ZooKeeper - Hadoop: The Definitive Guide - page 739

}
}
Example 21-3. A helper class that waits for the ZooKeeper connection to be established
public class ConnectionWatcher implements Watcher {
private static final int SESSION_TIMEOUT = 5000 ;
protected ZooKeeper zk ;
private CountDownLatch connectedSignal = new CountDownLatch ( 1 );
public void connect ( String hosts ) throws IOException ,
InterruptedException {
zk = new ZooKeeper ( hosts , SESSION_TIMEOUT , this );
connectedSignal . await ();
}
@Override
public void process ( WatchedEvent event ) {
if ( event . getState () == KeeperState . SyncConnected ) {
connectedSignal . countDown ();
}
}
public void close () throws InterruptedException {
zk . close ();
}
}
The code for JoinGroup is very similar to CreateGroup . It creates an ephemeral
znode as a child of the group znode in its join() method, then simulates doing work of
some kind by sleeping until the process is forcibly terminated. Later, you will see that
upon termination, the ephemeral znode is removed by ZooKeeper.
Listing Members in a Group
Now we need a program to find the members in a group (see Example 21-4 ) .
Example 21-4. A program to list the members in a group
public class ListGroup extends ConnectionWatcher {
public void list ( String groupName ) throws KeeperException ,
InterruptedException {
String path = "/" + groupName ;