ZooKeeper - Hadoop: The Definitive Guide - page 758

A client using the ZooKeeper object can receive notifications of the state transitions by
registering a Watcher object. On entering the CONNECTED state, the watcher receives a
WatchedEvent whose KeeperState value is SyncConnected .
NOTE
A ZooKeeper Watcher object serves double duty: it can be used to be notified of changes in the
ZooKeeper state (as described in this section), and it can be used to be notified of changes in znodes (de-
scribed in Watch triggers ). The (default) watcher passed into the ZooKeeper object constructor is used
for state changes, but znode changes may either use a dedicated instance of Watcher (by passing one in
to the appropriate read operation) or share the default one if using the form of the read operation that
takes a Boolean flag to specify whether to use a watcher.
The ZooKeeper instance may disconnect and reconnect to the ZooKeeper service, mov-
ing between the CONNECTED and CONNECTING states. If it disconnects, the watcher re-
ceives a Disconnected event. Note that these state transitions are initiated by the
ZooKeeper instance itself, and it will automatically try to reconnect if the connection is
lost.
The ZooKeeper instance may transition to a third state, CLOSED , if either the
close() method is called or the session times out, as indicated by a KeeperState of
type Expired . Once in the CLOSED state, the ZooKeeper object is no longer con-
sidered to be alive (this can be tested using the isAlive() method on States ) and
cannot be reused. To reconnect to the ZooKeeper service, the client must construct a new
ZooKeeper instance.