Zookeeper alternative

Childrenclose − close a connectionConnect to the ZooKeeper EnsembleThe ZooKeeper class provides connection functionality through its constructor. The signature of the constructor is as follows −ZooKeeper(String connectionString, int sessionTimeout, Watcher watcher)Where,connectionString − ZooKeeper ensemble host.sessionTimeout − session timeout in milliseconds.watcher − an object implementing “Watcher” interface. The ZooKeeper ensemble returns the connection status through the watcher object.Let us create a new helper class ZooKeeperConnection and add a method connect. The connect method creates a ZooKeeper object, connects to the ZooKeeper ensemble, and then returns the object.Here CountDownLatch is used to stop (wait) the main process until the client connects with the ZooKeeper ensemble.The ZooKeeper ensemble replies the connection status through the Watcher callback. The Watcher callback will be called once the client connects with the ZooKeeper ensemble and the Watcher callback calls the countDown method of the CountDownLatch to release the lock, await in the main process.Here is the complete code to connect with a ZooKeeper ensemble.Coding: ZooKeeperConnection.java// import java classesimport java.io.IOException;import java.util.concurrent.CountDownLatch;// import zookeeper classesimport org.apache.zookeeper.KeeperException;import org.apache.zookeeper.WatchedEvent;import org.apache.zookeeper.Watcher;import org.apache.zookeeper.Watcher.Event.KeeperState;import org.apache.zookeeper.ZooKeeper;import org.apache.zookeeper.AsyncCallback.StatCallback;import org.apache.zookeeper.KeeperException.Code;import org.apache.zookeeper.data.Stat;public class ZooKeeperConnection { // declare zookeeper instance to access ZooKeeper ensemble private ZooKeeper zoo; final CountDownLatch connectedSignal = new CountDownLatch(1); // Method to connect zookeeper ensemble. public ZooKeeper connect(String host) throws IOException,InterruptedException { zoo = new ZooKeeper(host,5000,new Watcher() { public void process(WatchedEvent we) { if (we.getState() == KeeperState.SyncConnected) { connectedSignal.countDown(); } } }); connectedSignal.await(); return zoo; } // Method to disconnect from zookeeper server public void close() throws InterruptedException { zoo.close(); }}Save the above code and it will be used in the next section for connecting the ZooKeeper ensemble.Create a ZnodeThe ZooKeeper class provides create method to create a new znode in the ZooKeeper ensemble. The signature of the create method is as follows −create(String path, byte[] data, List acl, CreateMode createMode)Where,path − Znode path. For example, /myapp1, /myapp2, /myapp1/mydata1, myapp2/mydata1/myanothersubdatadata − data to store in a specified znode pathacl − access control list of the node to be created. ZooKeeper API provides a static interface ZooDefs.Ids to get some of basic acl list. For example, ZooDefs.Ids.OPEN_ACL_UNSAFE returns a list of acl for open znodes.createMode − the type of node, either ephemeral, sequential, or both. This is an enum.Let us create a new Java application to check the create functionality of the ZooKeeper API. Create a file ZKCreate.java. In the main method, create an object of type ZooKeeperConnection and call the connect method to connect to

IST 2015mZxid = 0x7fmtime = Tue Sep 29 17:14:24 IST 2015pZxid = 0x7fcversion = 0dataVersion = 1aclVersion = 0ephemeralOwner = 0x0dataLength = 23numChildren = 0Remove a ZnodeRemoves a specified znode and recursively all its children. This would happen only if such a znode is available.Syntaxrmr /pathSamplermr /FirstZnodeOutput[zk: localhost:2181(CONNECTED) 10] rmr /FirstZnode[zk: localhost:2181(CONNECTED) 11] get /FirstZnodeNode does not exist: /FirstZnodeDelete (delete /path) command is similar to remove command, except the fact that it works only on znodes with no children.Zookeeper - APIZooKeeper has an official API binding for Java and C. The ZooKeeper community provides unofficial API for most of the languages (.NET, python, etc.). Using ZooKeeper API, an application can connect, interact, manipulate data, coordinate, and finally disconnect from a ZooKeeper ensemble.ZooKeeper API has a rich set of features to get all the functionality of the ZooKeeper ensemble in a simple and safe manner. ZooKeeper API provides both synchronous and asynchronous methods.ZooKeeper ensemble and ZooKeeper API completely complement each other in every aspect and it benefits the developers in a great way. Let us discuss Java binding in this chapter.Basics of ZooKeeper APIApplication interacting with ZooKeeper ensemble is referred as ZooKeeper Client or simply Client.Znode is the core component of ZooKeeper ensemble and ZooKeeper API provides a small set of methods to manipulate all the details of znode with ZooKeeper ensemble.A client should follow the steps given below to have a clear and clean interaction with ZooKeeper ensemble.Connect to the ZooKeeper ensemble. ZooKeeper ensemble assign a Session ID for the client.Send heartbeats to the server periodically. Otherwise, the ZooKeeper ensemble expires the Session ID and the client needs to reconnect.Get / Set the znodes as long as a session ID is active.Disconnect from the ZooKeeper ensemble, once all the tasks are completed. If the client is inactive for a prolonged time, then the ZooKeeper ensemble will automatically disconnect the client.Java BindingLet us understand the most important set of ZooKeeper API in this chapter. The central part of the ZooKeeper API is ZooKeeper class. It provides options to connect the ZooKeeper ensemble in its constructor and has the following methods −connect − connect to the ZooKeeper ensemblecreate − create a znodeexists − check whether a znode exists and its informationgetData − get data from a particular znodesetData − set data in a particular znodegetChildren − get all sub-nodes available in a particular znodedelete − get a particular znode and all its

And running the program will output the above created znodes.myfirstsubnodemysecondsubnodeDelete a ZnodeThe ZooKeeper class provides delete method to delete a specified znode. The signature of the delete method is as follows −delete(String path, int version)Where,path − Znode path.version − Current version of the znode.Let us create a new Java application to understand the delete functionality of the ZooKeeper API. Create a file ZKDelete.java. In the main method, create a ZooKeeper object zk using ZooKeeperConnection object. Then, call the delete method of zk object with the specified path and version of the node.The complete program code to delete a znode is as follows −Coding: ZKDelete.javaimport org.apache.zookeeper.ZooKeeper;import org.apache.zookeeper.KeeperException;public class ZKDelete { private static ZooKeeper zk; private static ZooKeeperConnection conn; // Method to check existence of znode and its status, if znode is available. public static void delete(String path) throws KeeperException,InterruptedException { zk.delete(path,zk.exists(path,true).getVersion()); } public static void main(String[] args) throws InterruptedException,KeeperException { String path = "/MyFirstZnode"; //Assign path to the znode try { conn = new ZooKeeperConnection(); zk = conn.connect("localhost"); delete(path); //delete the node with the specified path } catch(Exception e) { System.out.println(e.getMessage()); // catches error messages } }}Zookeeper - ApplicationsZookeeper provides a flexible coordination infrastructure for distributed environment. ZooKeeper framework supports many of the today's best industrial applications. We will discuss some of the most notable applications of ZooKeeper in this chapter.Yahoo!The ZooKeeper framework was originally built at “Yahoo!”. A well-designed distributed application needs to meet requirements such as data transparency, better performance, robustness, centralized configuration, and coordination. So, they designed the ZooKeeper framework to meet these requirements.Apache HadoopApache Hadoop is the driving force behind the growth of Big Data industry. Hadoop relies on ZooKeeper for configuration management and coordination. Let us take a scenario to understand the role of ZooKeeper in Hadoop.Assume that a Hadoop cluster bridges 100 or more commodity servers. Therefore, there’s a need for coordination and naming services. As computation of large number of nodes are involved, each node needs to synchronize with each other, know where to access services, and know how they should be configured. At this point of time, Hadoop clusters require cross-node services. ZooKeeper provides the facilities for cross-node synchronization and ensures the tasks across Hadoop projects are serialized and synchronized.Multiple ZooKeeper servers support large Hadoop clusters. Each client machine communicates with one of the ZooKeeper servers to retrieve and update its synchronization information. Some of the real-time examples are −Human Genome Project −

Querying in ClickHouse CloudThe data in this system table is held locally on each node in ClickHouse Cloud. Obtaining a complete view of all data, therefore, requires the clusterAllReplicas function. See here for further details.This table does not exist if ZooKeeper is not configured. The 'system.zookeeper_connection' table shows current connections to ZooKeeper (including auxiliary ZooKeepers). Each row shows information about one connection.Columns:name (String) — ZooKeeper cluster's name.host (String) — The hostname/IP of the ZooKeeper node that ClickHouse connected to.port (String) — The port of the ZooKeeper node that ClickHouse connected to.index (UInt8) — The index of the ZooKeeper node that ClickHouse connected to. The index is from ZooKeeper config.connected_time (DateTime) — When the connection was establishedsession_uptime_elapsed_seconds (UInt64) — Seconds elapsed since the connection was establishedis_expired (UInt8) — Is the current connection expired.keeper_api_version (String) — Keeper API version.client_id (UInt64) — Session id of the connection.xid (Int32) — Xid of the current session.Example: