For reliable ZooKeeper service, you should deploy ZooKeeper in a
cluster known as an ensemble. As long as a majority
of the ensemble are up, the service will be available. Because
Zookeeper requires a majority, it is best to use an odd number of
machines. For example, with four machines ZooKeeper can only handle
the failure of a single machine; if two machines fail, the
remaining two machines do not constitute a majority. However, with
five machines ZooKeeper can handle the failure of two machines.