Middleware

Introduction#

In Go Middleware can be used to execute code before and after handler function. It uses the power of Single Function Interfaces. Can be introduced at any time without affecting the other middleware. For Ex: Authentication logging can be added in later stages of development without disturbing the existing code.

Remarks#

The Signature of middleware should be (http.ResponseWriter, *http.Request) i.e. of http.handlerFunc type.

Normal Handler Function

func loginHandler(w http.ResponseWriter, r *http.Request) {
            // Steps to login
}


func main() {
    http.HandleFunc("/login", loginHandler)
    http.ListenAndServe(":8080", nil)
}

Middleware Calculate time required for handlerFunc to execute

// logger middlerware that logs time taken to process each request
func Logger(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        startTime := time.Now()
        h.ServeHttp(w,r)
        endTime := time.Since(startTime)
        log.Printf("%s %d %v", r.URL, r.Method, endTime)
    })
}

func loginHandler(w http.ResponseWriter, r *http.Request) {
            // Steps to login
}


func main() {
    http.HandleFunc("/login", Logger(loginHandler))
    http.ListenAndServe(":8080", nil)
}

CORS Middleware

func CORS(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        origin := r.Header.Get("Origin")
        w.Header().Set("Access-Control-Allow-Origin", origin)
        if r.Method == "OPTIONS" {
            w.Header().Set("Access-Control-Allow-Credentials", "true")
            w.Header().Set("Access-Control-Allow-Methods", "GET,POST")
            
            w.RespWriter.Header().Set("Access-Control-Allow-Headers", "Content-Type, X-CSRF-Token, Authorization")
            return
        } else {
            h.ServeHTTP(w, r)
        }
    })
}

func main() {
    http.HandleFunc("/login", Logger(CORS(loginHandler)))
    http.ListenAndServe(":8080", nil)
}

Auth Middleware

func Authenticate(h http.Handler) http.Handler {
    return CustomHandlerFunc(func(w *http.ResponseWriter, r *http.Request) {
        // extract params from req
        // post params | headers etc
        if CheckAuth(params) {
            log.Println("Auth Pass")
            // pass control to next middleware in chain or handler func
            h.ServeHTTP(w, r)
        } else {
            log.Println("Auth Fail")
            // Responsd Auth Fail
        }
    })
}

Recovery Handler to prevent server from crashing

func Recovery(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request){
        defer func() {
            if err := recover(); err != nil {
                // respondInternalServerError
            }
        }()
        h.ServeHTTP(w , r)
    })
}