Writing a simple app

Introduction

In order to play around with Kubernetes, we’re going to write a simple Ruby web server with Sinatra. Feel free to skip this section if you want to jump to Docker and Kubernetes. The complete app code can be found under applications/sinatra in this repo.

Mac

Verifying that we have Ruby installed

Most Mac’s come with Ruby installed. We can verify that with the following command:

> ruby -v
ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-darwin18]

If you don’t, thats ok too! Follow the installation guide at ruby-lang.org to install ruby on your system.

Installing RVM

We’re going to install rvm, a ruby version manager so that we can keep our dependencies isolated for this application. You can check to see if you have it installed by running this command.

> rvm -v
rvm 1.29.3 (latest) by Michal Papis, Piotr Kuczynski, Wayne E. Seguin [https://rvm.io]

If you don’t have it installed rvm has an install guide with instructions for various systems.

Anatomy of a Sinatra app

We’re going to define a couple of files in this section:

Creating our project

Create a directory called noobernetes with the mkdir command.

> mkdir noobernetes

cd into that directory as we’ll use that to write our application.

Writing our .ruby-version and .ruby-gemset

These files simply let us declare a separate ruby version and gemset so that we can isolate our apps dependencies and ruby version from the rest of our ruby apps.

We’ll start by creating a .ruby-version file. In our case, we’re going to use ruby-2.7.1.

2.7.1

Next we’ll create a .ruby-gemset file and create a name for where we want our gems to be installed.

noobernetes

Now that we’ve defined these files, we should be able to cd out of the directory and back into it so that rvm can use them to do its magic.

> cd ..
> cd noobernetes
Required ruby-2.7.1 is not installed.
To install do: 'rvm install "ruby-2.7.1"'

We see that ruby-2.7.1 is not yet installed. We can now install the version of ruby that we’ve specified in our .ruby-version without impacting our system installed Ruby.

> rvm install ruby-2.7.1
> cd ..
> cd noobernetes
ruby-2.7.1 - #gemset created /Users/mavenlink/.rvm/gems/ruby-2.7.1@noobernetes
ruby-2.7.1 - #generating noobernetes wrappers - please wait

Writing our Gemfile

We’re going to write a simple Gemfile so that we can include our sinatra dependency.

source "https://rubygems.org"

gem 'sinatra'

Once we’ve gotten our Gemfile written, we’re going to install our dependencies to use bundler to install our dependencies.

We’ll first need to install bundler. We’re specifying a version here so that we have a compatible bundler version for our Dockerfile in future steps.

> gem install bundler -v 1.16.2
Fetching: bundler-1.16.2.gem (100%)
Successfully installed bundler-1.16.2
Parsing documentation for bundler-1.16.2
Installing ri documentation for bundler-1.16.2
Done installing documentation for bundler after 6 seconds
1 gem installed

Then we can run bundle install to install Sinatra and its dependencies.

> bundle install
Fetching gem metadata from https://rubygems.org/.........
Using bundler 1.16.2
Fetching mustermann 1.0.2
Installing mustermann 1.0.2
Fetching rack 2.0.4
Installing rack 2.0.4
Fetching rack-protection 2.0.1
Installing rack-protection 2.0.1
Fetching tilt 2.0.8
Installing tilt 2.0.8
Fetching sinatra 2.0.1
Installing sinatra 2.0.1
Bundle complete! 1 Gemfile dependency, 6 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
Bundle complete! 1 Gemfile dependency, 85 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.

Writing our Web Server

We’re going to make a simple web application that has one route. It will build widgets, so that we can simulate CPU load in future tutorial steps. To start, create a file called web.rb with the following content.


require 'sinatra'

set :bind, '0.0.0.0'

def build_widget
  100.times do |i|
    100000.downto(1) do |j|
      Math.sqrt(j) * i / 0.2
    end
  end
end

get '/' do
  build_widget
  "Your widget was built!"
end

Running it locally

Now that we’ve got our app built, we can run it with the following command:

> ruby web.rb
[2018-05-20 15:23:10] INFO  WEBrick 1.3.1
[2018-05-20 15:23:10] INFO  ruby 2.3.7 (2018-03-28) [x86_64-darwin17]
== Sinatra (v2.0.1) has taken the stage on 4567 for development with backup from WEBrick
[2018-05-20 15:23:10] INFO  WEBrick::HTTPServer#start: pid=49289 port=4567

We can visit the app and see it running at localhost:4567

Conclusion

We now have a simple Ruby server running locally. Next we’re going to get it containerized with Docker!

Resources

Continue to Containerizing our app