Dockerize a Sinatra Microservice

Sinatra is a lean and fun framework to build simple web apps and microservices in Ruby. Traditionally developers would deploy to a platform like Heroku or self-managed server. But what if your team is working in more than one language and need a uniform way of deploying things to the cloud? This is, of course, a great use case for Docker.

Let's say your Sinatra app has a simple structure:

.
├── Gemfile
├── Gemfile.lock
├── README.md
├── app.rb
├── config.ru
└── spec
    ├── app_spec.rb
    └── spec_helper.rb

To package it in Docker, create a Dockerfile:

FROM ruby:2.5

RUN apt-get update -qq && apt-get install -y build-essential

ENV APP_HOME /app
RUN mkdir $APP_HOME
WORKDIR $APP_HOME

ADD Gemfile* $APP_HOME/
RUN bundle install --without development test

ADD . $APP_HOME

EXPOSE 4567

CMD ["bundle", "exec", "rackup", "--host", "0.0.0.0", "-p", "4567"]

Note the RUN bundle install --without development test line. I'm assuming that you're building a container for deployment purposes, and we don't need our development dependencies for that.

In case you want to run all development tasks and tests in Docker, you have two options:

1. Bundle everything and live with a slightly bloated container image in production
2. Create a Dockerfile.dev and use that for development and testing before building a production image with Dockerfile.

Either way, you can build and run the container locally:

$ docker build -t sinatra-demo .
$ docker run-p 80:4567 sinatra-demo
$ curl localhost
> hello world

That's great, now we have our app running in a standard container.

How can we deploy it? The best platform to run Docker containers today is Kubernetes. For a detailed tutorial on deployment, see my other post:

CI/CD for Microservices on DigitalOcean Kubernetes

Marko Anastasov ・ Mar 13 '19

#kubernetes #tutorial #devops #cicd