How I use Docker for Rails development: running services

Captain's log, stardate d568.y38/AB

One of the most frustrating moments in development projects is when I spend too much time with the project environment setup (especially when it fails). Let me share with you how I'm using Docker to improve this!

Cranes at night - Photo by EJ Yao on Unsplash

Last week, I resumed the development of a project in which I faced an excessibly complicated project setup.

On the one hand, the project has some external dependencies (S3 and integrations with other custom APIs) not trivial to set up. Even with a good documentation, it's easy to accidentally skip a step, causing everything to break.

In just one week, I experienced the following kind of issues:

  • The API I used required to create a user through the admin interface. I created an admin user instead of a regular user 🤦‍♂️
  • Sass installation only works with very specific versions of Node.js
  • When Chrome updates, it breaks Selenium-based tests. You have to manually install a new Chrome driver

For these reasons (and others, like being able to run MSSQL in any os), I'm excited about using Docker as a controlled development environment.

1. Docker configuration

I have a confession to do: I don't have neither Postgres nor MySQL installed on my Mac. Not even redis. And I don't plan to install them at all.

First thing I do, when starting a project, is to set up the services using docker compose. We start with a docker-compose.yml file:

version: '3.4'

    image: postgres:11.1
      - postgres:/var/lib/postgresql/data
      - 5432:5432

    image: redis:3.2-alpine
      - redis:/data
      - 6379:6379


Then I add a some scripts to package.json:

"scripts": {
  "docker:start": "docker-compose up -d",
  "docker:stop": "docker-compose down",
  "docker:purge": "docker-compose down --volumes",
  "docker:logs": "docker-compose logs -f"

With this setup, when I want to start development of the project, I run:

$ yarn docker:start

To test everything is running I use docker ps:

$ docker ps
CONTAINER ID        IMAGE                    ...  PORTS
bcfecc563fd5        redis:3.2-alpine    >6379/tcp
cb6eb58dfaa9        minio/minio         >9000/tcp
343aa530bbe9        kartoza/postgis:9.6-2.4>5432/tcp

At the end of the day (or when I change project, more on that below), I stop the services using:

$ yarn docker:stop

One important thing to notice is that, in this configuration, containers expose the ports to the host computer. You can access services from your app exactly the same way as if they were installed locally.

But this has two important consequences:

  • Can't start two containers with the same service at the same time. Since the port is in use, you can't start a service with the same port. That means that when you move to another project, you must stop containers first.
  • Can't start a container if you already have installed locally a service using the same port. If you have Postgres installed, you won't be able to start the Postgres container service.

Both problems can be solved by changing the ports mapping at docker compose configuration. See here:

  image: postgres:11.1
    - postgres:/var/lib/postgresql/data
    - 55432:5432

That will map the container's port 5432 to http://localhost:55432. This can be used to avoid port collisions.

2. Application configuration

In order to work with this setup, the Rails application must implement a twelve-factor compatible config.

That means storing the configuration using environment variables. For Rails I use the rails-dotenv gem:

# Gemfile
group :development, :test do
  gem 'dotenv-rails'

And then create a gitignored .env file with the following configuration:

DATABASE_URL: postgres://postgres:[email protected]:5432
REDIS_URL: redis://localhost:6379/

With this configuration, no database.yml file is (strictly) required (although we would need to add the database name in the DATABASE_URL variable).

Anyway, I think it is better to provide one and use it as documentation, so pretty please, add it to git!:

# Database is configured using DATABASE_URL environment variable.
# See .env or .env.sample file

  database: my_blog_development

  database: my_blog_production

  database: my_blog_test

Finally, If we want to use redis, we will need to install the gem first and load the configuration in an initializer:

# config/initializers/redis.rb

Redis.current = ENV["REDIS_URL"])

3. (Optional) Replace S3 with minio on development

If you want to replace your S3 instance with minio, just add it to services inside docker-compose.yml file:

    image: minio/minio
      - minio:/data
      MINIO_ACCESS_KEY: my-blog
      MINIO_SECRET_KEY: 2NVQWHTTT3asdasMgqapGchy6yAMZn
      - 9000:9000
    command: server /data


And configure ActiveStorage using storage.yml file (also included in git):

default: &default
  service: S3
  region: <%= ENV["AWS_REGION"] %>
  access_key_id: <%= ENV["AWS_ACCESS_KEY_ID"] %>
  secret_access_key: <%= ENV["AWS_SECRET_ACCESS_KEY"] %>
  bucket: <%= ENV["S3_BUCKET"] %>

  <<: *default
  bucket: my-blog-development
  endpoint: <%= ENV["S3_ENDPOINT"] %>
  force_path_style: <%= ENV["S3_FORCE_PATH"] %>

  <<: *default
  bucket: l-photo-booth-production

  service: Disk
  root: <%= Rails.root.join('tmp/storage') %>

Notice that, unlike real S3, minio needs two additional variables. You also need to add the following lines to the .env file:

AWS_REGION: eu-west-1
S3_BUCKET: my-blog-development
S3_FORCE_PATH: 'true'
S3_ENDPOINT: http://localhost:9000

Run yarn docker:start and connect to http://localhost:9000 to create your bucket.

Next up: Use Docker to run your application

This post is long enough. In this part we saw how to create services that can be used by your app and how to configure it all.

In the next part, I'm going to talk about how to run the application itself inside a container and what possibilities it opens.

Stay tuned!

Daniel Gómez

Daniel Gómez

Dani started playing with computers with his Oric 1, at least 100 years ago. He now juggles his programming skills with two bands and being a father of three. Legend says, it's him and his identical twin brothers working as a team.

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