Get Started, Part 4: Swarms

• 1: Orientation
• 2: Containers
• 3: Services
• 4: Swarms
• 5: Stacks
• 6: Deploy your app

Prerequisites

• Install Docker version 1.13 or higher.

• Get Docker Compose as described in Part 3 prerequisites.

• Get Docker Machine, which is pre-installed with Docker for Mac and Docker for Windows, but on Linux systems you need to install it directly. On pre Windows 10 systems without Hyper-V, use Docker Toolbox.

• Read the orientation in Part 1.

• Learn how to create containers in Part 2.

• Make sure you have published the friendlyhello image you created by pushing it to a registry. We’ll be using that shared image here.

• Be sure your image works as a deployed container. Run this command, slotting in your info for username, repo, and tag: docker run -p 80:80 username/repo:tag, then visit http://localhost/.

• Have a copy of your docker-compose.yml from Part 3 handy.

Introduction

In part 3, you took an app you wrote in part 2, and defined how it should run in production by turning it into a service, scaling it up 5x in the process.

Here in part 4, you deploy this application onto a cluster, running it on multiple machines. Multi-container, multi-machine applications are made possible by joining multiple machines into a “Dockerized” cluster called a swarm.

Understanding Swarm clusters

A swarm is a group of machines that are running Docker and joined into a cluster. After that has happened, you continue to run the Docker commands you’re used to, but now they are executed on a cluster by a swarm manager. The machines in a swarm can be physical or virtual. After joining a swarm, they are referred to as nodes.

Swarm managers can use several strategies to run containers, such as “emptiest node” – which fills the least utilized machines with containers. Or “global”, which ensures that each machine gets exactly one instance of the specified container. You instruct the swarm manager to use these strategies in the Compose file, just like the one you have already been using.

Swarm managers are the only machines in a swarm that can execute your commands, or authorize other machines to join the swarm as workers. Workers are just there to provide capacity and do not have the authority to tell any other machine what it can and cannot do.

Up until now, you have been using Docker in a single-host mode on your local machine. But Docker also can be switched into swarm mode, and that’s what enables the use of swarms. Enabling swarm mode instantly makes the current machine a swarm manager. From then on, Docker will run the commands you execute on the swarm you’re managing, rather than just on the current machine.

Set up your swarm

A swarm is made up of multiple nodes, which can be either physical or virtual machines. The basic concept is simple enough: run docker swarm init to enable swarm mode and make your current machine a swarm manager, then run docker swarm join on other machines to have them join the swarm as workers. Choose a tab below to see how this plays out in various contexts. We’ll use VMs to quickly create a two-machine cluster and turn it into a swarm.

Create a cluster

• Local VMs (Mac, Linux, Windows 7 and 8)
• Local VMs (Windows 10/Hyper-V)

$ docker-machine create --driver virtualbox myvm1
$ docker-machine create --driver virtualbox myvm2

$ docker-machine ssh myvm1 "docker swarm init"
Swarm initialized: current node <node ID> is now a manager.

To add a worker to this swarm, run the following command:

  docker swarm join \
  --token <token> \
  <ip>:<port>

$ docker-machine ssh myvm2 "docker swarm join \
--token <token> \
<ip>:<port>"

This node joined a swarm as a worker.

docker@myvm1:~$ docker node ls
ID                            HOSTNAME            STATUS              AVAILABILITY        MANAGER STATUS
brtu9urxwfd5j0zrmkubhpkbd     myvm2               Ready               Active              
rihwohkh3ph38fhillhhb84sk *   myvm1               Ready               Active              Leader

docker-machine ssh myvm1 "docker node ls"

$ docker-machine create -d hyperv --hyperv-virtual-switch "myswitch" myvm1
$ docker-machine create -d hyperv --hyperv-virtual-switch "myswitch" myvm2

$ docker-machine ssh myvm1 "docker swarm init"
Swarm initialized: current node <node ID> is now a manager.

To add a worker to this swarm, run the following command:

  docker swarm join \
  --token <token> \
  <ip>:<port>

$ docker-machine ssh myvm2 "docker swarm join \
--token <token> \
<ip>:<port>"

This node joined a swarm as a worker.

docker@myvm1:~$ docker node ls
ID                            HOSTNAME            STATUS              AVAILABILITY        MANAGER STATUS
brtu9urxwfd5j0zrmkubhpkbd     myvm2               Ready               Active              
rihwohkh3ph38fhillhhb84sk *   myvm1               Ready               Active              Leader

docker-machine ssh myvm1 "docker node ls"

Deploy your app on a cluster

The hard part is over. Now you just repeat the process you used in part 3 to deploy on your new swarm. Just remember that only swarm managers like myvm1 execute Docker commands; workers are just for capacity.

Copy the file docker-compose.yml you created in part 3 to the swarm manager myvm1’s home directory (alias: ~) by using the docker-machine scp command:

docker-machine scp docker-compose.yml myvm1:~

Now have myvm1 use its powers as a swarm manager to deploy your app, by sending the same docker stack deploy command you used in part 3 to myvm1 using docker-machine ssh:

docker-machine ssh myvm1 "docker stack deploy -c docker-compose.yml getstartedlab"

And that’s it, the app is deployed on a cluster.

Wrap all the commands you used in part 3 in a call to docker-machine ssh, and they’ll all work as you’d expect. Only this time, you’ll see that the containers have been distributed between both myvm1 and myvm2.

$ docker-machine ssh myvm1 "docker stack ps getstartedlab"

ID            NAME        IMAGE              NODE   DESIRED STATE
jq2g3qp8nzwx  test_web.1  username/repo:tag  myvm1  Running
88wgshobzoxl  test_web.2  username/repo:tag  myvm2  Running
vbb1qbkb0o2z  test_web.3  username/repo:tag  myvm2  Running
ghii74p9budx  test_web.4  username/repo:tag  myvm1  Running
0prmarhavs87  test_web.5  username/repo:tag  myvm2  Running

Accessing your cluster

You can access your app from the IP address of either myvm1 or myvm2. The network you created is shared between them and load-balancing. Run docker-machine ls to get your VMs’ IP addresses and visit either of them on a browser, hitting refresh (or just curl them). You’ll see five possible container IDs all cycling by randomly, demonstrating the load-balancing.

The reason both IP addresses work is that nodes in a swarm participate in an ingress routing mesh. This ensures that a service deployed at a certain port within your swarm always has that port reserved to itself, no matter what node is actually running the container. Here’s a diagram of how a routing mesh for a service called my-web published at port 8080 on a three-node swarm would look:

Having connectivity trouble?

Keep in mind that in order to use the ingress network in the swarm, you need to have the following ports open between the swarm nodes before you enable swarm mode:

• Port 7946 TCP/UDP for container network discovery.
• Port 4789 UDP for the container ingress network.

Iterating and scaling your app

From here you can do everything you learned about in part 3.

Scale the app by changing the docker-compose.yml file.

Change the app behavior by editing code.

In either case, simply run docker stack deploy again to deploy these changes.

You can join any machine, physical or virtual, to this swarm, using the same docker swarm join command you used on myvm2, and capacity will be added to your cluster. Just run docker stack deploy afterwards, and your app will take advantage of the new resources.

Cleanup

You can tear down the stack with docker stack rm. For example:

docker-machine ssh myvm1 "docker stack rm getstartedlab"

Keep the swarm or remove it?

At some point later, you can remove this swarm if you want to with docker-machine ssh myvm2 "docker swarm leave" on the worker and docker-machine ssh myvm1 "docker swarm leave --force" on the manager, but you’ll need this swarm for part 5, so please keep it around for now.

On to Part 5 »

Recap and cheat sheet (optional)

Here’s a terminal recording of what was covered on this page:

In part 4 you learned what a swarm is, how nodes in swarms can be managers or workers, created a swarm, and deployed an application on it. You saw that the core Docker commands didn’t change from part 3, they just had to be targeted to run on a swarm master. You also saw the power of Docker’s networking in action, which kept load-balancing requests across containers, even though they were running on different machines. Finally, you learned how to iterate and scale your app on a cluster.

Here are some commands you might like to run to interact with your swarm a bit:

docker-machine create --driver virtualbox myvm1 # Create a VM (Mac, Win7, Linux)
docker-machine create -d hyperv --hyperv-virtual-switch "myswitch" myvm1 # Win10
docker-machine env myvm1                # View basic information about your node
docker-machine ssh myvm1 "docker node ls"         # List the nodes in your swarm
docker-machine ssh myvm1 "docker node inspect <node ID>"        # Inspect a node
docker-machine ssh myvm1 "docker swarm join-token -q worker"   # View join token
docker-machine ssh myvm1   # Open an SSH session with the VM; type "exit" to end
docker-machine ssh myvm2 "docker swarm leave"  # Make the worker leave the swarm
docker-machine ssh myvm1 "docker swarm leave -f" # Make master leave, kill swarm
docker-machine start myvm1            # Start a VM that is currently not running
docker-machine stop $(docker-machine ls -q)               # Stop all running VMs
docker-machine rm $(docker-machine ls -q) # Delete all VMs and their disk images
docker-machine scp docker-compose.yml myvm1:~     # Copy file to node's home dir
docker-machine ssh myvm1 "docker stack deploy -c <file> <app>"   # Deploy an app