dbmusings

middleware wrapping in go

2 min read

Middlewares are used for tasks which are shared across your app handlers like

validating the incoming request, logging,
authentication, rate-limiting, etc.

Router → Middleware Handler → Application Handler

In case you have such a list of middlewares, you can wrap/chain them with your server handler.

Below is a small example of middleware wrapping.

Live demo here

middleware_wrapping.go
type MyServer struct {
	config string
}
 
func firstMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		fmt.Println("first middleware!")
		next.ServeHTTP(w, r)
	})
}
 
func secondMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		fmt.Println("second middleware!")
		next.ServeHTTP(w, r)
	})
}
 
func (s *MyServer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(http.StatusOK)
	fmt.Fprintf(w, fmt.Sprintf("%s from backend server!", s.config))
}
 
func main() {
 
	handler := &MyServer{
		config: "hello",
	}
 
	go func(s *MyServer) {
		http.Handle("/hello", firstMiddleware(secondMiddleware(s)))
		err := http.ListenAndServe(":8080", nil)
		if err != nil {
			fmt.Printf("Error for lb server on :8080; %s\n", err.Error())
			return
		}
	}(handler)
 
	serverAddress := "http://localhost:8080/hello"
	req, _ := http.NewRequest("GET", serverAddress, nil)
	resp, err := http.DefaultClient.Do(req)
 
	if err != nil {
		fmt.Printf("error: %+v in calling lb", err)
		return
	}
 
	defer resp.Body.Close()
	bytes, _ := io.ReadAll(resp.Body)
	fmt.Printf("server resp : %s", string(bytes))
}

The output being :

first middleware!
second middleware!
server resp: hello from backend server!

Notice the sequence of execution. The call gets resolved based on the ordering of the

middlewares (left to right).