Based on: https://github.com/docker/labs/blob/master/swarm-mode/beginner-tutorial/README.md https://docs.docker.com/engine/swarm/

Check out the documentation on Docker Swarm Mode for more information.

Since this example would require various machine it can be done online using: https://labs.play-with-docker.com/ you will need a Docker ID at https://hub.docker.com/signup

What is a Docker Swarm?

Docker swarm is a container orchestration tool, meaning that it allows the user to manage multiple containers deployed across multiple host machines.

• A swarm consists of multiple Docker hosts which run in swarm mode and act as managers (to manage membership and delegation) and workers (which run swarm services). A given Docker host can be a manager, a worker, or perform both roles. When you create a service, you define its optimal state (number of replicas, network and storage resources available to it, ports the service exposes to the outside world, and more). Docker works to maintain that desired state. For instance, if a worker node becomes unavailable, Docker schedules that node’s tasks on other nodes. A task is a running container which is part of a swarm service and managed by a swarm manager, as opposed to a standalone container.
• One of the key advantages of swarm services over standalone containers is that you can modify a service’s configuration, including the networks and volumes it is connected to, without the need to manually restart the service. Docker will update the configuration, stop the service tasks with the out of date configuration, and create new ones matching the desired configuration.
• When Docker is running in swarm mode, you can still run standalone containers on any of the Docker hosts participating in the swarm, as well as swarm services. A key difference between standalone containers and swarm services is that only swarm managers can manage a swarm, while standalone containers can be started on any daemon.

You can use the Docker CLI to create a swarm, deploy application services to a swarm, and manage swarm behavior. In the same way that you can use Docker Compose to define and run containers, you can define and run Swarm service stacks.

One of the key benefits associated with the operation of a docker swarm is the high level of availability offered for applications.

About data

Swarm Mode itself does not do anything different with volumes, it runs any volume mount command you provide on the node where the container is running. If your volume mount is local to that node, then your data will be saved locally on that node. There is no built in functionality to move data between nodes automatically.

We will not consider this detail here. If interested, a good starting point is this: https://docs.docker.com/engine/swarm/services/#give-a-service-access-to-volumes-or-bind-mounts or this: https://devopsian.net/posts/share-persistent-storage-volumes-in-swarm/

Creating the nodes and Swarm

We start creating 5 nodes (the maximum allowed) to get something like this (IP addresses will vary):

We will now create a swarm by initializing it on the first node (node1). This is done using the command:

[node1] $ docker swarm init --listen-addr <IP_node1> --advertise-addr <IP_node1>