NOTE
ZooKeeper High Availability
Majority-quorum availability, leader failover, ensemble sizing, and why more servers do not always increase fault tolerance.
This is a historical learning note and may contain outdated or incomplete understanding.
ZooKeeper needs a majority quorum to make progress. An ensemble of 2f + 1 voting members can tolerate up to f unavailable members while retaining a majority.
Adding an even-numbered voting server often increases quorum size without increasing the number of failures tolerated; odd-sized ensembles are therefore common.
High availability also depends on independent failure domains, stable low-latency storage/networking, correct timeouts, and operational recovery. Observers/non-voting roles can serve read scalability in suitable versions/designs without changing voting quorum in the same way.