What is Docker Swarm?

Docker Swarm provides native clustering to Docker. Clustering using Docker Swarm 0.2.0 provides a basic introduction to Docker Swarm, and how to create a simple three node cluster. As a refresher, the key components of Docker Swarm are shown below:

In short, Swarm Manager is a pre-defined Docker Host, and is a single point for all administration. Additional Docker hosts are identified as Nodes and communicate with the Manager using TCP. By default, Swarm uses hosted Discovery Service, based on Docker Hub, using tokens to discover nodes that are part of a cluster. Each node runs a Node Agent that registers the referenced Docker daemon, monitors it, and updates the Discovery Service with the node’s status. The containers run on a node.

That blog provide complete details, but a quick summary to create the cluster is shown below:

# Create cluster
TOKEN=`docker run swarm create`
# Creating Swarm master
docker-machine create -d virtualbox --swarm --swarm-master --swarm-discovery token://$TOKEN swarm-master
# Creating swarm node 01
docker-machine create -d virtualbox --swarm --swarm-discovery token://$TOKEN swarm-node-01
Create swarm node 02
docker-machine create -d virtualbox --swarm --swarm-discovery token://$TOKEN swarm-node-02

Listing the cluster shows:

NAME            ACTIVE   DRIVER       STATE     URL                         SWARM
swarm-master             virtualbox   Running   tcp://192.168.99.106:2376   swarm-master (master)
swarm-node-01            virtualbox   Running   tcp://192.168.99.107:2376   swarm-master
swarm-node-02   *        virtualbox   Running   tcp://192.168.99.108:2376   swarm-master

It has one master and two nodes.

Deploy a Java EE application to Docker Swarm

All hosts in the cluster are accessible using a single, virtual host. Swarm serves the standard Docker API, so any tool that communicates with a single Docker host communicate can scale to multiple Docker hosts by communicating to this virtual host.

Docker Container Linking Across Multiple Hosts explains how to link containers across multiple Docker hosts. It deploys a Java EE 7 application to WildFly on one Docker host, and connects it with a MySQL container running on a different Docker host. We can deploy both of these containers using the virtual host, and they will then be deployed to the Docker Swarm cluster.

Lets get started!

MySQL on Docker Swarm

1. Start the MySQL container
   

   
   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

2. Status of the container can be seen as:
   
   

   
   CONTAINER ID        IMAGE               COMMAND                CREATED             STATUS              PORTS                           NAMES
   b49d627a0431        mysql:latest        "/entrypoint.sh mysq   5 minutes ago       Up 4 minutes        192.168.99.107:3306->3306/tcp   swarm-node-01/mysqldb

3. It shows the container is running on swarm-node-01. Make sure you are connected to the Docker Swarm cluster using eval $(docker-machine env --swarm swarm-master).
4. Find IP address of the host where this container is started:
   
   

   
   ~> docker inspect --format '{{ .Node.Ip }}' $(docker ps -q --filter 'name=*mysqldb*')
   192.168.99.107

WildFly on Docker Swarm

~> docker ps
CONTAINER ID        IMAGE                                  COMMAND                CREATED             STATUS              PORTS                           NAMES
ab5717083812        arungupta/wildfly-mysql-javaee7:host   "/opt/jboss/wildfly/   25 minutes ago      Up 25 minutes       192.168.99.108:8080->8080/tcp   swarm-node-02/mywildfly   
b49d627a0431        mysql:latest                           "/entrypoint.sh mysq   34 minutes ago      Up 33 minutes       192.168.99.107:3306->3306/tcp   swarm-node-01/mysqldb

Enjoy!