docker.io - Docker linking between application & database containers

Question

I am trying to use my Scala-Akka application with my MySQL database on two separate Docker containers. I found out that Docker allows developers to link their application to their databases with the flag named --link. In my Dockerfiles in which I've used to create my images, I have add in EXPOSE 3306 8080 to it.

And this is how I run the containers:

docker run -d -p 3306:3306 --name mysql centos6mysql
docker run -d -p 8080:8080 --name scalaapp --link mysql:db centos6scala

After running the containers, I used docker ps and I am able to see the active containers. However, It seems like the application container is not using the database from the MySQL container. Anyone know what's wrong?

Answer 1

Linking in Docker allows network connections to be made between containers. Docker will define environmental variables to your linked containers for the URL, IP, port, and protocol. The names of these will be based on the name of your container. For instance:

DB_NAME=/web2/db
DB_PORT=tcp://172.17.0.5:5432
DB_PORT_5432_TCP=tcp://172.17.0.5:5432
DB_PORT_5432_TCP_PROTO=tcp
DB_PORT_5432_TCP_PORT=5432
DB_PORT_5432_TCP_ADDR=172.17.0.5

You can use these environmental variables to set up your Akka app container to connect to your DB container. However, you must manually configure the app container to do so. Docker will not make the connection for you automatically.

So, somewhere in your app, you will need to pass these values to your startup script, something that might look like:

./restcore --Ddb.default.db="jdbc:mysql//${DB_PORT_3306_TCP_ADDR}:${DB_PORT_3306_TCP_PORT"