The Raspberry Pi is a cheap and fun piece of kit for geeks. My QNAP NAS is running a few too many applications on Docker, and I have a few Raspberry Pi’s laying around, so I thought it’d be a fun weekend project to turn them into a Docker Swarm.

The Swarm

Before getting into the automation side of things, let’s set up a prototype of the swarm itself. I’ll be setting up;

  • A Raspberry Pi Rev 1 Model B,
  • A Raspberry Pi Rev 3 Model B,

The Raspberry Pi’s will be running Raspberry Pi OS Buster (2020-08-20). While the Rev 3 is 64 bit, I’ll be using the armhf on both. All the machines are on the same network.

Follow the official instructions to burn the image to the SD cards and spin up the Pi’s. We’ll be needing Docker, so follow the instructions for installing that too.

The Docker Swarm has two types of nodes; managers and workers. The managers keep the state of the swarm in a distributed store by using the Raft Consensus Algorithm. For best results, the amount of managers should be odd. I only have three machines to work with, so I’ll stick with a single manager for now. This cluster doesn’t need to be highly available. The Rev 3 is the most powerful RPi, so I’ll be electing it as the manager.

pi@manager:~$ sudo docker swarm init
Swarm initialized: current node (eseiud89bm9gl3eail1tnnznx) is now a manager.

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

    docker swarm join --token SWMTKN-1-6814zqm6wpp4q43fe9a9m2ezxcsecjnlmvcayvm5ydzpwkegdh-4n5qaw4ujge9xcmi2vcr0mcrn 192.168.0.2:2377

To add a manager to this swarm, run 'docker swarm join-token manager' and follow the instructions.

pi@manager:~$ sudo docker node ls
ID                            HOSTNAME            STATUS              AVAILABILITY        MANAGER STATUS      ENGINE VERSION
eseiud89bm9gl3eail1tnnznx *   manager             Ready               Active              Leader              19.03.13

Docker is nice enough to provide instructions for the workers. Let’s do that.

pi@worker:~$ sudo docker swarm join --token SWMTKN-1-6814zqm6wpp4q43fe9a9m2ezxcsecjnlmvcayvm5ydzpwkegdh-4n5qaw4ujge9xcmi2vcr0mcrn 192.168.0.2:2377
This node joined a swarm as a worker.

All management commands need to be run on the manager, so let’s check the list of nodes;

pi@manager:~$ sudo docker node ls
ID                            HOSTNAME            STATUS              AVAILABILITY        MANAGER STATUS      ENGINE VERSION
eseiud89bm9gl3eail1tnnznx *   manager             Ready               Active              Leader              19.03.13
bcm9cnme7ewvx224klfun1qfn     worker              Ready               Active                                  19.03.13

Now let’s run an application on the swarm. For demonstration purposes let’s just use some random image, like nginxdemos/hello.

pi@manager:~$ sudo docker pull nginxdemos/hello
Using default tag: latest
latest: Pulling from nginxdemos/hello
550fe1bea624: Pull complete
d421ba34525b: Pull complete
fdcbcb327323: Pull complete
bfbcec2fc4d5: Pull complete
0497d4d5654f: Pull complete
f9518aaa159c: Pull complete
a70e975849d8: Pull complete
Digest: sha256:f5a0b2a5fe9af497c4a7c186ef6412bb91ff19d39d6ac24a4997eaed2b0bb334
Status: Downloaded newer image for nginxdemos/hello:latest
docker.io/nginxdemos/hello:latest

pi@manager:~$ sudo docker service create -p 8080:80 nginxdemos/hello:0.2
image nginxdemos/hello:0.2 could not be accessed on a registry to record
its digest. Each node will access nginxdemos/hello:0.2 independently,
possibly leading to different nodes running different
versions of the image.

9ldqvus6y2zzp64pj187mmb52
overall progress: 1 out of 1 tasks
1/1: running   [==================================================>]
verify: Service converged

pi@manager:~$ sudo docker service ls
ID                  NAME                MODE                REPLICAS            IMAGE                  PORTS
9ldqvus6y2zz        jolly_rhodes        replicated          1/1                 nginxdemos/hello:0.2   *:8080->80/tcp

pi@manager:~$ sudo docker service ps jolly_rhodes
ID                  NAME                IMAGE                  NODE                DESIRED STATE       CURRENT STATE           ERROR               PORTS
hfebnhjrg0v7        jolly_rhodes.1      nginxdemos/hello:0.2   manager             Running             Running 7 minutes ago

Fantastic. We have a service running in the swarm. Let’s see if it works;

pi@manager:~$ links -dump http://localhost:8080
   NGINX Logo

   Server address: 10.0.0.35:80

   Server name: 2c704cbf842b

   Date: 12/Nov/2020:19:53:43 +0000

   URI: /

   [ ] Auto Refresh
                  Request ID: e620aa94f6e13add9a69cd03340b1551
                               © NGINX, Inc. 2018

Now you can do all the typical things with Docker Swarm; adding replicas, deploying stacks and adding more nodes. In the next post we’ll go through how this can be automated using Ansible to easily scale to more nodes.