Dockerizing Apache ZooKeeper with jplock/zookeeper

![ZooKeeper Logo](

ZooKeeper is a popular open-source distributed coordination service used for building distributed systems. It provides a simple and reliable way to implement coordination, synchronization, and configuration management in distributed applications.

In this article, we will explore how to use Docker to containerize ZooKeeper using the jplock/zookeeper image. We will cover the steps to set up a ZooKeeper cluster, run a sample application, and understand the essential concepts behind ZooKeeper.

What is Docker?

Docker is an open-source platform that allows you to automate the deployment, scaling, and management of applications using containerization. Containers are lightweight, isolated environments that package everything needed to run an application, including the code, runtime, dependencies, and system libraries.

Setting Up a ZooKeeper Cluster with Docker

To get started, make sure you have Docker installed on your machine. You can download and install Docker from the official website. Once Docker is installed, you can proceed with the following steps:

  1. Create a Docker network to enable communication between ZooKeeper nodes:
docker network create zookeeper
  1. Start the ZooKeeper cluster by running three instances of the jplock/zookeeper image:
docker run -d --net=zookeeper --name=zookeeper1 -e "ZOO_MY_ID=1" -e "ZOO_SERVERS=server.1=zookeeper1:2888:3888 server.2=zookeeper2:2888:3888 server.3=zookeeper3:2888:3888" jplock/zookeeper
docker run -d --net=zookeeper --name=zookeeper2 -e "ZOO_MY_ID=2" -e "ZOO_SERVERS=server.1=zookeeper1:2888:3888 server.2=zookeeper2:2888:3888 server.3=zookeeper3:2888:3888" jplock/zookeeper
docker run -d --net=zookeeper --name=zookeeper3 -e "ZOO_MY_ID=3" -e "ZOO_SERVERS=server.1=zookeeper1:2888:3888 server.2=zookeeper2:2888:3888 server.3=zookeeper3:2888:3888" jplock/zookeeper

Here, we are starting three ZooKeeper nodes (zookeeper1, zookeeper2, and zookeeper3) and specifying their IDs and the server addresses of all three nodes.

  1. Verify that the ZooKeeper cluster is running:
docker ps

You should see three running containers for the ZooKeeper cluster.

Understanding ZooKeeper Concepts

ZooKeeper is designed to be simple and easy to use. It provides a hierarchical namespace called znodes, which are similar to directories on a filesystem. ZooKeeper provides the following key concepts:

  1. Znode: A znode is similar to a file or directory in a filesystem. Each znode has a unique path in the ZooKeeper namespace and can store data.
  2. Watch: A watch is a one-time trigger that gets set on a znode. When the znode changes, the watch is triggered, allowing clients to be notified of changes.
  3. Event: An event is a notification mechanism used to communicate changes and updates in the ZooKeeper cluster. Clients can register for different types of events, such as node creation, deletion, or data changes.
  4. Ephemeral Node: An ephemeral node is a znode that exists only as long as the session that created it is active. When the session ends, ephemeral nodes are automatically deleted.
  5. ZooKeeper Session: A session is a connection between a client and the ZooKeeper server. Sessions are used to maintain state and ensure reliable communication between clients and the server.

Running a Sample Application with ZooKeeper

To demonstrate the usage of ZooKeeper, let's run a sample application that uses ZooKeeper for coordination. We will use the official ZooKeeper client library for Java.

First, create a new directory on your local machine for the sample application:

mkdir zookeeper-sample
cd zookeeper-sample

Create a new file SampleApp.java and add the following code:

import org.apache.zookeeper.*;

public class SampleApp {
    public static void main(String[] args) throws Exception {
        String connectionString = "zookeeper1:2181,zookeeper2:2181,zookeeper3:2181";
        int sessionTimeout = 5000;
        ZooKeeper zooKeeper = new ZooKeeper(connectionString, sessionTimeout, null);

        String path = "/sample";
        String data = "Hello, ZooKeeper!";
        zooKeeper.create(path, data.getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);

        byte[] retrievedData = zooKeeper.getData(path, false, null);
        System.out.println("Retrieved data: " + new String(retrievedData));

        zooKeeper.close();
    }
}

This code creates a connection to the ZooKeeper cluster, creates a znode with the path /sample and the data "Hello, ZooKeeper!", retrieves the data from the znode, and prints it.

Compile the code and