What principle is the Watcher mechanism of ZooKeeper based on?

The Watcher mechanism in ZooKeeper is based on the principle of the publish/subscribe pattern. In ZooKeeper, clients can register a Watcher to monitor the state changes of a specified node. When the state of the node changes, ZooKeeper notifies the registered clients with the Watcher, allowing them to promptly detect the changes in node state and take appropriate actions. This notification method based on the Watcher mechanism enables real-time data synchronization and event notification, making ZooKeeper widely applicable in coordinating and managing tasks in distributed systems.