Why it matters

ZooKeeper is deployed everywhere. Understanding is essential for operating any Hadoop-adjacent system.

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The architecture

Ensemble: 3 or 5 servers. Majority quorum required.

Namespace: znodes (hierarchical). Persistent or ephemeral.

ZooKeeper primitivesEnsemble3 or 5 serversZnodeshierarchicalWatches + ephemeralfor coordinationEphemeral znodes disappear on client session end; foundation for HA leader election
ZooKeeper features.
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How it works end to end

ZAB: total-order broadcast. Leader accepts writes; followers replicate.

Sessions: client-server with timeout. Ephemeral znodes tied to session.

Watches: one-shot notification on znode change.

Recipes: leader election, distributed lock, membership.