ZooKeeper - Hadoop: The Definitive Guide - page 737

When the main() method is run, it creates a CreateGroup instance and then calls its
connect() method. This method instantiates a new ZooKeeper object, which is the
central class of the client API and the one that maintains the connection between the client
and the ZooKeeper service. The constructor takes three arguments: the first is the host ad-
dress (and optional port, which defaults to 2181) of the ZooKeeper service; [ 142 ] the second
is the session timeout in milliseconds (which we set to 5 seconds), explained in more de-
tail later; and the third is an instance of a Watcher object. The Watcher object receives
callbacks from ZooKeeper to inform it of various events. In this scenario,
CreateGroup is a Watcher , so we pass this to the ZooKeeper constructor.
When a ZooKeeper instance is created, it starts a thread to connect to the ZooKeeper
service. The call to the constructor returns immediately, so it is important to wait for the
connection to be established before using the ZooKeeper object. We make use of Java's
CountDownLatch class (in the java.util.concurrent package) to block until
the ZooKeeper instance is ready. This is where the Watcher comes in. The Watcher
interface has a single method:
public void process ( WatchedEvent event );
When the client has connected to ZooKeeper, the Watcher receives a call to its pro-
cess() method with an event indicating that it has connected. On receiving a connection
event (represented by the Watcher.Event.KeeperState enum, with value Syn-
cConnected ), we decrement the counter in the CountDownLatch , using its coun-
tDown() method. The latch was created with a count of one, representing the number of
events that need to occur before it releases all waiting threads. After calling coun-
tDown() once, the counter reaches zero and the await() method returns.
The connect() method has now returned, and the next method to be invoked on the
CreateGroup is the create() method. In this method, we create a new ZooKeeper
znode using the create() method on the ZooKeeper instance. The arguments it takes
are the path (represented by a string), the contents of the znode (a byte array null here),
an access control list (or ACL for short, which here is completely open, allowing any cli-
ent to read from or write to the znode), and the nature of the znode to be created.
Znodes may be ephemeral or persistent. An ephemeral znode will be deleted by the
ZooKeeper service when the client that created it disconnects, either explicitly or because
the client terminates for whatever reason. A persistent znode, on the other hand, is not de-
leted when the client disconnects. We want the znode representing a group to live longer
than the lifetime of the program that creates it, so we create a persistent znode.