'Hello, World!' in Golang - How to Develop a Simple Web App in Go

The Go programming language, often referred to as "golang", has gained a lot of well-deserved traction in the DevOps community. Many of the most popular tools such as Docker, Kubernetes, and Terraform are written in Go, but it can also be a great choice for building web applications and APIs.

Go provides you with all the speed and performance of a compiled language, but feels like coding in an interpreted language. This comes down to the great tooling that you get out of the box with a compiler, runner, test suite and code formatter provided by default.

Add to that the robust and easily comprehensible approach to concurrency, to take maximum advantage of today's multi-core or multi-cpu execution environments, and it's clear why the Go community has grown so rapidly.

Go feels like it was created with specific goals in mind, leading to a deceptively simple language design that's straightforward to learn, without compromising on capabilities.

In this post, I'm going to show you how easy it is to develop a simple web application in Go, package it as a lightweight Docker image, and deploy it to Heroku. I'll also show a fairly new feature of Go: built-in package management.

Go Modules

I'm going to use Go's built-in module support for this article.

Go 1.0 was released in March 2012. Up until version 1.11 (released in August 2018), developing Go applications involved managing a GOPATH for each "workspace", analogous to java's

JAVA_HOME

, and all of your Go source code and any third-party libraries were stored below the

GOPATH

I always found this a bit off-putting, compared to developing code in languages like Ruby or Javascript where I could have a simpler directory structure isolating each project. In both of those languages, a single file (

Gemfile

for Ruby,

package.json

for Javascript) lists all the external libraries, and the package manager keeps track of managing and installing dependencies for me.

I'm not saying you can't manage the
GOPATH
environment variable to isolate projects from one another. I particularly find the package manager approach is easier.

Thankfully, Go now has excellent package management built in, so this is no longer a problem. However, you might find

GOPATH

mentioned in many older blog posts and articles, which can be a little confusing.

Hello, World!

Let's get started on our web application. As usual, this is going to be a very simple "Hello, World!" app, because I want to focus on the development and deployment process, and keep this article to a reasonable length.

Pre-requisites

You'll need:

A recent version of golang (I'm using 1.14.9.)
Docker
A Heroku account (The free account works fine for this example.)
The Heroku command-line client
git

Go mod init

To create our new project, we need to create a directory for it, and use the

go mod init

command to initialize it as a Go module.

mkdir helloworld
cd helloworld
go mod init digitalronin/helloworld

It's common practice to use your github username to keep your project names globally unique, and avoid name conflicts with any of your project dependencies, but you can use any name you like.

You'll see a

go.mod

file in the directory now. This is where Go will track any project dependencies. If you look at the contents of the file, they should look something like this:

module digitalronin/helloworld

go 1.14

Let's start committing our changes:

git init
git add *
git commit -m "Initial commit"

gin

We're going to use gin for our web application. Gin is a lightweight web framework, similar to Sinatra for Ruby, express.js for Javascript, or Flask for Python.

Create a file called

hello.go

containing this code:

package main

import "github.com/gin-gonic/gin"

func main() {
	r := gin.Default()

	r.GET("/hello", func(c *gin.Context) {
		c.String(200, "Hello, World!")
	})

	r.Run(":3000")
}

Let's break this down a little:

r := gin.Default()

This creates a router object,

, using the built-in defaults that come with gin.

Then, we assign a handler function to be called for any HTTP GET requests to the path

/hello

, and to return the string "Hello, World!" and a 200 (HTTP OK) status code:

	r.GET("/hello", func(c *gin.Context) {
		c.String(200, "Hello, World!")
	})

Finally, we start our webserver and tell it to listen on port 3000:

r.Run(":3000")

To run this code, execute:

go run hello.go

You should see output like this:

go: finding module for package github.com/gin-gonic/gin
go: found github.com/gin-gonic/gin in github.com/gin-gonic/gin v1.6.3
[GIN-debug] [WARNING] Creating an Engine instance with the Logger and Recovery middleware already attached.

[GIN-debug] [WARNING] Running in "debug" mode. Switch to "release" mode in production.
 - using env:   export GIN_MODE=release
 - using code:  gin.SetMode(gin.ReleaseMode)

[GIN-debug] GET    /hello                    --> main.main.func1 (3 handlers)
[GIN-debug] Listening and serving HTTP on :3000

Now if you visit

http://localhost:3000/hello

in your web browser, you should see the message "Hello, World!"

Notice that we didn't have to install gin separately, or even edit our

go.mod

file to declare it as a dependency. Go figures that out and makes the necessary changes for us, which is what's happening when we see these lines in the output:

go: finding module for package github.com/gin-gonic/gin
go: found github.com/gin-gonic/gin in github.com/gin-gonic/gin v1.6.3

If you look at the

go.mod

file, you'll see it now contains this:

module digitalronin/helloworld

go 1.14

require github.com/gin-gonic/gin v1.6.3 // indirect

You will also see a

go.sum

file now. This is a text file containing references to the specific versions of all the package dependencies, and their dependencies, along with a cryptographic hash of the contents of that version of the relevant module.

The

go.sum

file serves a similar function to

package-lock.json

for a Javascript project, or

Gemfile.lock

in a Ruby project, and you should always check it into version control along with your source code.

Let's do that now:

git add *
git commit -m "Add 'Hello world' web server"

Serving HTML and JSON

I'm not going very far into what you can build with gin, but I do want to demonstrate a little more of its functionality. In particular, sending JSON responses and serving static files.

Let's look at JSON responses first. Add the following code to your

hello.go

file, right after the

r.GET

block:

api := r.Group("/api")

api.GET("/ping", func(c *gin.Context) {
  c.JSON(200, gin.H{
    "message": "pong",
  })
})

Here we're creating a "group" of routes behind the path

/api

with a single path

/ping

which will return a JSON response.

With this code in place, run the server with

go run

and then hit the new API endpoint:

curl http://localhost:3000/api/ping

You should get the response:

{"message":"pong"}

Finally, let's make our webserver serve static files. Gin has an additional library for this.

Change the import block at the top of the

hello.go

file to this:

import (
    "github.com/gin-gonic/contrib/static"
    "github.com/gin-gonic/gin"
)

The most popular code editors have golang support packages you can install which will take care of the
import
declarations for you automatically, updating them for you whenever you use a new module in your code.

Then, add this line inside the

main

function:

r.Use(static.Serve("/", static.LocalFile("./views", true)))

The full code for our web application now looks like this:

hello.go

package main

import (
	"github.com/gin-gonic/contrib/static"
	"github.com/gin-gonic/gin"
)

func main() {
	r := gin.Default()

	r.GET("/hello", func(c *gin.Context) {
		c.String(200, "Hello, World!")
	})

	api := r.Group("/api")

	api.GET("/ping", func(c *gin.Context) {
		c.JSON(200, gin.H{
			"message": "pong",
		})
	})

	r.Use(static.Serve("/", static.LocalFile("./views", true)))

	r.Run()
}

The

r.Use(static.Serve...

line enables our webserver to serve any static files from the

views

directory, so let's add a few:

mkdir -p views/css

views/css/stylesheet.css

body {
  font-family: Arial;
}

h1 {
  color: red;
}

views/index.html

<html>
  <head>
    <link rel="stylesheet" href="/css/stylesheet.css" />
  </head>
  <body>
    <h1>Hello, World!</h1>
  </body>
</html>

Now restart the webserver using

go run hello.go

and visit

http://localhost:3000

and you should see the styled message:

Dockerize

We've written our go web application, now let's package it up as a Docker image. We could create it as a Heroku buildpack, but one of the nice features of Go is that you can distribute your software as a single binary file. This is an area where Go really shines, and using a Docker-based Heroku deployment lets us take advantage of that. Also, this technique isn't limited to Go applications: You can use Docker-based deployment to Heroku for projects in any language. So, it's a good technique to understand.

So far, we've been running our code with the

go run

command. To compile it into a single, executable binary, we simply run:

go build

This will compile all our Go source code and create a single file. By default, the output file will be named according to the module name, so in our case it will be called

helloworld

We can run this:

./helloworld

And we can hit the same HTTP endpoints as before, either with

curl

or our web browser.

The static files are not compiled into the binary, so if you put the
helloworld
file in a different directory, it will not find the
views
directory to serve our HTML and CSS content.

That's all we need to do to create a binary for whatever platform we're developing on (in my case, my Mac laptop). However, to run inside a Docker container (for eventual deployment to Heroku) we need to compile a binary for whatever architecture our Docker container will run on.

I'm going to use alpine linux, so let's build our binary on that OS. Create a

Dockerfile

with the following content:

FROM golang:1.14.9-alpine
RUN mkdir /build
ADD go.mod go.sum hello.go /build/
WORKDIR /build
RUN go build

In this image, we start with the

golang

base image, add our source code, and run

go build

to create our

helloworld

binary.

We can build our Docker image like this:

docker build -t helloworld .

Don't forget the trailing
.
at the end of that command. It tells Docker we want to use the current directory as the build context.

This creates a Docker image with our

helloworld

binary in it, but it also contains all the Go tools needed to compile our code, and we don't want any of that in our final image for deployment, because it makes the image unnecessarily large. It can also be a security risk to install unnecessary executables on your Docker images.

We can see the size of our Docker image like this:

$ docker images helloworld
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
helloworld          latest              9657ec1ca905        4 minutes ago       370MB

For comparison, the

alpine

image (a lightweight linux distribution, often used as a base for docker images) is much smaller:

$ docker images alpine
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
alpine              latest              caf27325b298        20 months ago       5.53MB

On my Mac, the

helloworld

binary is around 14MB, so the golang image is much bigger than it needs to be.

What we want to do is use this Dockerfile to build our helloworld binary to run on alpine linux, then copy the compiled binary into an alpine base image, without all the extra golang tools.

We can do this using a "multistage" Docker build. Change the Dockerfile to look like this:

FROM golang:1.14.9-alpine AS builder
RUN mkdir /build
ADD go.mod go.sum hello.go /build/
WORKDIR /build
RUN go build

FROM alpine
RUN adduser -S -D -H -h /app appuser
USER appuser
COPY --from=builder /build/helloworld /app/
COPY views/ /app/views
WORKDIR /app
CMD ["./helloworld"]

On the first line, we label our initial Docker image

AS builder

Later, we switch to a different base image

FROM alpine

and then copy the

helloworld

binary from our builder image like this:

COPY --from=builder /build/helloworld /app/

Build the new Docker image:

docker build -t helloworld .

Now, it's the size you would expect for a base alpine image plus our helloworld binary:

$ docker images helloworld
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
helloworld          latest              1d6d9cb64c7e        8 seconds ago       20.7MB

We can run our webserver from the Docker image like this. (If you have another version running using

go run hello.go

./helloworld

, you'll need to stop that one first, to free up port 3000.)

docker run --rm -p 3000:3000 helloworld

The dockerized webserver should behave just like the
go run hello.go
and
./helloworld
versions except that it has its own copies of the static files. So, if you change any of the files in
views/
you won't see the changes until you rebuild the Docker image and restart the container.

Deploy to Heroku

Now that we have our dockerized web application, let's deploy it to Heroku. Heroku is a PaaS provider that makes it simple to deploy and host an application. You can set up and deploy your application through the Heroku UI, or through the Heroku CLI. For this example, we'll use the Heroku command-line application.

Getting PORT from an environment variable

We've hard-coded our webserver to run on port 3000, but that won't work on Heroku. Instead, we need to alter it to run on whichever port number is specified in the

PORT

environment variable, which Heroku will supply automatically.

To do this, alter the

r.Run

line near the bottom of our

hello.go

file, and remove the

":3000"

string value so the line becomes:

r.Run()

The default behavior of gin is to run on whatever port is in the

PORT

environment variable (or port 8080 if nothing is specified). This is exactly the behavior Heroku needs.

Setting up our Heroku app

First, log into Heroku:

heroku login

Now, create an app:

heroku create

Tell Heroku we want to build this project using a Dockerfile, rather than a buildpack:

heroku stack:set container

To do this, we also need to create a

heroku.yml

file like this:

build:
  docker:
    web: Dockerfile

run:
  web: ./helloworld

The

heroku.yml

file is a manifest that defines our app and allows us to specify add-ons and config vars to use during app provisioning.

Next, add git and commit these files, then push to heroku to deploy:

git push heroku main

My git configuration uses
main
as the default branch. If your default branch is called
master
, then run
git push heroku master
instead.

You should see Heroku building the image from your Dockerfile, and pushing it to the Heroku Docker registry. Once the command completes, you can view the deployed application in your browser by running:

heroku open

Conclusion

To recap, here's a summary of what we covered today:

Creating a golang web application, using go modules and the gin web framework to serve strings, JSON, and static files
Using a multistage Dockerfile to create a lightweight Docker image
Deploying a Docker-based application to Heroku using
```
heroku stack:set container
```
and a
```
heroku.yml
```
file

I've only scratched the surface in this article on how to use Go to build web applications and APIs. I hope this gives you enough to get started on your own golang web applications.

Also published at https://dev.to/heroku/deploying-your-first-golang-webapp-11b3