Good Zookeeper Hello world Program with Java client

Question

I was trying to use Zookeeper in our project. Could run the server..Even test it using zkcli.sh .. All good.. But couldn't find a good tutorial for me to connect to this server using Java ! All I need in Java API is a method

public String getServiceURL ( String serviceName ) 

I tried https://cwiki.apache.org/confluence/display/ZOOKEEPER/Index --> Not good for me.

http://zookeeper.apache.org/doc/trunk/javaExample.html : Sort of ok; but couldnt understand concepts clearly ! I feel it is not explained well..

Answer 1

Finally, this is the simplest and most basic program I came up with which will help you with ZooKeeper "Getting Started":

package core.framework.zookeeper;

import java.util.Date;
import java.util.List;
import java.util.concurrent.CountDownLatch;

import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.Watcher.Event.KeeperState;
import org.apache.zookeeper.ZooDefs.Ids;
import org.apache.zookeeper.ZooKeeper;

public class ZkConnect {
    private ZooKeeper zk;
    private CountDownLatch connSignal = new CountDownLatch(0);

    //host should be 127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002
    public ZooKeeper connect(String host) throws Exception {
        zk = new ZooKeeper(host, 3000, new Watcher() {
            public void process(WatchedEvent event) {
                if (event.getState() == KeeperState.SyncConnected) {
                    connSignal.countDown();
                }
            }
        });
        connSignal.await();
        return zk;
    }

    public void close() throws InterruptedException {
        zk.close();
    }

    public void createNode(String path, byte[] data) throws Exception
    {
        zk.create(path, data, Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
    }

    public void updateNode(String path, byte[] data) throws Exception
    {
        zk.setData(path, data, zk.exists(path, true).getVersion());
    }

    public void deleteNode(String path) throws Exception
    {
        zk.delete(path,  zk.exists(path, true).getVersion());
    }

    public static void main (String args[]) throws Exception
    {
        ZkConnect connector = new ZkConnect();
        ZooKeeper zk = connector.connect("54.169.132.0,52.74.51.0");
        String newNode = "/deepakDate"+new Date();
        connector.createNode(newNode, new Date().toString().getBytes());
        List<String> zNodes = zk.getChildren("/", true);
        for (String zNode: zNodes)
        {
           System.out.println("ChildrenNode " + zNode);   
        }
        byte[] data = zk.getData(newNode, true, zk.exists(newNode, true));
        System.out.println("GetData before setting");
        for ( byte dataPoint : data)
        {
            System.out.print ((char)dataPoint);
        }

        System.out.println("GetData after setting");
        connector.updateNode(newNode, "Modified data".getBytes());
        data = zk.getData(newNode, true, zk.exists(newNode, true));
        for ( byte dataPoint : data)
        {
            System.out.print ((char)dataPoint);
        }
        connector.deleteNode(newNode);
    }

}

Answer 2

This blog post, Zookeeper Java API examples , includes some good examples if you are looking for Java examples to start with. Zookeeper also provides a client API library( C and Java) that is very easy to use.

Zookeeper is one of the best open source server and service that helps to reliably coordinates distributed processes. Zookeeper is a CP system (Refer CAP Theorem) that provides Consistency and Partition tolerance. Replication of Zookeeper state across all the nods makes it an eventually consistent distributed service.

Answer 3

This post has almost all operations required to interact with Zookeeper. https://www.tutorialspoint.com/zookeeper/zookeeper_api.htm

1. Create ZNode with data
2. Delete ZNode
3. Get list of ZNodes(Children)
4. Check an ZNode exists or not
5. Edit the content of a ZNode...

Answer 4

If you are on AWS; now We can create internal ELB which supports redirection based on URI .. which can really solve this problem with High Availability already baked in.

Answer 5

This is about as simple as you can get. I am building a tool which will use ZK to lock files that are being processed (hence the class name):

package mypackage;

import java.io.IOException;
import java.util.List;

import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.Watcher;

public class ZooKeeperFileLock {

  public static void main(String[] args) throws IOException, KeeperException, InterruptedException {

    String zkConnString = "<zknode1>:2181,<zknode2>:2181,<zknode3>:2181";

    ZooKeeperWatcher zkWatcher = new ZooKeeperWatcher();
    ZooKeeper client = new ZooKeeper(zkConnString, 10000, zkWatcher);

    List<String> zkNodes = client.getChildren("/", true);

    for(String node : zkNodes) {
      System.out.println(node);
    }
  }

  public static class ZooKeeperWatcher implements Watcher {

    @Override
    public void process(WatchedEvent event) {
    }

  }