Getting Started with Docker (Tech Tip #39)

If the numbers of articles, meetups, talk submissions at different conferences, tweets, and other indicators are taken into consideration, then seems like Docker is going to solve world hunger. It would be nice if it would, but apparently not. But it does solve one problem really well!

Lets hear it from @solomonstre – creator of Docker project!

In short, Docker simplifies software delivery by making it easy to build and share images that contain your application’s entire environment, or application operating system.

What does it mean by application operating system ?

Your application typically require a specific version of operating system, application server, JDK, database server, may require to tune the configuration files, and similarly multiple other dependencies. The application may need binding to specific ports and certain amount of memory. The components and configuration together required to run your application is what is referred to as application operating system.

You can certainly provide an installation script that will download and install these components. Docker simplifies this process by allowing to create an image that contains your application and infrastructure together, managed as one component. These images are then used to create Docker containers which run on the container virtualization platform, provided by Docker.

What are the main components of Docker ?

Docker has two main components:

  • Docker: the open source container virtualization platform
  • Docker Hub: SaaS platform for sharing and managing Docker images

Docker uses Linux Containers to provide isolation, sandboxing, reproducibility, constraining resources, snapshotting and several other advantages. Read this excellent piece at InfoQ on Docker Containers for more details on this.

Images are “build component” of Docker and a read-only template of application operating system. Containers are runtime representation, and created from, images. They are “run component” of Docker. Containers can be run, started, stopped, moved, and deleted. Images are stored in a registry, the “distribution component” of Docker.

Docker in turn contains two components:

  • Daemon runs on a host machine and does the heavy lifting of building, running, and distributing Docker containers.
  • Client is a Docker binary that accepts commands from the user and communicates back and forth with daemon

How do these work together ?

Client communicates with Daemon, either co-located on the same host, or on a different host. It requests the Daemon to pull an image from the repository using pull command. The Daemon then downloads the image from Docker Hub, or whatever registry is configured. Multiple images can be downloaded from the registry and installed on Daemon host.

docker-architecture-techtip39

Client can then start the Container using run command. The complete list of client commands can be seen here.

Client communicates with Daemon using sockets or REST API.

Because Docker uses Linux Kernel features, does that mean I can use it only on Linux-based machines ?

Docker daemon and client for different operating systems can be installed from docs.docker.com/installation/. As you can see, it can be installed on a wide variety of platforms, including Mac and Windows.

For non-Linux machines, a lightweight Virtual Machine needs to be installed and Daemon is installed within that. A native client is then installed on the machine that communicates with the Daemon. Here is the log from booting Docker daemon on Mac:

For example, Docker Daemon and Client can be installed on Mac following the instructions at docs.docker.com/installation/mac.

The VM can be stopped from the CLI as:

And then restarted again as:

And logged in as:

The complete list of boot2docker commands are available in help:

Enough talk, show me an example ?

Some of the JBoss projects are available as Docker images at www.jboss.org/docker and can be installed following the commands explained on that page. For example, WildFly Docker image can be installed as:

The image can be verified using the command:

Once the image is downloaded, the container can be started as:

By default, Docker containers do not provide an interactive shell and input from STDIN. So if WildFly Docker container is started using the command above, it cannot be terminated using Ctrl + C.  Specifying -i option will make it interactive and -t option allocated a pseudo-TTY.

In addition, we’d also like to make the port 8080 accessible outside the container, i.e. on our localhost. This can be achieved by specifying -p 80:8080 where 80 is the host port and 8080 is the container port.

So we’ll run the container as:

Container’s IP address can be found as:

The started container can be verified using the command:

And now the WildFly server can now be accessed on your local machine as http://192.168.59.103 and looks like as shown:

Finally the container can be stopped by hitting Ctrl + C, or giving the command as:

The container id obtained from “docker ps” is passed to the command here.

More detailed instructions to use this image, such as booting in domain mode, deploying applications, etc. can be found at github.com/jboss/dockerfiles/blob/master/wildfly/README.md.

What else would you like to see in the WildFly Docker image ? File an issue at github.com/jboss/dockerfiles/issues.

Other images that are available at jboss.org/docker are:

 

Did you know that Red Hat is amongst one of the top contributors to Docker, with 5 Red Hatters from Project Atomic working on it ?

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