Zookeeper

The Zookeeper is a software system that allows distributed applications to maintain a centralized registry of configuration data, which is stored in a distributed hierarchical repository. It is typically used to provide high availability, scalability, and fault tolerance for distributed applications. It is designed to be simple to use, highly reliable, and to provide efficient access to the data that is stored in its distributed hierarchical repository. Zookeeper provides a hierarchical namespace that is similar to a filesystem. The namespace is made up of nodes and paths, which are identified by a path string. Nodes can be named, stored and retrieved, and each node can contain data and children nodes. Paths are used to traverse the hierarchy and to access the data in the nodes. For example, the path "/zk" would refer to the root node in the namespace, while "/zk/app1" would refer to the node named "app1" which is a child of the root node. Zookeeper also provides synchronization primitives, such as locks and barriers, that enable applications to coordinate operations across multiple nodes. This can be used to ensure that distributed tasks are completed in the correct order, and that resources are not over-utilized or under-utilized.

import org.apache.zookeeper.*;

public class MyZookeeper {

    private static final int SESSION_TIMEOUT = 30000;  // 30 seconds
    private static ZooKeeper zk;
    
    public static void main(String[] args) throws Exception {
        zk = new ZooKeeper("localhost:2181", SESSION_TIMEOUT, new Watcher() {
            public void process(WatchedEvent we) {
                // Process the event
            }
        });

        // Create a node
        String path = zk.create("/mynode", "data".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
        
        // Read the data
        byte[] data = zk.getData(path, false, null);
        System.out.println("Data: " + new String(data));

        // Update the data
        zk.setData(path, "updated data".getBytes(), -1);
        data = zk.getData(path, false, null);
        System.out.println("Data: " + new String(data));

        // Delete the node
        zk.delete(path, -1);
    }

}

The code above shows a basic example of using Zookeeper's API. It creates a node in the root of the namespace, reads the data from the node, updates the data, and then deletes the node. These operations can be used to create, retrieve, update and delete data in a distributed application. They can also be used to coordinate operations across multiple nodes, to ensure that tasks are completed in the correct order and resources are not over-utilized or under-utilized.