Deploy a Rails App with Docker

2019 Aug 14

This site is deployed using Docker containers on a small DigitalOcean Droplet.

Since I never have a private Docker registry, the original deploy flow was,

  • push the source code to the remote Git repository
  • on the Droplet, fetch the source code from the Git remote
  • build an Docker image from the source code
  • start the Docker container

Since some time ago, the docker build has kept failing on the bundle install instruction on the Droplet. During bundle install, the Bundler fails to build the sassc gem. The Bundler complains that,

virtual memory exhausted: Cannot allocate memory

, while building the sassc gem.

A workaround I tried was,

  • build the Docker image on a more powerful machine, like the local dev machine
  • save the image into a tar archive (docker save img-foo | bzip2 > foo.img.bz)
  • copy the tar to the Droplet
  • load the image from the tar (bunzip2 foo.img.bz && docker load -i foo.img)
  • start the Docker container

One problem of the workaround was the volume to transfer is a bit large (the image is around 1G before compression, and 370M after compression). It was time consuming over a slow network.

Current Deploy Workflow

Actually, Bundler supports fetching gems from local cache, not only from rubygems.org. Using bundle package --all can,

Copy all of the .gem files needed to run the application into the vendor/cache directory.

In the future, when running bundle install, use the gems in the cache in preference to the ones on rubygems.org.

The sassc gem is built on the local machine while bundle package, and put into vendor/cache directory.

Now, the deploy flow is like,

  • run bundle package --all if the application’s dependencies change
  • check in vendor/cache folder and push it to the Git remote
  • on the Droplet, fetch the source code from the Git remote
  • build an Docker image from the source code
  • start the Docker container

The Dockerfile is like,

USER app
RUN mkdir -p /home/app/source-code
WORKDIR /home/app/source-code
COPY vendor/cache ./vendor/cache/
COPY Gemfile* ./
# use RVM, so "bash -lc ..."
RUN bash -lc 'bundle install --deployment --without development test'
# copy the app source code
COPY . ./

Other Notes

If a gem is located at a particular git repository using the :git parameter, like

gem "rails", "2.3.8", :git => "https://github.com/rails/rails.git"

Then the --all option should be used in bundle package.

Some gems are built using CMake. When run bundle package --all, CMakeCache.txt files are created and stored in vendor/cache. The problem with CMakeCache.txt files is that they use hardcoded paths of the machine runs bundle package --all. When build the Docker image in the remote server, the hardcoded paths fail the bundle install. Therefore, in the .dockerignore file, add the following line.

vendor/cache/*/CMakeCache.txt
bundle