This is an attempt at tackling a problem of implementing docker swarm native clustering for Keycloak. I have seen several examples online, but none of them have quite satisfied me.
I hope this will prove useful for those of you who want to implement some sort of high availability setup.
The main goal was to set up a working Keycloak cluster while avoiding a typical Docker Swarm anti-pattern, namely, defining several docker services different in name only (like "keycloak1", "keycloak2", etc.). A setup like this would require an additional configuration of load balancer or solving the problem of having several endpoints programmatically. There is no need to add an extra instance of haproxy when you can simply delegate the task of load balancing to Docker.
But here's the thing: to make sure that Keycloak containers know whom to call to request clustering, one would have to provide perspective peers' ip-addresses, or other alternatives, on startup. For that reason, I decided to teach Keycloak containers how to discover peers dynamically. And to do that they just have to ask Docker about it using the host utility. An example of that is provided in my version of jgroups.sh script (the part that's added to the original image's script is marked).
This setup works like a charm. In fact, scaling Keycloak service defined this way is as easy as using docker service scale, both up and down.
As an example, I have added a postgres service which is what we use for our project.
To deploy Keycloak cluster you are going to need a proper image described in Dockerfile. It features a customised version of jgroups.sh and a discovery settings file TCPPING.cli (which I have not written myself. It is a version I found somewhere on Keycloak Blog or Forum).
Here's how you build this image with a name and a tag of your choosing:
docker build . -t name_of_your_choice:any_tag
Alternatively, there is a pre-built image uploaded to DockerHub, antonaag/keycloak:12.0.4. It is referenced in keycloak-stack.yaml.
To learn in detail about Docker Swarm, visit the official Docker website.
To deploy a stack, run the following command on a manager node of your swarm cluster:
docker stack deploy -c keycloak-stack.yaml stack
This command will deploy two services to your swarm: stack_postgres and stack_keycloak. After you make sure that Keycloak is functional (for example by visiting its Web Interface or sending a request to its API), you can scale Keycloak by running the following:
docker service scale stack_keycloak=2
Obviously, you can scale it to more than 2 replicas. In terms of high-availability, it makes sense to set max_replicas_per_node equal to 1 (as it is done in keycloak-stack.yaml). In this case, you are limited by a number of nodes in your swarm.