docker-1.12

First Release Candidate of Docker 1.12 was announced over two weeks ago. Several new features are
planned for this release.

This blog will show how to create a Distributed Application Bundle from Docker Compose and deploy it as Docker Stack in Docker Swarm Mode. Many thanks to @friism for helping me understand these concepts.

Let's look at the features first:

  • Built-in orchestration: A typical application is defined using a Docker Compose file. This definition consists of multiple containers and deployed on multiple hosts. This avoids Single Point of Failure (SPOF) and keeps your application
    resilient. Multiple orchestration frameworks such as Docker Swarm, Kubernetes and Mesos allow you to orchestrate these applications. However it is such an important characteristic of the application, Docker Engine now has built-in orchestration.
    More details on this topic in a later blog.
  • Service: A replicated, distributed and load balanced service can be easily created using docker service create command. A “desired state” of the application, such as run 3 containers of Couchbase, is provided and the
    self-healing Docker engine ensures that that many containers are running in the cluster. If a container goes down, another container is started. If a node goes down, containers on that node are started on a different node. More on this in a later
    blog.
  • Zero-configuration Security: Docker 1.12 comes with mutually authenticated TLS, providing authentication, authorization and encryption to the communications of every node participating in the swarm, out of the box. More on this in
    a later blog.
  • Docker Stack and Distributed Application Bundle: Distributed Application Bundle, or DAB, is a multi-services distributable image format. Read further for more details.

So far, you can take a Dockerfile and create an image from it using the docker build command. A container can be started using the docker run command. Multiple containers can be easily started by giving
that command multiple times. Or you can also use Docker Compose file and scale up your containers using the docker-compose scale command.
docker-lifecycle

Image is a portable format for a single container. Distributed Application Bundle, or DAB, is a new concept introduced in Docker 1.12, is a portable format for multiple containers. Each bundle can be then deployed as
a Stack at runtime.

docker-stack-lifecycle

Learn more about DAB at docker.com/dab. For simplicity, here is an analogy that can be drawn:

Dockerfile -> Image -> Container

Docker Compose -> Distributed Application Bundle -> Docker Stack

Let's use a Docker Compose file, create a DAB from it, and deploy it as a Docker Stack.

Its important to note that this is an experimental feature in 1.12-RC2.

Create a Distributed Application Bundle from Docker Compose

Docker Compose CLI adds a new bundle command. More details can be found:

Now, let's take a Docker Compose definition and create a DAB from it. Here is our Docker Compose definition:

This Compose file starts a WildFly and a Couchbase server. A Java EE application is pre-deployed in the WildFly server that connects to the Couchbase server and allows to perform CRUD operations using the REST API. The source for this file is at: 
github.com/arun-gupta/oreilly-docker-book/blob/master/hello-javaee/docker-compose.yml. Generate an application bundle with it:

depends_on only creates dependency between two services and makes them start in a specific order. This only ensures that the Docker container is started but the application within the container may take longer to start. So this attribute only
partially solves the problem. container_name gives a specific name to the container. Relying upon a specific container name is tight coupling and does not allow to scale the container.  So both the warnings can be ignored,
for now. This command generates a file using the Compose project name, which is the directory name. So in our case, hellojavaee.dsb file is generated. This file extension has been renamed to .dab in RC3. The generated
application bundle looks like:

This file provides complete description of the services included in the application. I'm not entirely sure if Distributed Application Bundle is the most appropriate name, discuss this in #24250. It
would be great if other container formats, such as Rkt, or even VMs can be supported here. But for now, Docker is the only supported format.

Initialize Swarm Mode in Docker

As mentioned above, “desired state” is now maintained by Docker Swarm. And this is now baked into Docker Engine already. Docker Swarm concepts have evolved as well and can be read at Swarm mode key concepts. A
more detailed blog on this will be coming later. But for this blog, a new command docker swarm is now added:

Initialize a Swarm node (as a manager) in the Docker Engine:

More details about this node can be found using docker node inspect self command.

The detailed output is verbose but the relevant section is:

The output shows that the node is a manager. For a single-node cluster, this node will also act as a worker.

 

More details about the cluster can be obtained using the docker swarm inspect command.

AcceptancePolicy shows that other worker nodes can join this cluster, but a manager requires explicit approval.

Deploy a Docker Stack

Create a stack using docker deploy command:

The command usage can certainly be simplified as discussed in #24249. See the list of services:

The output shows that two services, WildFly and Couchbase, are running. Services is also a new concept introduced in Docker 1.12. There is what gives you the
“desired state” and Docker Engine works to give you that. docker ps shows the list of containers running:

WildFly container starts up before the Couchbase container is up and running. This means the Java EE application tries to connect to the Couchbase server and fails. So the application never boots successfully.

Self-healing Docker Service

Docker Service maintains the “desired state” of an application. In our case, the desired state is to ensure that one, and only one, container for the service is running. If we remove the container, not the service, then the service will
automatically start the container again. Remove the container as:

Note, you've to give -f because the container is already running. Docker 1.12 self-healing mechanisms kick in and automatically restart the container. Now if you list the containers again:

This shows that a new container has been started. Inspect the WildFly service:

Swarm assigns a random port to the service, or this can be manually updated using docker service update command. In our case, port 8080 of the container is mapped to 30004 port on the host.

Verify the Application

Check that the application is successfully deployed:

Add a new book to the application:

Verify the books again:

Learn more about this Java EE application at github.com/arun-gupta/oreilly-docker-book/tree/master/hello-javaee.

This blog showed how to create a Distributed Application Bundle from Docker Compose and deploy it as Docker Stack in Docker Swarm Mode.

Docker Service and Stack References

 

Posted by Arun Gupta, VP, Developer Advocacy, Couchbase

Arun Gupta is the vice president of developer advocacy at Couchbase. He has built and led developer communities for 10+ years at Sun, Oracle, and Red Hat. He has deep expertise in leading cross-functional teams to develop and execute strategy, planning and execution of content, marketing campaigns, and programs. Prior to that he led engineering teams at Sun and is a founding member of the Java EE team. Gupta has authored more than 2,000 blog posts on technology. He has extensive speaking experience in more than 40 countries on myriad topics and is a JavaOne Rock Star for three years in a row. Gupta also founded the Devoxx4Kids chapter in the US and continues to promote technology education among children. An author of several books on technology, an avid runner, a globe trotter, a Java Champion, a JUG leader, NetBeans Dream Team member, and a Docker Captain, he is easily accessible at @arungupta.

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