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Running Rails inside a Container

In the previous_step we built the image and run a container ready for a Rails app. Now we are going to run the app inside it. We won’t dive into Compose yet and use just Docker to do that.

Binding the working directory

First thing to do is copying the application files inside the container. We could do that by using an ADD instruction in the Dockerfile, but rebuilding the image after any change during development would not be sustainable.

So we bind the working directory of the project to the one in the container with the -v (source:destination) option. When source is a path it is bound to the destination in the container (you can use $PWD as the source path if you cd to the application path)

$ docker run --rm -it -v /path-to-my-awesome-app:/app my-awesome-app bash rails@container-id:/app$

If you ls you should see all the application files, including the Gemfile.

Start the application

You can now bundle as you usually would

rails@container-id:/app$ bundle Fetching gem metadata from Fetching rake 12.3.1 Installing rake 12.3.1 Fetching concurrent-ruby 1.0.5 . . . Bundle complete! 18 Gemfile dependencies, 78 gems now installed. Bundled gems are installed into `/usr/local/bundle`

Migrate the database

rails@container-id:/app$ rails db:migrate

And finally run the application

rails@container-id:/app$ rails s => Booting Puma => Rails 5.2.0 application starting in development => Run `rails server -h` for more startup options Puma starting in single mode... * Version 3.11.4 (ruby 2.5.0-p0), codename: Love Song * Min threads: 5, max threads: 5 * Environment: development * Listening on tcp:// Use Ctrl-C to stop

Note To receive requests from outside the container rails server should not be listening on localhost. If that is the case you should bind the server to all interfaces with the option -b ‘’. I found it is generally better to always specify binding and port for the server, for example rails s -b -p 3000

Publish the ports

If you open a browser and navigate to localhost:3000 you’ll find you can’t connect to the service. That’s because right now the container is running isolated from the host machine. To connect to the service inside it, we need to publish the port used by it.

The -p host-port:container-port option does that, mapping a port on the container to one on the host machine. So we can stop rails server

^C- Gracefully stopping, waiting for requests to finish === puma shutdown: 2018-05-01 08:25:17 +0000 ===

  • Goodbye! Exiting

Exit the container

rails@4a4a10dbde3e:/app$ exit exit $

And run again it with the ports option, using rails default port both on the container and the host machine (you can actually use whatever ports you want, as long as the second one is the rails server one)

$ docker run --rm -it -v /path-to-my-awesome-app:/app -p 3000:3000 my-awesome-app bash rails@container-id:/app$

Bundle volume

Trying to start the server won’t work now

rails@container-id:/app$ rails s bash: rails: command not found

What’s happening here is we need to bundle again since every time we docker run, a new container is spawned that does not share anything with the previous (or future) ones.

I can feel the pain waiting for nokogiri to download, again.

Docker lets sharing files between containers using named volumes, with the -v name:destination option. When the first argument is a string instead of a path, -v creates a named volume and mount it in the destination path inside the container.

Named volumes are persisted by default when the container is stopped and even removed. This can be used to share bundle files between runs of the container.

Running bundle config we check the path for the bundle

rails@container-id:/app$ bundle config Settings are listed in order of priority. The top value will be used. path Set via BUNDLE_PATH: "/usr/local/bundle" ...

Then we can exit the container, and run it adding the volume option (-v app-bundle:bundle-path)

docker run --rm -it -v $PWD:/app -v app-bundle:/usr/local/bundle -p 3000:3000 my-awesome-app bash

Check it out

We can finally bundle and start the rails server. Connection to the service should be available on host at localhost:3000.

If we remove and run again the container, with the same options as last time, bundler should now use the gems from the volume when possible, completing the bundle in no time.

A little verbose

We just started adding options and the command docker run --rm -it -v $PWD:/app -v app-bundle:/usr/local/bundle -p 3000:3000 my-awesome-app bash is already starting to become long, clumsy and difficult to read.

To overcome this, docker-compose can be used to set docker configuration in a file, instead of the command.

Next step: Compose _(coming soon)

Previous step: Adding the Dockerfile

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