Middleware

Software that acts as a bridge between an operating system or database and applications, especially on a network.

Middleware is software that sits between the operating system and application layer, providing services and capabilities that applications use but do not implement themselves. In web development, the term more commonly refers to functions that process requests in a pipeline, executing logic before or after the main request handler.

In web frameworks like Express.js, middleware functions have access to the request and response objects and can modify them, execute code, end the request-response cycle, or pass control to the next middleware in the chain. Common middleware tasks include parsing request bodies, handling CORS headers, logging, and session management.

API gateways function as a form of middleware at the infrastructure level. They intercept API requests and apply processing logic -- such as authentication, rate limiting, request transformation, and response caching -- before forwarding requests to backend services. In serverless API gateways, middleware-like functionality is configured declaratively rather than written as code, allowing teams to compose request processing pipelines without managing server infrastructure.