[keycloak-dev] Clustering and user sessions

Had an idea with regards to clustering and user sessions. Instead of sending messages to the cluster when a individual user session is refreshed all nodes send a periodic update message. Obviously that's only for user sessions and not for admin updates, where we should still send invalidation messages for each update. Each node would keep a note of all user sessions where it has updated LastSessionRefresh, and once every period it would send this list to all nodes. This should mean that instead of sending a message every time a single session is updated, we send a single message per node every 60 seconds or so (should be configurable). When receiving a message from the cluster the node would go through the list and update the user sessions where the received LastSessionRefresh is higher than the one it has itself. Nodes still use the mem user sessions store, but with the cache on top.