Middleware

Learn essential concepts of middleware in FastAPI.

What You'll Learn

This section covers advanced FastAPI patterns, including:

• Middleware for request/response processing

• Dependency injection for reusable logic

• Background tasks and async processing

• WebSockets for real-time communication

• CORS and performance optimization

Core Concepts

Advanced patterns enable building production-grade APIs with:

• Request/response middleware hooks

• Cross-cutting concerns (logging, auth, CORS)

• Dependency injection for clean code

• Async task processing

• Real-time bidirectional communication

Middleware Example

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

app = FastAPI()

class TimingMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        start_time = time()
        response = await call_next(request)
        process_time = time() - start_time
        response.headers["X-Process-Time"] = str(process_time)
        return response

app.add_middleware(TimingMiddleware)

@app.get("/items")
async def list_items():
    return {"items": []}

Dependency Injection Pattern

from fastapi import FastAPI, Depends

app = FastAPI()

async def get_query(q: str = ""):
    return q

@app.get("/search")
async def search(query: str = Depends(get_query)):
    return {"search": query}

Real-World Usage

Advanced patterns are essential for:

• **Middleware**: CORS, authentication, logging, request timing

• **Dependency Injection**: Shared business logic, database sessions

• **Background Tasks**: Email sending, report generation, file processing

• **WebSockets**: Chat, real-time notifications, live dashboards

• **Performance**: Async operations, connection pooling, caching

🔑 Key Takeaways

• ✅ Understand the purpose of middleware

• ✅ Know when to apply this pattern

• ✅ Follow best practices consistently

• ✅ Test thoroughly in production scenarios

Next step: Explore the advanced section for production patterns and optimization techniques.