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Assignment 13 - A seamless experience

Customizing your experience

You might have realized that the rails console is not preserving our history. On your local system you might also use an .irbrc, shell customizations or other rc-files to enrich your development experience. These are all things that are currently missing in our containers. Not an issue tho - we can simply bind mount our rc-files into our containers!

    volumes:
      - ./.irbrc:/root/.irbrc

The .irbrc might look something like this:

require 'irb/completion'
require 'irb/ext/save-history'
IRB.conf[:SAVE_HISTORY] = 1000
IRB.conf[:HISTORY_FILE] = "/usr/src/app/.irb-history"

And we can use environment variables in our docker-compose.yml to change the behaviour of containers as well. For example we can set the history file used by bash using the HISTFILE environment variable:

HISTFILE=/usr/src/app/.bash_history

Note: You might want to add .bash_history and .irb_history to your .gitignore.

Note: The rc-files don't have the live in the repo - you can place them anywhere on your file system. You can even use environment variables within the docker-compose.yml to reference them. For example ${HOME}/.irbrc:/root/.irbrc.

Streamlining the execution of commands

When it comes to developing applications with Docker and Compose, we often have to make a few choice before running a command. Should we use exec or run? And which service do we have to specify for the command at hand? Here are a few examples:

  • docker-compose exec --rm app rspec: We run the test suite using exec because we want to utilize Spring running provided the bye app service.
  • docker-compose run --rm bundle: We install gems using run because missing gems might prevent our application from starting and exec won't work.
  • docker-compose exec pg psql -U postgres: We use exec to start a psql session for our Postgres Database. We want to use exec here because Postgres has to be running in order to connect to it. It also allows us to omit the hostname of the database server since Postgres is running on "localhost" in the context of the pg service.
  • docker-compose exec redis redis-cli: We use exec to connect to Redis via the redis-cli. We have to use the redis service here because our other containers won't have redis-cli installed. Using exec instead of run allows us to omit the hostname and connect to "localhost" in the context of the redis service.

This list is neither complete nor will it be identical across projects. This is where Donner is helpful. Donner allows you to define how to execute given commands in a simple yaml-config. Instead of having to decide whether ot use run vs exec and which service to use, you can just execute

  • donner run rspec
  • donner run bundle
  • donner run psql -U postgres
  • donner run redis-cli

Donner will figure out how to execute the command for you based on the config.

Donner even allows you to set aliases for the specified commands. This way all you have to type is rspec or psql -U postgres!

Check out the README and integrate Donner into your project if you are interested.

Accelerating bind mounts on MacOS

If you are a MacOS user, you might notice that interacting with your applications seams slower compared to a version that runs locally. This is due to the overhead that comes with osxfs, a shared file system that allows you to bind mount file and folders into Docker containers.

To work around this, we can use Blitz, a zero dependency source code synchronizer for Docker for Mac. Instead of relying on osxfs and bind mounts to get your source code into your containers, Blitz synchronises files and folder from you Mac with a Doocker Volume. That means that your containers can access the files at native speed!

Check out the README and try setting it up if you are using MacOS.

What changed

You can find our changes in the scripts_to_rule_them_all branch. Compare it to the previous branch to see what changed.

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