Packaging an App from Scratch with Scaffolding

Hey there, fellow developer, let’s make ourselves a quick Express app and wrap it up in a Habitat package. Ready? Me too, I like to learn.

We’re going to want Habitat installed on our system, but that’s not too hard. Follow the instructions on the Set up your environment page and come on back here. Can you run hab --version now in a terminal? Excellent.

Okay, we’re going to use the Express application generator, so make sure you have this npm packaged installed globally:

npm install express-generator -g

Oh right, you need Node.js installed on your workstation–but you already do don’t you? Nice.

Next, we’ll use the generator to make our amazing webapp and then write a quick Habitat Plan.

express --git expresso
cd expresso
mkdir habitat
$EDITOR habitat/plan.sh

Here’s what you want in your plan.sh:

pkg_origin=fnichol
pkg_name=expresso
pkg_version=0.1.0
pkg_scaffolding=core/scaffolding-node

Notice that I used my Habitat origin of fnichol (that’s the origin and key I generated when I ran hab setup earlier on). The last line is where the good stuff is–this tells the Habitat build system to use the core/scaffolding-node Scaffolding package to build your app.

I’m sure you’re as antsy as I am to commit this to version control–let’s do that now:

git init
git add .
git commit -m 'Expresso: a typo or pun?'

Finally, we’ll enter a Habitat Studio where we can build an isolated package for our app:

hab studio enter
build .

After a lot of output flies by, we should have our first Habitat package built and installed! Mine is called fnichol/expressobecause my origin name is part of the Package Identifier.

Now let’s start our app! When you enter a Habitat Studio these days, a Supervisor will be running in the background waiting to start services, so we can start ours with:

hab svc start fnichol/expresso

By the way, if you want to tail the Supervisor output, run sup-log and hit Ctrl+c when you’re done.

To quickly check our webapp, we can use wget to fetch the app’s index page. There isn’t much software in a Studio by default to keep our builds isolated and lean, so we’ll stick with good ‘ol wget for now:

wget -q -O - http://localhost:8000

I’m hoping you’re looking at your fabulously generated HTML, just as I am:

<!DOCTYPE html><html><head><title>Express</title><link rel="stylesheet" href="/stylesheets/style.css"></head><body><h1>Express</h1><p>Welcome to Express</p></body></html>

Finally, how about performing a next-level Habitat trick like changing your app’s listen port using a versioned runtime configuration injected into Habitat’s gossip ring? The Node Scaffolding has provided us with a configuration setting called app.port which we can change, so let’s set it to 3000 instead:

echo 'app = { port = 3000 }' | hab config apply expresso.default $(date +'%s')

This command looks more complicated than it is. We’re piping in some inline TOML on stdin for the hab config applycommand and targeting all “expresso” apps running in the default group (in Habitat we call this a Service Group). The $(date +'%s') part gives us the number of seconds since the Unix epoch and acts as our version number.

If you want to see the Habitat Supervisor reacting to this change, run sup-log and hit Ctrl+c when you’re done. You can verify the port change was applied with wget:

wget -q -O - http://localhost:3000

You can exit your Studio session with exit (I guessed this too), and you will find a Habitat package waiting for you in a ./results/ directory. The .hart file extension is for “Habitat ARTifact”. This is because we take puns seriously. We’re punstoppable.

The End

There’s much more to Habitat once you have a runnable package. Check out the Habitat website for more documentation, tutorials, community events, and more.

Author Fletcher Nichol

Fletcher Nichol is a software developer from Edmonton, Canada who has worked in jobs ranging from systems administrator to web application developer. He enjoys playing the drums and taking walks on warm summer days with an audiobook in his ears. He is active in many automation and testing projects such as Chef and Docker and is the lead developer of Test Kitchen.