when a request is processed?

A "After response"

B "Response sent"

C "Before request"

D "Request received"

💡 Hint

Check step 2 in the execution_table where middleware starts processing.

At which step does the route handler generate the response?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look for when the response object is created in the execution_table.

If middleware does not call 'call_next', what happens?

A The middleware returns a response directly, skipping the route

B An error occurs automatically

C The request is forwarded to the route

D The response is delayed but eventually sent

💡 Hint

Refer to key_moments about middleware behavior when skipping call_next.

Concept Snapshot

Custom middleware in FastAPI intercepts requests and responses.
Define a class inheriting BaseHTTPMiddleware.
Override async dispatch(request, call_next).
Call 'await call_next(request)' to continue.
Modify request or response as needed.
Add middleware with app.add_middleware().

Full Transcript

Custom middleware in FastAPI acts like a checkpoint between the client and your route handlers. When a request comes in, the middleware's dispatch method runs first. It can do things before the request reaches your route, like logging or modifying the request. Then it calls 'await call_next(request)' to pass the request to the next handler, usually your route. After the route creates a response, the middleware can again process or change the response before sending it back to the client. This lets you add features like logging, headers, or authentication checks in one place. Remember, if you don't call 'call_next', the request won't reach your route and the middleware must return a response directly. This flow ensures you can control how requests and responses move through your app.