NOTE
ZooKeeper Watches
Watch notifications, one-shot/re-registration semantics, ordering, disconnect gaps, and cache reconciliation.
This is a historical learning note and may contain outdated or incomplete understanding.
ZooKeeper watches notify clients that watched state changed. Traditional watches are one-shot: after receiving a notification, the client normally re-reads state and re-establishes the required watch (newer APIs also provide persistent-watch variants depending on version).
A notification says “something changed”, not necessarily the complete final state. Multiple changes can occur before a client processes/re-reads.
Correct consumers maintain a local cache by watch → re-read/reconcile, and handle disconnect/session-expiration cases where assumptions about watch coverage or ephemeral ownership may no longer hold.