## Deploy Java EE 7 Application (Container Linking)

<<JavaEE7_PreBuilt_WAR>> explained how to use an in-memory database with the application server. This gets you started rather quickly but becomes a bottleneck soon as the database is only in-memory. This means that any changes made to your schema and data are lost after the application server shuts down. In this case, you need to use a database server that resides outside the application server. For example, MySQL as the database server and WildFly as the application server.

This section will show how https://docs.docker.com/userguide/dockerlinks/[Docker Container Linking] can be used to connect to a service running inside a Docker container via a network port.

. Start MySQL server as:

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docker run --name mysqldb -e MYSQL_USER=mysql -e MYSQL_PASSWORD=mysql -e MYSQL_DATABASE=sample -e MYSQL_ROOT_PASSWORD=supersecret -p 3306:3306 -d mysql

`-e` define environment variables that are read by the database at startup and allow us to access the database with this user and password.

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. Start WildFly and deploy Java EE 7 application as:

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docker run -it --name mywildfly --link mysqldb:db -p 8080:8080 arungupta/wildfly-mysql-javaee7

`--link` takes two parameters - first is name of the container we're linking to and second is the alias for the link name.

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.Container Linking

[NOTE]

Creating a link between two containers creates a conduit between a source container and a target container and securely transfer information about source container to target container.

In our case, target container (WildFly) can see information about source container (MySQL). When containers are linked, information about a source container can be sent to a recipient container. This allows the recipient to see selected data describing aspects of the source container. For example, IP address of MySQL server is expoed at $DB_PORT_3306_TCP_ADDR and port of MySQL server is exposed at $DB_PORT_3306_TCP_PORT. These are then used to create the JDBC resource.

See more about container communication on the Docker website link:https://docs.docker.com/userguide/dockerlinks/[Linking Containers Together]

. See the output as:

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> curl http://$(docker-machine ip lab):8080/employees/resources/employees

<?xml version="1.0" encoding="UTF-8" standalone="yes"?><collection><employee><id>1</id><name>Penny</name></employee><employee><id>2</id><name>Sheldon</name></employee><employee><id>3</id><name>Amy</name></employee><employee><id>4</id><name>Leonard</name></employee><employee><id>5</id><name>Bernadette</name></employee><employee><id>6</id><name>Raj</name></employee><employee><id>7</id><name>Howard</name></employee><employee><id>8</id><name>Priya</name></employee></collection>