Live Reload Go Projects with wgo

Let’s put this in more context with a short example, which you can see below.

package main

import (
	"io"
	"log"
	"net/http"
	"regexp"
)

func main() {
	mux := http.NewServeMux()
	mux.HandleFunc("POST /", func(writer http.ResponseWriter, request *http.Request) {
		err := request.ParseForm()
		if err != nil {
			http.NotFound(writer, request)
		}

		subject := request.PostForm.Get("subject")
		pattern := "^(?i:Ref(?i:erence)? ID: )(?P<refid>[0-9a-zA-Z]{14})$"
		subjectRegex := regexp.MustCompile(pattern)
		if !subjectRegex.MatchString(subject) {
			io.WriteString(writer, "Subject did not match the required format")
			return
		}
		log.Printf("%s matches pattern %s\n", subject, pattern)
		io.WriteString(writer, "Subject matched the required format")
	})

	log.Print("Starting server on :4000")
	err := http.ListenAndServe(":4000", mux)
	log.Fatal(err)
}

The code sets up a small web application that has one route which accepts POST requests. If the POST data contains an attribute named subject, whose value matches the regular expression in pattern, it will set the body of the response to: Subject matched the required format. Otherwise, the response’s body will be set to: Subject did not match the required format.

If you’re not that familiar with regular expressions, don’t worry. subject will match if it starts with either Reference: or Ref: and is followed by a 14 character string containing a combination of lower and uppercase letters, and digits between 0 and 9. For example: Ref ID: a4FGhd321ewqeU.

To test it out, you’d start the app running with go run main.go, and then send a curl request, such as the one below.

curl -X POST \
    -d subject="Ref ID: a4FGhd321ewqeU" \
    http://localhost:4000

You’d then see Subject matched the required format printed to the terminal.

However, if the regular expression or some other aspect of the code was refactored, you’d need to restart the app by pressing Ctrl + C and run go run main.go again.

As I said near the top of the post, that’s fine – at first. But, it rapidly becomes annoying when you’re doing it numerous times a day.

Live Reloading solves this problem

It does so by removing the need to manually stop and restart the app, because it automatically recompiles and restarts the code when it detects a change to a source file.

Live reloading is common in other languages, such as JavaScript with Webpack. There are a number of ways to do it in Go, but the simplest way that I’ve found is by using wgo (watcher-go).

To quote the project repository:

Too many file watchers either force you to wrap your commands into strings, require config files or log tons of noisy output to your stdout. In contrast, wgo is dead simple and silent by default. The implementation is also really short, most of it resides in just two files (wgo_cmd.go and main.go). You can read the entire codebase in one sitting, start here.

To use it is trivial, as all you need to do is use wgo where you would have used the standard Go binary in the past. For example:

wgo run main.go

Keep in mind that, depending on your $PATH configuration, you may have to provide the absolute path to wgo.

So, continuing with the previous example, let’s say that when subject matches the regular expression, you want to include its value in the response’s body. To do that, you’d change the call to io.WriteString() at the end of the route’s handler to the following:

io.WriteString(writer, fmt.Sprintf("Subject [%s] matched the required format", subject))

And, you’d include "fmt" in the import list. Then, when you save the changes, in the terminal tab or window where you ran wgo you’ll see output similar to the following, showing the app being recompiled and restarted.

2024/04/19 17:29:46 Starting server on :4000
2024/04/19 17:29:48 Starting server on :4000
2024/04/19 17:29:50 Starting server on :4000

Now, if you make the same curl request as before:

curl -X POST -d subject="Ref ID: a4FGhd321ewqeU" http://localhost:4000

You’ll see the following response:

Subject [Ref ID: a4FGhd321ewqeU] matched the required format

That’s how to use Live Reloading in Go

By using wgo (watcher-go) – for very little effort – you can save yourself a lot of time and effort restarting Go code to see the effects of the changes you make. It’s almost like you’re using a scripting language with all the benefits of a statically compiled one.

If you’ve not tried it already, I strongly recommend giving it a go at least to see what you think.