What Is Middleware in Go and How to Write It

Middleware in Go is a function that wraps an http.Handler to modify the request before it reaches the handler or the response after it returns. It implements the http.Handler interface by calling ServeHTTP, allowing you to chain logic like logging, authentication, or compression around your core business logic.

func LoggingMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        start := time.Now()
        next.ServeHTTP(w, r)
        log.Printf("%s %s %v", r.Method, r.URL.Path, time.Since(start))
    })
}

// Usage
http.Handle("/", LoggingMiddleware(http.HandlerFunc(myHandler)))

Middleware is like a security guard or a toll booth that every visitor must pass through before reaching your main office. It lets you perform tasks like checking IDs, logging who entered, or cleaning up the room before the main event happens. You use it to keep your main code clean by moving shared tasks like authentication or error handling into these reusable wrappers.