Why middleware processes requests globally in FastAPI - Performance Evidence

Performance: Why middleware processes requests globally

MEDIUM IMPACT

Middleware affects the time it takes for every request to be processed before reaching route handlers, impacting overall request latency.

Adding logging or authentication checks to requests

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.middleware("http")
async def fast_middleware(request, call_next):
    # Non-blocking async operation example
    response = await call_next(request)
    return response

Avoids blocking calls and runs lightweight code, minimizing delay on all requests.

📈 Performance GainReduces request delay to near zero, improving INP and user experience.

Adding logging or authentication checks to requests

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.middleware("http")
async def slow_middleware(request, call_next):
    import time
    time.sleep(0.1)  # blocking delay
    response = await call_next(request)
    return response

Blocking operations in middleware delay every request, causing slow response times.

📉 Performance CostBlocks rendering for 100ms on every request, increasing INP significantly.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Blocking middleware with sleep	N/A	N/A	Delays response arrival	[X] Bad
Non-blocking async middleware	N/A	N/A	Minimal delay on response	[OK] Good

Rendering Pipeline

Middleware runs before route handlers, so it affects the server's response time which impacts how quickly the browser receives data to render.

→Request Processing

→Response Generation

⚠️ BottleneckMiddleware blocking or slow operations delay the entire request lifecycle.

Core Web Vital Affected

INP

Middleware affects the time it takes for every request to be processed before reaching route handlers, impacting overall request latency.

Optimization Tips

1Middleware runs on every request, so keep it fast and non-blocking.

2Avoid heavy or blocking operations inside middleware to prevent slowing all requests.

3Use async code in middleware to improve server responsiveness and user experience.

Performance Quiz - 3 Questions

Why does middleware affect all requests in FastAPI?

A Because middleware only runs on error responses

B Because middleware runs globally on every request before route handlers

C Because middleware runs after the response is sent

D Because middleware only runs on static files

DevTools: Network

How to check: Open DevTools, go to Network tab, reload page, and check the Time column for request delays.

What to look for: Look for high waiting (TTFB) times indicating slow server response caused by middleware.

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