Rails with Databases

Initial Dockerfile

Below is a minimal, multi-stage build, Dockerfiler that is capable of supporting both ActiveRecord and Action Cable:

# syntax = docker/dockerfile:1

FROM ruby:slim as build

RUN apt-get update &&\
    apt-get install --yes build-essential git pkg-config redis

RUN gem install rails
RUN rails new demo --css tailwind

FROM ruby:slim

COPY --from=build /demo /demo
COPY --from=build /usr/local/bundle /usr/local/bundle

COPY <<-"EOF" /demo/config/routes.rb
Rails.application.routes.draw { root "rails/welcome#index" }

ENV RAILS_ENV=production
RUN bin/rails assets:precompile
RUN bin/rails db:prepare

CMD bin/rails server

This Dockerfile contains a number of small changes from the minimal one:

  • For Active Record (or more precisely, sqlite3) to work, we need to add pkg-config to the list of packages installed via apt-get.
  • For Action Cable to work, we need to add redis to the list of packages installed via apt-get.
  • --minimal and --skip-active-record are removed from the rails new line.
  • Since the example below uses Tailwindcss, we need to add --css tailwind to the rails new line.
  • We need to add RUN bin/rails assets:precompile and RUN bin/rails db:prepare immediately after the ENV RAILS_ENV=production line.
  • In order to see the log messages, add ENV RAILS_LOG_TO_STDOUT=true immediately before the CMD bin/rails server line.
  • We need to serve the assets. Chose one of the following two methods:

    • Add to the bottom of fly.toml:

      guest_path = "/demo/public"
      url_prefix = "/"

In general, requests that never make it to Rails are served fastest, so the second approach where fly serves the static files is the better approach.

Example application

In order to demonstrate Action Cable and Active Record functionality, we need a Rails application. Following is a simple visitors counter.

Replace the three lines starting with COPY <<-"EOF" config/routes.rb with the following:

# syntax = docker/dockerfile:1

FROM ruby:slim as build

RUN apt-get update &&\
    apt-get install --yes build-essential git pkg-config redis

RUN gem install rails
RUN rails new demo --css tailwind

FROM ruby:slim

COPY --from=build /demo /demo
COPY --from=build /usr/local/bundle /usr/local/bundle

RUN bin/rails generate model Visitor counter:integer &&\
    bin/rails generate channel counter

COPY <<-"EOF" app/controllers/visitors_controller.rb
class VisitorsController < ApplicationController
  def counter
    @visitor = Visitor.find_or_create_by(id: 1)
    @visitor.update! counter: @visitor.counter.to_i + 1
    @visitor.broadcast_replace_later_to 'counter',
      partial: 'visitors/counter'

COPY <<-"EOF" app/views/visitors/counter.html.erb
<%= turbo_stream_from 'counter' %>

<div class="absolute top-0 left-0 h-screen w-screen mx-auto mb-3 bg-navy px-20 py-14 rounded-[20vh] flex flex-row items-center justify-center" style="background-color:rgb(36 24 91)">
  <img src="https://fly.io/static/images/brand/brandmark-light.svg" class="h-[50vh]" style="margin-top: -15px" alt="The monochrome white Fly.io brandmark on a navy background" srcset="">

  <div class="text-white" style="font-size: 40vh; padding: 10vh" data-controller="counter">
    <%= render "counter", visitor: @visitor %>

COPY <<-"EOF" app/views/visitors/_counter.html.erb
<%= turbo_frame_tag(dom_id visitor) do %>
  <%= visitor.counter.to_i %>
<% end %>

COPY <<-"EOF" config/routes.rb
Rails.application.routes.draw { root "visitors#counter" }

ENV RAILS_ENV=production
RUN bin/rails assets:precompile
RUN bin/rails db:prepare

CMD bin/rails server

Since this isn’t a Rails tutorial, an overview of the contents will suffice:

  • A Vistor model with a single counter column.
  • A Counter channel to send realtime updates.
  • A controller method that updates the model, broadcasts the results, and renders the initial page.
  • HTML that connects to the channel, displays the fly.io logo, and renders a partial.
  • A partial that shows the counter inside a turbo frame.
  • A route that connects the root to the controller method we defined above.

Setting up Redis

Before we deploy this application, we need to create the redis database. We will be using Upstash for Redis, and creating the database and connecting it to the demo application is a matter of issuing two commands:

flyctl redis create
flyctl secrets set \

Replace the secret with the private URL provided by the redis create command. And you don’t need to split the secrets set line, that was merely done to fit in your browser window.

If you have deployed the minimal application previously, at this point, your application has been redeployed, and you can visit the page to see a counter. If you open multiple windows, each will be updated simultaneously. If you haven’t launched and deployed the application previously do it now.

Making a persistent sqlite3 database

The counter itself needs to be saved in a database, and by default that database is sqlite3 for Rails. For that database to be kept intact across deploys it needs to be placed on a volume.

We can create a volume using the CLI:

flyctl volumes create sqlite3_volume

To use this volume, we are going to need to mount it and set an environment variable relocating the database to be on that volume. We can do both by editing fly.toml:

  DATABASE_URL = "sqlite3:///mnt/volume/production.sqlite3"

  source = "sqlite3_volume"
  destination = "/mnt/volume"

One last problem remains: the db:prepare step needs to be run after the volume is mounted, which means that it needs to run on the deployment machine, not on the build machine. This means that in your Dockerfile this needs to be done by the CMD statement not on a RUN statement.

Start by removing RUN bin/rails db:prepare, then replace the CMD statement with:

CMD ["/bin/bash", "-c", "bin/rails db:prepare; bin/rails server"]

The issue here is that Dockerfiles are limited to one CMD statement. An alternative solution is to create a script (perhaps using a “here” document), and have the CMD run that script.

The next time you deploy after this change you will start with a new database. After that, all future deploys will use the same database.

Before proceeding to the next step, run flyctl volumes list to find the volume id. Delete the volume using flyctl volumes delete. Also remove the DATABASE_URL from the [env] section and delete the [mounts] section.

Changing the database to Postgresql

Postgresql is an alternative to sqlite3. To use it, you will need to install a library and a gem:

  • Add libpq-dev to the apt-get install line.
  • Add --database postgresql to the rails new line.

Once again, knowing what libraries to install is a matter of knowing Debian conventions: libps-dev. Digging deeper into the filelist, there is one file, libpq.so that needs to be available at deploy time.

One approach is to add a copy line:

COPY --from=build /usr/lib/x86_64-linux-gnu/ /usr/lib/x86_64-linux-gnu/

Another is to install something that includes it that might be needed anyway at deploy time, such as postgresql-client. Since this will be on the deploy image the multi-stage build approach of install and copy only what you need needs to be reversed: install and remove what you don’t need. That’s why you will often see lines like the following in Dockerfiles:

RUN apt-get update -qq && \
    apt-get install --no-install-recommends -y \
    postgresql-client \
    && rm -rf /var/lib/apt/lists /var/cache/apt/archives

Now you are ready to create a postgres cluster:

flyctl postgres create

When done, you will find a link to This Is Not Managed Postgres . That information may be useful in the future so bookmark the link (or return back to this page), but Fly Postgres is fine for this demo.

Your postgres app will be assigned a name if you didn’t provide one. Find that name and then run:

flyctl postgres attach happy-store-1234

… replacing the name with the one assigned or selected by you.

Before proceeding, there is one more change that might be worth making. If you ever plan to scale up to multiple application machines it would be better to run db:prepare exactly once per deploy rather than once per vm. You can accomplish this by adding the following to your fly.toml:

  release_command = "bin/rails db:prepare"

Then restore the CMD line in your Dockerfile:

CMD bin/rails server

Changing the database to MySQL

MySQL on Fly is more of a do it yourself kinda thing at the moment. Follow that guide to set up a MySQL application.

Once you have it up and running, the instructions are roughly the same as with postgresql, with the following modifications:

  • On the rails new line, specify --database mysql instead of --database postgresql
  • default-libmysqlclient-dev is the package containing the mysql libraries.
  • default-mysql-client is the package containing the mysql client.
  • Add the deploy release command to fly.toml from the postgresql instructions above.
  • You need to run fly secrets set DATABASE_URL= in your application. For postgresql this is taken care of for you by the flyctl postgres attach command. This will look something like the following:

     fly secrets set DATABASE_URL=mysql2://root:password@mysql-app-name.internal:3306/


  • Adding databases require a lot of configuration:
    • Much of the configuration involve actions outside your Dockerfile: i.e, setting up volumes, secrets, virtual machines, etc.
    • While Dockerfiles don’t reduce the number of steps required, they do provide a place where these actions can be captured concisely and reproducible executed.
  • Knowing what packages to add and where shared libraries are to be found is once again requires more knowledge of operating system conventions (in this case Debian) than Docker knowledge.