docker use case

http://www.wouterdanes.net/2014/04/11/continuous-integration-using-docker-maven-and-jenkins.html

Docker

Docker is a tool allows you to run multiple processes in their own containers on a machine without the overhead of virtual machines. It does give process separation and to the processes it looks like they're running in their own linux environment. Docker was created in Go and is basically an interface to Linux Containers (LXC), a feature of the linux kernel that was introduced in the 3.8 release.

Docker works well under Ubuntu and Red Hat is working to make it enterprise ready for RHEL.

Docker allows you to specify containers using Dockerfiles. These can be compared to Vagrantfiles that are used to provision virtual machines. These docker files can be composed of other docker files, creating a sort of inheritance / composition of containers.

Creating an Integration Server Using Docker

I created a sample project on Github that shows how you can run integration tests on a sample Hippo project using docker. If you have Vagrant, you can see the Jenkins server in action with a simple vagrant up from the root of the project. Using the composition command in the Dockerfiles (FROM), I've built an oracle jdk 7 image that is used as the base of a tomcat image. I then use that tomcat image as the base for my integration environments. The Dockerfile for the integration image resides in the root of the project and is as follows:

FROM wouterd/tomcatMAINTAINER Wouter Danes "https://github.com/wouterd"ADD myhippoproject.tar.gz /tmp/app-distribution/RUN for i in $(ls /tmp/app-distribution/) ; do mkdir -p /var/lib/tomcat6/${i} && cp -f /tmp/app-distribution/${i}/* /var/lib/tomcat6/${i}/ ; done

The wouterd/tomcat image is built as follows:

FROM        wouterd/oracle-jre7VOLUME      ["/var/log/tomcat6"]MAINTAINER  Wouter Danes "https://github.com/wouterd"RUN         apt-get install -y tomcat6CMD         JAVA_HOME=/usr/lib/jvm/java-7-oracle CATALINA_BASE=/var/lib/tomcat6 CATALINA_HOME=/usr/share/tomcat6 /usr/share/tomcat6/bin/catalina.sh runEXPOSE      8080

The wouterd/oracle-jre7 has some magic to install jdk 7 from oracle on a clean ubuntu VM. The code for this container and the two above is in the docker-images directory in the Github project. The wouterd/tomcat image does the following:

• FROM wouterd/oracle-jre7 takes the container wouterd/oracle-jre7 (a container with Ubuntu + oracle java7 installed)
• VOLUME ["/var/log/tomcat6"] tells the container to expose that path to the outside world. Docker actually "physically" places this path outside the container so that other containers can also reach it. Further down I will show why this is great. (Sneak peak: syslog deprecated?)
• RUN apt-get install -y tomcat6 installs tomcat6 using the ubuntu repository
• CMD [some bash] sets the command that gets executed when this container is run, setting two environment variables and starting catalina.sh
• EXPOSE 8080 tells docker to expose port 8080 in the container to the host system.

The docker container that gets built during the integration test simply does two things:

• Takes the wouterd/tomcat container
• ADD [file] [destination] copies the tar.gz with the distribution from the maven project to a temporary folder and extracts it if it's a tar.
• The RUN line is a convoluted way to copy the contents of the tar over the TOMCAT_HOME directory, for some reason simply adding the tar to the TOMCAT_HOME folder didn't let it extract properly. It would probably work if TOMCAT_HOME was just empty.
• It inherits the CMD directive from the wouterd/tomcat container, so if you "run" the integration container using docker run, it will start up tomcat and start deploying the distribution.