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Assignment 7 - Glueing things together

Preparing the Compose file

Now that we have a clean environment, let's put the docker-compose.yml in place:

cp _examples/docker-compose.yml .

Take some time to get familiar with the content of the file.

Starting the services

Open the file and make the changes in the appropriate places. This should just be the name of the image that will be used. Once you are done, go ahead and start the environment with:

docker-compose up -d

The -d flag will start all services in the background. This allows us to keep working in our current shell.

We can make sure that all services are running with

docker-compose ps

You should see two containers. The State column should say Up.

In order to see what our containers tell us on STDOUT we can use the following command:

docker-compose logs

We can also append some additional flags to the docker-compose logs command. Use the --help option to find out more. Getting to know the docker-compose logs command will be very help for debugging any issues that come up.

Getting the app up and running

Try browsing http://localhost:3000 - you will see that the database has not been migrated. That makes sense, we just created a whole new set of containers and volumes.

Note: Docker Compose automatically prefixes all resources with the project name (the name of the current directory per default).

So let's run the migrations. Just as with the Docker CLI, we can run arbitrary commands in the context of a service:

docker-compose run --rm app rake db:create db:migrate db:test:prepare

Now the web application should work as expected. The --rm in the command deletes the container right after it terminates. This is useful because we don't have to clean up after ourselves. Go ahead and browse http://localhost:3000 to verify it works as expected.

Let's try some other things - this time we will run the test suite:

docker-compose run --rm app rspec

And we can easily start a rails console as well:

docker-compose run --rm app rails c

Note: We don't have to specify the -it flags with docker-compose run. Docker Compose takes care of that for us automatically when using the run command.

Shutting things down

You can stop everything by running

docker-compose down

This will delete the container, but keep the volumes intact. Use the -v flag if you also want to delete the volumes defined in the docker-compose.yml.

Building images

Docker Compose also enables us to build our images with a single command:

docker-compose build

Docker Compose will build the image for the service app because we specified the build directive. It automatically tags the image with the name specified by the image directive.

We can combine building the image with the up command as well:

docker-compose up -d --build

Bonus

  • What will happen if you just run the following command? Any idea why? 
    docker-compose run --rm app
  • Import seed data by running rake db:seed
  • What happens when you shut down the services with docker-compose down and then run the test suite?

What changed

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

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