Engineering Craft › Design Patterns

Middleware

A pipeline of functions wrapped around request handling.

Also known as: middleware, request pipeline, Express middleware

Middleware is a pipeline of functions wrapped around request handling. Each function receives the request, can do something before or after the real handler, and then passes control to the next one.

// Express
app.use((req, res, next) => {          // runs for every request
  console.log(req.method, req.url);
  next();                              // hand over to the next step
});

app.use(requireLogin);                 // may stop the chain with a 401
app.get("/orders", listOrders);        // the actual handler

request ─► logging ─► auth ─► parse body ─► handler ─► response
               └──────────── each step can act before and after ────────┘

What it’s used for

Cross-cutting concerns that apply to many routes, so they don’t need to be repeated in each handler:

• Logging and request IDs.
• Authentication and authorization.
• Parsing JSON bodies and cookies.
• CORS headers and compression.
• Rate limiting.
• Error handling and response formatting.

How it works

Each middleware is a function that either:

• calls next() to continue down the chain,
• or ends the request itself (sending a 401, or a cached response).

Order matters. Auth must run before handlers that need it, and error handlers usually go last.

# Python (ASGI-style, simplified)
async def timing(request, call_next):
    start = time.time()
    response = await call_next(request)
    response.headers["X-Time"] = str(time.time() - start)
    return response

Cautions

• Forgetting next() leaves the request hanging.
• Order bugs: a body parser placed after the route never sees the body.
• Too much work in middleware slows every request.
• Hidden behavior: when something odd happens to a request, check the middleware stack (request lifecycle).

The idea is related to the decorator and chain of responsibility patterns.