Middleware

Middleware is software that sits in the middle of a processing pipeline. In web frameworks it usually means functions that run on each request before the final handler, and on the response afterward, handling shared concerns such as logging, authentication or compression.

Why it matters

Middleware keeps cross-cutting logic out of individual routes. Instead of repeating a login check in fifty handlers, one middleware does it once and either passes the request along or stops it with an error. Order matters because each step can change what the next step sees. Express, Django, ASP.NET and Laravel all use the pattern. The word is also used more broadly for software connecting separate systems, such as message brokers.

Example

app.use((req, res, next) => {
  console.log(req.method, req.url);
  next();                       // pass to the next step
});

app.use((req, res, next) => {
  if (!req.headers.authorization) return res.status(401).end();
  next();
});

Common mistakes

• Forgetting to call next() or send a response, so requests hang.
• Putting middleware in the wrong order, such as authentication after the route it should protect.
• Doing slow work in middleware that runs on every request.

Related

• API
• Web server
• Rate limiting
• Design pattern

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