Docker in development: Episode 3

In the last issue, we explained how to containerize Ruby on Rails applications. In this issue we'll explore how to perform everyday tasks in containers.

Run Rake tasks and Rails commands

Running Rake tasks is easy. After you build the image, you can use docker-compose to run commands in it. For example, if you want to view your application's routes:

<code class="language-bash">$ docker-compose run web rails routes</code>

Similarly, if you want to create a database, migrate and populate data:

<code class="language-bash">$ docker-compose run web rails db:create db:migrate db:seed</code>

If you want to run the test suite, you must create a test database:

<code class="language-bash">$ docker-compose run web rails db:create db:migrate RAILS_ENV=test</code>

Then run the test suite (assuming rake runs rake test by default):

<code class="language-bash">$ docker-compose run web rake</code>

Tip: Create custom scripts/aliases

I run docker-compose run web rails ... hundreds of times a day, and to make things easier I put this script inside my $PATH:

<code class="language-bash">#!/bin/bash
docker-compose run web rails "$@"</code>

Please note that this script assumes that your docker-compose.yml file has a web service. If not, it won't work.

Perform other tasks

So far all the commands are very simple and straightforward, you just need to run them in the web service. So what do you do with some of the more difficult tasks like loading a pre-existing database into a container's database? This was one of the tasks that took me a little longer to figure out.

In PostgreSQL, there are two ways to do this, depending on the format of the dump file. In the past, we had to deal with --format=c dumps and regular dumps.

Suppose you have a latest.dump file containing a PostgreSQL dump in c format, and you want to load it into a (running) container, first you have to find out the container ID. You can do this by running the following command:

<code class="language-bash">$ docker container ls</code>

or

<code class="language-bash">$ docker ps</code>

Once you have the container ID (in this example, we will use 80f8041db4b4), you can run the following command to restore the dump in the container:

<code class="language-bash">$ docker exec -i 80f8041db4b4 pg_restore -d app_development -U postgres</code>

If you have a regular dump (e.g. latest.sql), you can restore it using:

<code class="language-bash">$ docker exec -i 80f8041db4b4 psql -d app_development -U postgres</code>

If you are using docker-compose, things get easier:

<code class="language-bash">$ docker-compose exec -T db pg_restore -d app_development -U postgres</code>

Everything is containerized!

I use Elastic Beanstalk a lot. I usually install it using Homebrew, but it installs a lot of its own dependencies such as Python, SQLite, etc. I don't want all of this on my system, especially since I always have issues with the Python version. Instead, I containerize it: docker-awsebcli.

Stay tuned for the next issue!

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