In latest Server Installation documentation, in section 3.2. Standalone Clustered Mode
https://www.keycloak.org/docs/latest/server_installation/index.html#_operating-mode there is no information about setting synchronization between nodes.
Although there is section 8.5. Multicast Network Setup which describes one of the methods, it is not referenced from 3.2. Besides, both sections describe nearly same topic, IMO should be somehow merged.
As the text reads, I've started two nodes using docker image (jboss/keycloak:6.0.0) on two machines in a swarm and expected it to work. Unfortunately, after logging to administrative console I've got error on many XHR requests which needed authorization, but were redirected to different node. I'm aware that setup with solely swarm ingress load balancer is not common on production, but users may try to exercise this way.
Recently a very good post about this was posted to Keycloak blog, https://www.keycloak.org/2019/05/keycloak-cluster-setup.html. I followed it and it works like a charm.
I think some parts of it, if not whole, should be included in documentation, or at least should be referenced in a 'note' section.