Skip to content

Latest commit

 

History

History
112 lines (87 loc) · 5.56 KB

assignment_05.md

File metadata and controls

112 lines (87 loc) · 5.56 KB
Raw

Assignment 5 - integrating Postgres

Before we start, let's clean up. Make sure to stop all containers and optionally delete them. Either use docker container ls -a and docker container rm OR docker container prune (if you don't care about any of the containers on your system)

Configuring the application

Currently, our Rails app is using SQLite as a DBMS. We're going to switch to PostgresSQL and need to update our database.yml. An updated database.yml can be found in the config directory

cp config/database.yml.postgres config/database.yml

Make sure to take a look at the updated database.yml. As you can see, we are utilizing environment variables to configure our connection to PostgreSQL.

Since we made changes to the source code of our application, we need to rebuild the image:

docker image build -t your_docker_id/rails_app:v1 .

Running Postgres

Now we can start our Postgres instance:

docker container run --name pg -d \
  -v ws-pg-data:/var/lib/postgresql/data \
  -e POSTGRES_USER=postgres -e POSTGRES_PASSWORD=secret postgres:10.6-alpine

Let's dig a little into the flags we used to start the container:

  • The --name flag gives our container a name. This way we can easily start/stop it and reference it from other containers.
  • -d starts the container in detatched mode. That means we won't be attached to STDOUT of the container. Try playing around docker container attach and docker container logs to see what is going on. Don't forget the --help flag if you want to find out more about those commands
  • With -v ws-pg-data:/var/lib/postgresql/data, we make sure that the data that Postgres writes to disk is persisted outside of the container. Docker will create a volume named ws-pg-data and mount it into the container.
  • -e allows us to pass in environment variables that will be available to our container. We use this flag to configure the credentials for Postgres.

Make sure that the container is up and running by running the docker container ls command. You should see a container with the name pg in the output:

$ docker container ls
CONTAINER ID        IMAGE                  COMMAND                  CREATED             STATUS              PORTS                              NAMES
66bbf21aedc0        postgres:10.6-alpine   "docker-entrypoint.s…"   17 seconds ago      Up 16 seconds       5432/tcp                           pg

Talking to the service

Now that we have our Postgres container up and running, we need to start our Rails application. We already built the new image with the correct database.yml in place. If you take a quick look at the database.yml, you will see that we set the connection information via environment variables. We need to pass the correct environment variables to our docker container run command using the -e flag:

docker container run -it -p 127.0.0.1:3000:3000 --link pg \
  -e POSTGRES_HOST=pg -e POSTGRES_USER=postgres -e POSTGRES_PASSWORD=secret \
  your_docker_id/rails_app:v1

The environment variables that we pass in are used to configure the connection to our Postgres database. These environment variables are read in out database.yml.

You might also have spotted the --link flag in our command. This flag allows us to talk to our Postgres container by its name pg.

Note: We don't need to tell Docker which command we want to execute in the container because we made rails server the default in the prior assignment.

Note: --link is deprecated and we will learn about a better mechanism later in the workshop. However, it is the easiest way to get the app up and running for now.

If you open your browser and go to http://localhost:3000, you will see that we need to migrate our database. So let's do that in another container (in another shell):

docker container run -it --link pg \
  -e POSTGRES_HOST=pg -e POSTGRES_USER=postgres -e POSTGRES_PASSWORD=secret \
  your_docker_id/rails_app:v1 rake db:create db:migrate

With the database schema in place you should be able to to interact with our application through the web interface.

Feel free to terminate your Rails and Postgres containers and restart them. Your data will still be there!

Let's start by stopping Postgres

docker container stop pg
docker container rm pg

The volume ws-pg-data will still be there. You can verify this via:

docker volume ls

Bonus

  • Try to run the test suite with the Postgres connection in place.
  • Start a Rails console in a second container and create some records. They should show up in the Web UI.

Troubleshooting

Things are going wrong? Let's make them work! Here are a couple of things to try

  • Make sure you copied the database.yml AND rebuild the image?

  • Remove the Postgres container and volume:

    docker container rm -f pg
docker volume rm ws-pg-data

    And then try it again

  • Check the logs for the Postgres container

    docker container logs pg

  • Check if the expected containers are up and running

    docker container ls -a
docker volume ls

  • Delete all the containers and volumes and start over (be careful - this will do exactly that):

    docker container ls -a | xargs docker container rm
docker volume ls | xargs docker volume rm

What changed

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

Back to the overview