Understanding Mux and Handler by Writing Your Own RESTful Mux

All examples of code can be found here.

In this article, we will try to understand two crucial Go concepts - mux and Handler. But before we start writing any code let us first understand what we need to run a simple web service.

• First of all, we need a server itself that will run on some port, listening for the requests and providing responses for those requests.
• The next thing we need is a router. This entity is responsible for routing requests to corresponding handlers. In the Go world, servemux is actually an analog of a router.
• The last thing we need is a handler. It is responsible for processing a request, executing the business logic of your application, and providing a response for it.

Implementing a Handler Interface

Let’s start our journey with pure Go code, no libraries or frameworks. To run a server, we need to implement Handler interface:

type Handler interface{
   ServeHTTP(ResponseWriter, *Request)
}

To achieve that we need to create an empty struct and provide a method for it:

package main

import (
    "fmt"
    "net/http"
)

type handler struct{} 

func (t *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) { 
    fmt.Fprintf(w, "ping %s\n", r.URL.Query().Get("name"))
}

func main() {
    h := &handler{} //1
    http.Handle("/", h) //3
    http.ListenAndServe(":8000", nil) //4

}

1. We creating an empty struct that implements http.Handler interface
2. http.Handle will register our handler for a given pattern, in our case, it is “/”. Why pattern and not URI? Because under the hood when your server is running and getting any request - it will find the closest pattern to the request path and dispatch the request to the corresponding handler. That means that if you will try to call 'http://localhost:8000/some/other/path?value=foo' it will still be dispatched to our registered handler, even if it is registered under the “/” pattern.
3. On the last line with http.ListenAndServe we starting server on the port 8000. Keep in mind the second argument, which is nil for now, but we will consider it in detail in a few moments.

Let us check how it works using curl:

❯ curl "localhost:8000?name=foo"
ping foo

Using handler function

The next example looks very similar to the previous one, but it is a little bit shorter and easier to develop. We don’t need to implement any interface, just to create a function with has a similar signature as Handler.ServeHTTP

package main

import ( 
    "fmt"
    "net/http" 
)

func handler(w http.ResponseWriter, r *http.Request) { 
    fmt.Fprintf(w, "ping %s\n", r.URL.Query().Get("name"))
}

func main() {
    http.HandleFunc("/", handler) 
    http.ListenAndServe(":8000", nil) 
}

Notice that under the hood it is just syntax sugar for avoiding the creation of Handler instances on each pattern. HandleFunc is an adapter that converts it to a struct with serveHTTP method. So instead of this:

type root struct{} 
func (t *root) ServeHTTP(w http.ResponseWriter, r *http.Request) {...}

type home struct{} 
func (t *home) ServeHTTP(w http.ResponseWriter, r *http.Request) {...}

type login struct{} 
func (t *login) ServeHTTP(w http.ResponseWriter, r *http.Request) {...}
//...
http.Handle("/", root)
http.Handle("/home", home)
http.Handle("/login", login)

We can just use this approach:

func root(w http.ResponseWriter, r *http.Request) {...}
func home(w http.ResponseWriter, r *http.Request) {...}
func login(w http.ResponseWriter, r *http.Request) {...}
...
http.HandleFunc("/", root)
http.HandleFunc("/home", home)
http.HandleFunc("/login", login)

Creating own ServeMux

Remember we passed nil to http.ListenAndServe? Well, under the hood http package will use default ServeMux and bind handlers to it with http.Handle and http.HandleFunc . In production, it is not a good pattern to use default serveMux because it is a global variable thus any package can access it and register a new router or something worse. So let’s create our own serveMux. To do this we will use http.NewServeMux function. It returns an instance of ServeMux which also got Handle and HandleFunc methods.

mux := http.NewServeMux()
mux.HandleFunc("/", handlerFunc)
http.ListenAndServe(":8000", mux)

One interesting thing here is that our mux is a Handler too. Signature of http.ListenAndServe waiting for a Handler as a second argument and after receiving a request our HTTP server will call serveHTTP method of our mux which in turn call serveHTTP method of registered handlers.

So, it is not obligate to provide a mux from http.NewServeMux() . To understand this let’s create our own instance of a router.

package custom_router

import (
    "fmt"
    "net/http"
)

type router struct{}

func (r *router) ServeHTTP(w http.ResponseWriter, req *http.Request) {
    switch req.URL.Path {
    case "/foo":
        fmt.Fprint(w, "here is /foo")
    case "/bar":
        fmt.Fprint(w, "here is /bar")
    case "/baz":
        fmt.Fprint(w, "here is /baz")
    default:
        http.Error(w, "404 Not Found", 404)
    }
}

func main() {
    var r router
    http.ListenAndServe(":8000", &r)
}

And let’s check this:

❯ curl "localhost:8000/foo"
here is /foo

Keep in mind that servemux, provided by Go will process each request in a separate goroutine. You may try to implement a custom router acting like a Go mux using goroutines per request.

Creating own server

What will happen if a customer will call our endpoint which needs to get info from the DB, but DB is not responding for a long time? Will the customer wait for the response? If so, probably it is not so user-friendly API. So, to avoid this situation and provide a response after some time of waiting we may instantiate http.Server, which has ListenAndServe function. In production, we often need to tune our server, e.g. provide a non-standard logger or set timeouts.

srv := &http.Server{
   Addr:":8000",
   Handler: mux,
     // ReadTimeout is the maximum duration for reading the entire
   // request, including the body.
     ReadTimeout: 5 * time.Second,
     // WriteTimeout is the maximum duration before timing out
   // writes of the response.
     WriteTimeout: 2 * time.Second,
}
srv.ListenAndServe()

Behaviour of timeouts might seems not obvious. What will happen when the timeout will be exceeded? Will the request be forced to quit or responded to? Let’s create a handler that will sleep for 2 seconds and take a look at how our server will behave with WriteTimeout for 1 second.

func handler(w http.ResponseWriter, r *http.Request) {
    time.Sleep(2 * time.Second)
    fmt.Fprintf(w, "ping %s\n", r.URL.Query().Get("name"))
}

func main() {
    mux := http.NewServeMux()
    mux.HandleFunc("/", handler)
    srv := &http.Server{
        Addr:    ":8000",
        Handler: mux,
        WriteTimeout: 1 * time.Second,
    }
    srv.ListenAndServe()
}

Now let’s call curl with the help of the time util to measure how much time will take our request.

❯ time curl "http://localhost:8000?name=foo"
curl: (52) Empty reply from server
curl "http://localhost:8000?name=foo"  0.00s user 0.01s system 0% cpu 2.022 total

We see no reply from the server and request took 2 seconds instead of 1. This is because timeouts are just a mechanism that restricts specific actions after timeout was exceeded. In our case, writing anything to the response was restricted after 1 second has passed. I provided 2 links to awesome articles at the end.

And still, we have an open question: how to force our handler to quit after some period of time?

To achieve that we can simply use http method TimeoutHandler:

func TimeoutHandler(h Handler, dt time.Duration, msg string) Handler

Let’s rewrite our example with timeout handler. Don’t forget to increase time.sleep and timeout for +1 sec each, otherwise there will be still no response:

func handler(w http.ResponseWriter, r *http.Request) {
    time.Sleep(3 * time.Second)
    fmt.Fprintf(w, "ping %s\n", r.URL.Query().Get("name"))
}

func main() {
    mux := http.NewServeMux()
    mux.Handle("/", http.TimeoutHandler(http.HandlerFunc(handler), time.Second * 1, "Timeout"))
    srv := &http.Server{
        Addr:    ":8000",
        Handler: mux,
        WriteTimeout: 2 * time.Second,
    }
    srv.ListenAndServe()
}

Now our curl works exactly as we expected:

❯ time curl "http://localhost:8000?name=foo"
Timeoutcurl "http://localhost:8000?name=foo"  0.01s user 0.01s system 1% cpu 1.022 total

RESTful routing

Servemux provided by Go hasn’t some convenient way to support HTTP methods. Of course we always can achieve the same result using *http.Request, which contains all required information about the response, including HTTP method:

package main

import"net/http"

func createUser(w http.ResponseWriter, r *http.Request) {
    if r.Method != http.MethodPost {
        http.Error(w, "Method Not Allowed", 405)
    }
    w.Write([]byte("New user has been created"))
}

func main() {
   mux := http.NewServeMux()
   mux.HandleFunc("/users", createUser)
   http.ListenAndServe(":3000", mux)
}

Custom RESTful router

Now let’s try to make something more interesting. We need our router to be able to register handlers using handleFunc method which will accept 3 parameters:

• method string,
• pattern string,
• f func(w http.ResponseWriter, req *http.Request)

To achieve that we need to write a small bunch of code :)

Let’s start with types. First we need a router itself. It should have a map that will store registered URL pattern(e.g. /users ) and all information (or rules) that we want to apply to it:

type urlPattern string

type router struct {
    routes map[urlPattern]routeRules
}

func New() *router {
    return &router{routes: make(map[urlPattern]routeRules)}
}

Next let’s define what is a routeRules. In case of REST we want to store registered HTTP methods and related handlers:

type httpMethod string

type routeRules struct {
    methods map[httpMethod]http.Handler
}

Now we want our router to has a HandleFunc method:

/*
    method - string, e.g. POST, GET, PUT
    pattern - URL path for which we want to register a handler
    f - handler 
*/
func (r *router) HandleFunc(method httpMethod, pattern urlPattern, f func(w http.ResponseWriter, req *http.Request)) {
    rules, exists := r.routes[pattern]
    if !exists {
        rules = routeRules{methods: make(map[httpMethod]http.Handler)}
        r.routes[pattern] = rules
    }
    rules.methods[method] = http.HandlerFunc(f)
}

The last thing we need is that our router should implement Handler interface. So we need to implement ServeHTTP(w http.ResponseWriter, req *http.Request) method:

func (r *router) ServeHTTP(w http.ResponseWriter, req *http.Request) {
    // first we will try to find a registered URL pattern
    foundPattern, exists := r.routes[urlPattern(req.URL.Path)]
    if !exists {
        http.NotFound(w, req)
        return
    }
    // next we will try to check if such HTTP method was registered
    handler, exists := foundPattern.methods[httpMethod(req.Method)]
    if !exists {
        notAllowed(w, req, foundPattern)
        return
    }
    // finally we will call registered handler
    handler.ServeHTTP(w, req)
}

// small helper method
func notAllowed(w http.ResponseWriter, req *http.Request, r routeRules) {
    methods := make([]string, 1)
    for k := range r.methods {
        methods = append(methods, string(k))
    }
    w.Header().Set("Allow", strings.Join(methods, " "))
    http.Error(w, "Method Not Allowed", http.StatusMethodNotAllowed)
}

And that’s it. Now let’s register simple handler:

func handler(w http.ResponseWriter, req *http.Request) {
    w.Write([]byte("hello"))
}

func main() {
    r := New()
    r.HandleFunc(http.MethodGet, "/test", handler)
    http.ListenAndServe(":8000", r)
}

And check how it works:

❯ curl -X GET -i "http://localhost:8000/test"
HTTP/1.1 200 OK
Date: Wed, 13 Jul 2022 14:24:43 GMT
Content-Length: 5
Content-Type: text/plain; charset=utf-8

hello

❯ curl -X POST -i "http://localhost:8000/test"
HTTP/1.1 405 Method Not Allowed
Allow: GET
Content-Type: text/plain; charset=utf-8
X-Content-Type-Options: nosniff
Date: Wed, 13 Jul 2022 14:24:14 GMT
Content-Length: 19

Method Not Allowed

Awesome. We just wrote a router that may easily register a handler for specific HTTP method. The full code looks like:

package main

import (
    "net/http"
    "strings"
)

type httpMethod string
type urlPattern string

type routeRules struct {
    methods map[httpMethod]http.Handler
}

type router struct {
    routes map[urlPattern]routeRules
}

func (r *router) ServeHTTP(w http.ResponseWriter, req *http.Request) {
    foundRoute, exists := r.routes[urlPattern(req.URL.Path)]
    if !exists {
        http.NotFound(w, req)
        return
    }
    handler, exists := foundRoute.methods[httpMethod(req.Method)]
    if !exists {
        notAllowed(w, req, foundRoute)
        return
    }
    handler.ServeHTTP(w, req)
}

func (r *router) HandleFunc(method httpMethod, pattern urlPattern, f func(w http.ResponseWriter, req *http.Request)) {
    rules, exists := r.routes[pattern]
    if !exists {
        rules = routeRules{methods: make(map[httpMethod]http.Handler)}
        r.routes[pattern] = rules
    }
    rules.methods[method] = http.HandlerFunc(f)
}

func notAllowed(w http.ResponseWriter, req *http.Request, r routeRules) {
    methods := make([]string, 1)
    for k := range r.methods {
        methods = append(methods, string(k))
    }
    w.Header().Set("Allow", strings.Join(methods, " "))
    http.Error(w, "Method Not Allowed", http.StatusMethodNotAllowed)
}

func New() *router {
    return &router{routes: make(map[urlPattern]routeRules)}
}

func handler(w http.ResponseWriter, req *http.Request) {
    w.Write([]byte("hello"))
}

func main() {
    r := New()
    r.HandleFunc(http.MethodGet, "/test", handler)
    http.ListenAndServe(":8000", r)
}

Why not use it in prod? Well, because there are several libraries that offer you such features and a bunch more! Restrictions by host header, handling paths with path parameters, query parameters, pattern matching(we implement only exact equals), and a lot more.

Gorilla mux

One of the most popular libraries for that is Gorilla/mux.

❯ go get "github.com/gorilla/mux"

Here is a simple example of a GET handler.

func handler(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "Hello World!")
}

func main() {
    r := mux.NewRouter()
    r.HandleFunc("/test", handler).Methods("GET")
    http.ListenAndServe(":8000", r)
}

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Let’s check this endpoint and see the result:

❯ curl -X GET "http://localhost:8000/test"
Hello World!

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And if we try to send the POST method we will get 405:

❯ curl -X POST -i "http://localhost:8000/test"
HTTP/1.1 405 Method Not Allowed
Date: Wed, 13 Jul 2022 12:54:22 GMT
Content-Length: 0

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Gorilla mux:

https://github.com/gorilla/mux

Timeouts articles:

https://ieftimov.com/posts/make-resilient-golang-net-http-servers-using-timeouts-deadlines-context-cancellation/

https://blog.cloudflare.com/the-complete-guide-to-golang-net-http-timeouts/