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Chapter 2.3.

Web Application Basics

Now that everything is set up correctly let’s make the first iteration of our web application. We’ll begin with the three absolute essentials:

Let’s put these components together in the main.go file to make a working application.

File: main.go
package main

import (
    "log"
    "net/http"
)

// Define a home handler function which writes a byte slice containing
// "Hello from Snippetbox" as the response body.
func home(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("Hello from Snippetbox"))
}

func main() {
    // Use the http.NewServeMux() function to initialize a new servemux, then
    // register the home function as the handler for the "/" URL pattern.
    mux := http.NewServeMux()
    mux.HandleFunc("/", home)

    // Use the http.ListenAndServe() function to start a new web server. We pass in
    // two parameters: the TCP network address to listen on (in this case ":4000")
    // and the servemux we just created. If http.ListenAndServe() returns an error
    // we use the log.Fatal() function to log the error message and exit.
    log.Println("Starting server on :4000")
    err := http.ListenAndServe(":4000", mux)
    log.Fatal(err)
}

When you run this code, it should start a web server listening on port 4000 of your local machine. Each time the server receives a new HTTP request it will pass the request on to the servemux and — in turn — the servemux will check the URL path and dispatch the request to the matching handler.

Let’s give this a whirl. Save your main.go file and then try running it from your terminal using the go run command.

$ cd $HOME/code/snippetbox
$ go run main.go
2018/08/02 10:08:07 Starting server on :4000

While the server is running, open a web browser and try visiting http://localhost:4000. If everything has gone to plan you should see a page which looks a bit like this:

02.03-01.png

If you head back to your terminal window, you can stop the server by pressing Ctrl+c on your keyboard.

Additional Information

Network Addresses

The TCP network address that you pass to http.ListenAndServe() should be in the format "host:port". If you omit the host (like we did with ":4000") then the server will listen on all your computer’s available network interfaces. Generally, you only need to specify a host in the address if your computer has multiple network interfaces and you want to listen on just one of them.

In other Go projects or documentation you might sometimes see network addresses written using named ports like ":http" or ":http-alt" instead of a number. If you use a named port then Go will attempt to look up the relevant port number from your /etc/services file when starting the server, or will return an error if a match can’t be found.

Using go run

During development the go run command is a convenient way to try out your code. It’s essentially a shortcut that compiles your code, creates an executable binary in your /tmp directory, and then runs this binary in one step.

It accepts either a space-separated list of .go files, the path to a specific package (where the . character represents your current directory), or the full module path. For our application at the moment, the three following commands are all equivalent:

$ go run main.go
$ go run .
$ go run alexedwards.net/snippetbox