Go

HTTP Server

Remarks#

http.ServeMux provides a multiplexer which calls handlers for HTTP requests.

Alternatives to the standard library multiplexer include:

HTTP Hello World with custom server and mux

package main

import (
    "log"
    "net/http"
)

func main() {

    // Create a mux for routing incoming requests
    m := http.NewServeMux()

    // All URLs will be handled by this function
    m.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("Hello, world!"))
    })

    // Create a server listening on port 8000
    s := &http.Server{
        Addr:    ":8000",
        Handler: m,
    }

    // Continue to process new requests until an error occurs
    log.Fatal(s.ListenAndServe())
}

Press Ctrl+C to stop the process.

Hello World

The typical way to begin writing webservers in golang is to use the standard library net/http module.

There is also a tutorial for it here.

The following code also uses it. Here is the simplest possible HTTP server implementation. It responds "Hello World" to any HTTP request.

Save the following code in a server.go file in your workspaces.

package main

import (
    "log"
    "net/http"
)

func main() {
    // All URLs will be handled by this function
    // http.HandleFunc uses the DefaultServeMux
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("Hello, world!"))
    })

    // Continue to process new requests until an error occurs
    log.Fatal(http.ListenAndServe(":8080", nil))
}

You can run the server using:

$ go run server.go

Or you can compile and run.

$ go build server.go
$ ./server

The server will listen to the specified port (:8080). You can test it with any HTTP client. Here’s an example with cURL:

curl -i https://localhost:8080/
HTTP/1.1 200 OK
Date: Wed, 20 Jul 2016 18:04:46 GMT
Content-Length: 13
Content-Type: text/plain; charset=utf-8

Hello, world!

Press Ctrl+C to stop the process.

Using a handler function

HandleFunc registers the handler function for the given pattern in the server mux (router).

You can pass define an anonymous function, as we have seen in the basic Hello World example:

http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintln(w, "Hello, world!")
}

But we can also pass a HandlerFunc type. In other words, we can pass any function that respects the following signature:

func FunctionName(w http.ResponseWriter, req *http.Request)

We can rewrite the previous example passing the reference to a previously defined HandlerFunc. Here’s the full example:

package main

import (
    "fmt"
    "net/http"
)

// A HandlerFunc function
// Notice the signature of the function
func RootHandler(w http.ResponseWriter, req *http.Request) {
    fmt.Fprintln(w, "Hello, world!")
}

func main() {
    // Here we pass the reference to the `RootHandler` handler function
    http.HandleFunc("/", RootHandler)
    panic(http.ListenAndServe(":8080", nil))
}

Of course, you can define several function handlers for different paths.

package main

import (
    "fmt"
    "log"
    "net/http"
)

func FooHandler(w http.ResponseWriter, req *http.Request) {
    fmt.Fprintln(w, "Hello from foo!")
}

func BarHandler(w http.ResponseWriter, req *http.Request) {
    fmt.Fprintln(w, "Hello from bar!")
}

func main() {
    http.HandleFunc("/foo", FooHandler)
    http.HandleFunc("/bar", BarHandler)

    log.Fatal(http.ListenAndServe(":8080", nil))
}

Here’s the output using cURL:

➜  ~ curl -i localhost:8080/foo
HTTP/1.1 200 OK
Date: Wed, 20 Jul 2016 18:23:08 GMT
Content-Length: 16
Content-Type: text/plain; charset=utf-8

Hello from foo!

➜  ~ curl -i localhost:8080/bar
HTTP/1.1 200 OK
Date: Wed, 20 Jul 2016 18:23:10 GMT
Content-Length: 16
Content-Type: text/plain; charset=utf-8

Hello from bar!

➜  ~ curl -i localhost:8080/
HTTP/1.1 404 Not Found
Content-Type: text/plain; charset=utf-8
X-Content-Type-Options: nosniff
Date: Wed, 20 Jul 2016 18:23:13 GMT
Content-Length: 19

404 page not found

Create a HTTPS Server

Generate a certificate

In order to run a HTTPS server, a certificate is necessary. Generating a self-signed certificate with openssl is done by executing this command:

openssl req -x509 -newkey rsa:4096 -sha256 -nodes -keyout key.pem -out cert.pem -subj "/CN=example.com" -days 3650`

The parameters are:

  • req Use the certificate request tool
  • x509 Creates a self-signed certificate
  • newkey rsa:4096 Creates a new key and certificate by using the RSA algorithms with 4096 bit key length
  • sha256 Forces the SHA256 hashing algorithms which major browsers consider as secure (at the year 2017)
  • nodes Disables the password protection for the private key. Without this parameter, your server had to ask you for the password each time its starts.
  • keyout Names the file where to write the key
  • out Names the file where to write the certificate
  • subj Defines the domain name for which this certificate is valid
  • days Fow how many days should this certificate valid? 3650 are approx. 10 years.

Note: A self-signed certificate could be used e.g. for internal projects, debugging, testing, etc. Any browser out there will mention, that this certificate is not safe. In order to avoid this, the certificate must signed by a certification authority. Mostly, this is not available for free. One exception is the “Let’s Encrypt” movement: https://letsencrypt.org

The necessary Go code

You can handle configure TLS for the server with the following code. cert.pem and key.pem are your SSL certificate and key, which where generated with the above command.

package main

import (
    "log"
    "net/http"
)

func main() {
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("Hello, world!"))
    })

    log.Fatal(http.ListenAndServeTLS(":443","cert.pem","key.pem", nil))
}

Responding to an HTTP Request using Templates

Responses can be written to a http.ResponseWriter using templates in Go. This proves as a handy tool if you wish to create dynamic pages.

(To learn how Templates work in Go, please visit the Go Templates Documentation page.)

Continuing with a simple example to utilise the html/template to respond to an HTTP Request:

package main

import(
    "html/template"
    "net/http"
    "log"
)

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

type User struct{
    Name string
    nationality string //unexported field.
}

func check(err error){
    if err != nil{
        log.Fatal(err)
    }
}

func WelcomeHandler(w http.ResponseWriter, r *http.Request){
    if r.Method == "GET"{
        t,err := template.ParseFiles("welcomeform.html")
        check(err)
        t.Execute(w,nil)
    }else{
        r.ParseForm()
        myUser := User{}
        myUser.Name = r.Form.Get("entered_name")
        myUser.nationality = r.Form.Get("entered_nationality")
        t, err := template.ParseFiles("welcomeresponse.html")
        check(err)
        t.Execute(w,myUser)
    }
}

Where, the contents of

  1. welcomeform.html are:

    Help us greet you
    Enter Name: Enter Nationality:

  2. welcomeresponse.html are:

    Greetings, {{.Name}} Greetings, {{.Name}}.
    We know you are a {{.nationality}}!

Note:

  1. Make sure that the .html files are in the correct directory.

  2. When https://localhost:8080/ can be visited after starting the server.

  3. As it can be seen after submitting the form, the unexported nationality field of the struct could not be parsed by the template package, as expected.

Serving content using ServeMux

A simple static file server would look like this:

package main

import (
    "net/http"
)

func main() {
    muxer := http.NewServeMux()
    fileServerCss := http.FileServer(http.Dir("src/css"))
    fileServerJs := http.FileServer(http.Dir("src/js"))
    fileServerHtml := http.FileServer(http.Dir("content"))
    muxer.Handle("/", fileServerHtml)
    muxer.Handle("/css", fileServerCss)
    muxer.Handle("/js", fileServerJs)
    http.ListenAndServe(":8080", muxer)
}

Handling http method, accessing query strings & request body

Here are a simple example of some common tasks related to developing an API, differentiating between the HTTP Method of the request, accessing query string values and accessing the request body.

Resources

  • http.Handler interface

  • http.ResponseWriter

  • http.Request

  • Available Method and Status constants

    package main

    import ( “fmt” “io/ioutil” “log” “net/http” )

    type customHandler struct{}

    // ServeHTTP implements the http.Handler interface in the net/http package func (h customHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {

     // ParseForm will parse query string values and make r.Form available
 r.ParseForm()

 // r.Form is map of query string parameters
 // its' type is url.Values, which in turn is a map[string][]string
 queryMap := r.Form

 switch r.Method {
 case http.MethodGet:
     // Handle GET requests
     w.WriteHeader(http.StatusOK)
     w.Write([]byte(fmt.Sprintf("Query string values: %s", queryMap)))
     return
 case http.MethodPost:
     // Handle POST requests
     body, err := ioutil.ReadAll(r.Body)
     if err != nil {
         // Error occurred while parsing request body
         w.WriteHeader(http.StatusBadRequest)
         return
     }
     w.WriteHeader(http.StatusOK)
     w.Write([]byte(fmt.Sprintf("Query string values: %s\nBody posted: %s", queryMap, body)))
     return
 }

 // Other HTTP methods (eg PUT, PATCH, etc) are not handled by the above
 // so inform the client with appropriate status code
 w.WriteHeader(http.StatusMethodNotAllowed)

    }

    func main() { // All URLs will be handled by this function // http.Handle, similarly to http.HandleFunc // uses the DefaultServeMux http.Handle(”/”, customHandler{})

     // Continue to process new requests until an error occurs
 log.Fatal(http.ListenAndServe(":8080", nil))

    }

Sample curl output:

$ curl -i 'localhost:8080?city=Seattle&state=WA' -H 'Content-Type: text/plain' -X GET
HTTP/1.1 200 OK
Date: Fri, 02 Sep 2016 16:36:24 GMT
Content-Length: 51
Content-Type: text/plain; charset=utf-8

Query string values: map[city:[Seattle] state:[WA]]%

$ curl -i 'localhost:8080?city=Seattle&state=WA' -H 'Content-Type: text/plain' -X POST -d "some post data"
HTTP/1.1 200 OK
Date: Fri, 02 Sep 2016 16:36:35 GMT
Content-Length: 79
Content-Type: text/plain; charset=utf-8

Query string values: map[city:[Seattle] state:[WA]]
Body posted: some post data%

$ curl -i 'localhost:8080?city=Seattle&state=WA' -H 'Content-Type: text/plain' -X PUT
HTTP/1.1 405 Method Not Allowed
Date: Fri, 02 Sep 2016 16:36:41 GMT
Content-Length: 0
Content-Type: text/plain; charset=utf-8
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