NOTE
ZooKeeper Atomic Broadcast (ZAB)
ZooKeeper's leader-based atomic broadcast protocol, proposal ordering, quorum acknowledgment, recovery, and its distinction from Paxos/Raft.
This is a historical learning note and may contain outdated or incomplete understanding.
ZAB is ZooKeeper’s leader-based atomic broadcast protocol for totally ordering state-changing transactions and recovering safely across leader changes.
A leader establishes an epoch/leadership context, proposes ordered updates, collects quorum acknowledgments, commits them, and followers apply the committed order. Recovery synchronizes histories before normal broadcast resumes.
ZAB is related to the consensus/replicated-state-machine problem family but should not be described simply as “Paxos” or “Raft”. Its terminology and recovery protocol are ZooKeeper-specific.