tl;dr: Zynga uses zookeeper.

I was extremely disappointed with this blog post. It reads like a ninth grade composition assignment, and is extremely lacking in details.

Well I'd kind of like to know how they extended ZooKeeper to do that.

Maybe just zk clients that listen to updates (watchers) on specific znodes? Business/validation logic can be on clients that can either be embedded in the app or write out to a config file consumed by another process.