How to Implement Custom Middleware for Observability in Go

Implement custom observability middleware by wrapping your http.Handler to log request details and timing before and after the handler executes.

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))
    })
}

Apply it to your server using http.Handle or mux.Use:

mux := http.NewServeMux()
mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("Hello"))
})
http.ListenAndServe(":8080", LoggingMiddleware(mux))

Middleware is a function that sits between your web server and your actual code to inspect or modify requests and responses. You use it to add features like logging, authentication, or metrics without changing your core business logic. Think of it like a security guard at a building entrance who checks IDs and logs who enters before letting them see the main office.