ZooKeeper - Hadoop: The Definitive Guide - page 748

▪ A watch set on an exists operation will be triggered when the znode being
watched is created, deleted, or has its data updated.
▪ A watch set on a getData operation will be triggered when the znode being
watched is deleted or has its data updated. No trigger can occur on creation be-
cause the znode must already exist for the getData operation to succeed.
▪ A watch set on a getChildren operation will be triggered when a child of the
znode being watched is created or deleted, or when the znode itself is deleted.
You can tell whether the znode or its child was deleted by looking at the watch
event type: NodeDeleted shows the znode was deleted, and NodeChil-
drenChanged indicates that it was a child that was deleted.
The combinations are summarized in Table 21-3 .

Table 21-3. Watch creation operations and their corresponding triggers

Watch trigger

Watch cre-
ation

create znode create child delete znode delete child setData

exists NodeCreated NodeDeleted NodeDataChanged

getData NodeDeleted NodeDataChanged

getChildren NodeChildrenChanged NodeDeleted NodeChildrenChanged

A watch event includes the path of the znode that was involved in the event, so for
NodeCreated and NodeDeleted events, you can tell which node was created or de-
leted simply by inspecting the path. To discover which children have changed after a
NodeChildrenChanged event, you need to call getChildren again to retrieve the
new list of children. Similarly, to discover the new data for a NodeDataChanged event,
you need to call getData . In both of these cases, the state of the znodes may have
changed between receiving the watch event and performing the read operation, so you
should bear this in mind when writing applications.

ACLs

A znode is created with a list of ACLs, which determine who can perform certain opera-
tions on it.

ACLs depend on authentication, the process by which the client identifies itself to
ZooKeeper. There are a few authentication schemes that ZooKeeper provides: