What is a Middleware?

In web application architecture, middleware is code that runs in a pipeline before or after the actual route handler, applied to every request that matches its scope rather than being duplicated inside each handler individually. Common middleware responsibilities include parsing request bodies, verifying authentication tokens, enforcing rate limits, logging requests, and handling errors uniformly, the kind of concern that's the same everywhere and would be repetitive and error-prone to reimplement per endpoint. More broadly, middleware also describes any connective software layer between two systems that otherwise wouldn't speak the same language or trust each other directly, such as an integration layer that translates one system's data format into another's, or a broker that mediates between a legacy system and a modern API. In both senses, the defining trait is the same: middleware doesn't implement the core business logic, it implements the connective and cross-cutting logic around it. Well-designed middleware is composable and ordered deliberately — an auth-check middleware needs to run before a handler that assumes an authenticated user, for instance — and a common source of bugs is middleware ordering that silently changes behavior when reorganized.