https://4f0a9a3b.wangpeng.pages.dev/en/tags/zab/Recent content in ZAB on Zhuifeng NotesHugoenTue, 25 Aug 2026 00:00:00 +0000https://4f0a9a3b.wangpeng.pages.dev/en/tech/zookeeper/model-zab/Tue, 25 Aug 2026 00:00:00 +0000https://4f0a9a3b.wangpeng.pages.dev/en/tech/zookeeper/model-zab/<p>ZooKeeper&rsquo;s reliability comes from its simple data model and the strongly-consistent ZAB protocol. Understanding both is a prerequisite for using ZK correctly.</p> <h2 id="the-znode-data-model">The znode Data Model </h2> <p>ZK maintains a hierarchical tree similar to a file system; each node is a <strong>znode</strong> that can hold both data and children. Its type determines lifecycle and concurrency semantics:</p> <ul> <li><strong>Persistent / Ephemeral</strong>: ephemeral nodes are auto-deleted when the session disconnects, forming the basis of distributed locks and leader election.</li> <li><strong>Plain / Sequential</strong>: sequential nodes get a monotonically increasing sequence number appended on creation.</li> <li>Each znode carries <code>version</code>, <code>cversion</code>, <code>aversion</code>; versions increment on modification, providing optimistic-lock semantics.</li> </ul> <h2 id="session-and-watcher">Session and Watcher </h2> <p>A client establishes a <strong>session</strong> with the server, kept alive via heartbeats. On session timeout the server cleans up its ephemeral nodes. A client can register a <strong>Watcher</strong> on a node and receives a one-time notification when the node changes (data write, children change); it must re-register to keep watching.</p>