Contents

Inter-container communication

Before we continue let’s go back to our Java project and switch to a different branch and build the project and the image again

$ git checkout mysqldb
(make sure you're on this branch)

$ Linux: ./build.sh | Windows: build.bat
(wait for build to complete)

$ docker build -t greetings-ws:mysqldb .

Let’s run a container

$ docker run -d -p 8080:8080 --name crazy-cow greetings-ws:mysqldb
                                                              ^
                                                              |
                                                  be sure to specify the tag

$ docker ps
<container is not present>

$ docker logs -f crazy-cow

As we can see the container failed to start because it cannot connect to the database.

Then let’s spin up a database container from Docker Hub: https://hub.docker.com/_/mysql

$ docker run --name mysqldb -e MYSQL_DATABASE=greetings -e MYSQL_ROOT_PASSWORD=greetings -d mysql:8

Now let’s try to run the application container again

$ docker run -d -p 8080:8080 --name crazy-cow greetings-ws:mysqldb

Strange, it still cannot connect to the database even though they’re both on localhost, or … maybe not?

Network isolation!

When a docker container starts only the ports mapped to the host are accessible. When a computer tries to go to localhost it actually goes to its own localhost. So how can 2 containers communicate?

Two or more containers are accessible by their names and can communicate only if they are in the same overlay network.

Let’s create a docker overlay network

$ docker network create greetings-network

To run a docker container in a network we need to specify the ` –network` parameter

$ docker rm mysqldb crazy-cow (cleanup the old containers)

$ docker run -d --name mysqldb --network=greetings-network -e MYSQL_DATABASE=greetings -e MYSQL_ROOT_PASSWORD=greetings mysql:8

$ docker run -d --name crazy-cow --network=greetings-network -e MYSQL_HOST=mysqldb -p 8080:8080 greetings-ws:mysqldb

If you run now

$ docker logs -f crazy-cow

The logs should look fine. You can go to http://localhost:8080/greetings/new?name=CrazyCow to give it a try