Implementing a cache with distributed consistency using Apache ZooKeeper and HashMap in Java

23 Oct 2023

In this article, we will explore how to implement a cache with distributed consistency using Apache ZooKeeper and HashMap in Java. Caching is a common technique used in many software applications to improve performance by storing frequently accessed data in memory. However, when dealing with distributed systems, maintaining consistency across multiple nodes can be a challenge. Apache ZooKeeper, a distributed coordination service, can help us overcome this challenge by providing a reliable and consistent shared configuration and synchronization service.

Introduction
Setting up Apache ZooKeeper
Implementing the Cache
Using the Cache
Conclusion

Introduction

Caching data in a distributed environment requires ensuring that all nodes have an up-to-date and consistent view of the cached data. Apache ZooKeeper provides a distributed coordination service that helps in maintaining shared configuration information and synchronization across multiple nodes. By using ZooKeeper, we can implement a cache with distributed consistency.

Setting up Apache ZooKeeper

To get started, we need to set up Apache ZooKeeper. You can download the ZooKeeper distribution from the official Apache website (https://zookeeper.apache.org/) and follow the installation instructions provided. Once installed, start the ZooKeeper server by running the appropriate command.

Implementing the Cache

Next, we can start implementing the cache with distributed consistency. We will be using a combination of Apache ZooKeeper and HashMap in Java to achieve this.

First, we need to create a cache class that will handle the operations for adding, retrieving, and removing data from the cache. Here is an example implementation:

import org.apache.zookeeper.*;
import org.apache.zookeeper.data.Stat;
import java.util.HashMap;

public class DistributedCache {

    private ZooKeeper zooKeeper;
    private HashMap<String, String> cache;

    public DistributedCache(String serverAddress) throws Exception {
        zooKeeper = new ZooKeeper(serverAddress, 2000, event -> {
            // Handle ZooKeeper events
        });
        cache = new HashMap<>();
    }

    public String get(String key) throws Exception {
        if (cache.containsKey(key)) {
            return cache.get(key);
        } else {
            // Retrieve data from ZooKeeper
            String data = getDataFromZooKeeper(key);
            cache.put(key, data);
            return data;
        }
    }

    public void put(String key, String value) throws Exception {
        cache.put(key, value);
        // Update data in ZooKeeper
        updateDataInZooKeeper(key, value);
    }

    public void remove(String key) throws Exception {
        cache.remove(key);
        // Delete data from ZooKeeper
        deleteDataFromZooKeeper(key);
    }

    private String getDataFromZooKeeper(String key) throws Exception {
        byte[] data = zooKeeper.getData("/cache/" + key, false, new Stat());
        return new String(data);
    }

    private void updateDataInZooKeeper(String key, String value) throws Exception {
        Stat stat = zooKeeper.exists("/cache/" + key, false);
        if (stat != null) {
            zooKeeper.setData("/cache/" + key, value.getBytes(), stat.getVersion());
        } else {
            zooKeeper.create("/cache/" + key, value.getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
        }
    }

    private void deleteDataFromZooKeeper(String key) throws Exception {
        zooKeeper.delete("/cache/" + key, -1);
    }
}

In the above code, we initialize the ZooKeeper connection in the constructor and create a HashMap to store the cached data. The get() method checks if the data is available in the HashMap. If not, it retrieves the data from ZooKeeper and stores it in the HashMap before returning it.

The put() method adds the data to the HashMap and updates the data in ZooKeeper. The remove() method removes the data from the HashMap and deletes it from ZooKeeper.

Using the Cache

Now that we have implemented the cache class, we can use it in our application. Here is an example of how to use the cache:

public class Main {
    public static void main(String[] args) {
        try {
            DistributedCache cache = new DistributedCache("localhost:2181");
            cache.put("key1", "value1");
            cache.put("key2", "value2");

            System.out.println(cache.get("key1")); // Output: value1
            System.out.println(cache.get("key2")); // Output: value2

            cache.remove("key1");

            System.out.println(cache.get("key1")); // Output: null
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

In the above code, we create an instance of the DistributedCache class and pass the server address of the ZooKeeper ensemble. We then add some key-value pairs to the cache using the put() method. We can retrieve the values using the get() method and remove a key-value pair using the remove() method.

Conclusion

Implementing a cache with distributed consistency can be challenging in a distributed system. However, by leveraging Apache ZooKeeper and HashMap in Java, we can achieve this efficiently. The example code provided in this article serves as a starting point for building a cache with distributed consistency in your own applications.

References:

#distributedsystems #caching

Table of Contents

Introduction

Setting up Apache ZooKeeper

Implementing the Cache

Using the Cache

Conclusion