Cache using Hazelcast InMemory Data Grid

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Hazelcast is an open source In-Memory Data Grid (IMDG). It provides elastically scalable distributed In-Memory computing, widely recognized as the fastest and most scalable approach to application performance. Hazelcast makes distributed computing simple by offering distributed implementations of many developer-friendly interfaces from Java such as Map, Queue, ExecutorService, Lock and JCache. It is released under Apache license. 

Hazelcast is comparable to Redis, Memcache, it speaks Memcached protocol. It provides a convenient set of APIs to access the CPUs in your cluster for maximum processing speed. Hazelcast is highly scalable and available. Distributed applications can use Hazelcast for distributed caching, synchronization, clustering, processing, pub/sub messaging, etc. Hazelcast is implemented in Java and has clients for Java, C/C++, .NET, REST, Python, Go and Node.js. 

In this article we will explore how Map, MultiMap, Queue and Lock works with Hazelcast in a distributed environment. Most of the cloud applications are distributed and we need a caching service to store the frequently accessed objects across the services.

Download Hazelcast binaries and extract the archive. In command line, go to the hazelcast bin folder and start the service. Hazelcast service will get started.


$ cd C:\Hazelcast\hazelcast-3.11.3\bin

$ start.bat


$ cd /opt/hazelcast/hazelcast-3.11.3/bin

$ ./

We will take simple producer consumer as an example. Producer process add elements to data structure and Consumer process will retrieve it. Both the clients are in Java. In subsequent article, we will explain how to access Hazelcast from C++ and Go. 

Add below dependency to pom.xml



Create hazelcast instance, it will connect to localhost Hazelcast server which is running in the port 5701. 

   HazelcastInstance hzInstance = HazelcastClient.newHazelcastClient();


If the Hazelcast server is running in different machines or in different port, then you can mention its IP and connect to the server.

   ClientConfig clientConfig = new ClientConfig();
   ClientNetworkConfig networkConfig = clientConfig.getNetworkConfig();
   networkConfig.addAddress("", "");

   /* Create a client side HazelcastInstance */
   HazelcastInstance hzInstance = HazelcastClient.newHazelcastClient( clientConfig );

While exiting the process, shutdown the Hazelcast instance.



First we explore Map, below code will add items to Map and read the same. 

public void addItemsToMap() {
   System.out.println("Write entries to Map");

   IMap<String, String> map = hzInstance.getMap("my-distributed-map");
   map.put("title", "Find best open source");

   // Put if absent
   String loginId = ""; // get from database.
   map.putIfAbsent(loginId, "123-xxxx-xxx");

   //replace item in the map.
   map.replace("sessionId-" + loginId, "123-xxxx-xxx", "123-456-xxxx-xxx");

public void readItemsFromMap() {
   System.out.println("Read entries from Map");

   IMap<String, String> map = hzInstance.getMap("my-distributed-map");
   System.out.println("Title from Map: " + map.get("title"));

   // Put if absent
   String loginId = ""; // get from database.
   System.out.println("loginId from Map " + map.get(loginId));

MultiMap is a map which can multiple values for a same key. 

public void addItemsToMultiMap() {
   System.out.println("Write entries to MultiMap");

   MultiMap<String, String> subscriberMap = hzInstance.getMultiMap("my-subscriber-multimap");
   subscriberMap.put("channel1", "subscriber1");
   subscriberMap.put("channel1", "subscriber2");

public void readItemsFromMultiMap() {

  System.out.println("Read entries from MultiMap");

  MultiMap<String, String> subscriberMap = hzInstance.getMultiMap("my-subscriber-multimap");
  Collection<String> values = subscriberMap.get("channel1");

Queue is a data structure where data first in will be first out (FIFO). Here we talk about distributed queue where one process adds items to the queue and other polls it. White reading from the queue, instead of queue.poll(), if queue.take() function is used then it will block the queue till it receives data. 

 public void addItemsToQueue() throws InterruptedException {
   System.out.println("Write entries to Queue");

   BlockingQueue<String> queue = hzInstance.getQueue("my-distributed-queue");
   for(int i =0; i < 10; i++) {
     queue.put("item--" + i);

 public void readItemsFromQueue() throws InterruptedException {
   System.out.println("Read entries from Queue");

   BlockingQueue<String> queue = hzInstance.getQueue("my-distributed-queue");
   System.out.println("Entries in queue " + queue.size());

   String item = null;
   do {
     item = queue.poll(1, TimeUnit.MINUTES);    /* waits for a minute and if there is no item it returns null. */

    System.out.print("Items in queue " + item + " , ");
   } while(item != null);

Lock - Acquire a resource lock. In case of distributed environment, there might be a situation where only one service should access a particular resource. In that case this distributed lock will help.

public void tryLock() throws InterruptedException {
   Lock lock = hzInstance.getLock("resource-lock");
   if (lock.tryLock() ) {
       System.out.println("Producer process acquired lock.");
       Thread.sleep(1000 * 30);

   else {
     System.out.println("Producer process does not acquire lock.");

Below is the sample code for Producer and Consumer class. Copy the respective function above and add to Producer and Consumer class.

public class Producer {

  private HazelcastInstance hzInstance = null;

  public Producer() {
     hzInstance = HazelcastClient.newHazelcastClient();

   public void shutdown() {

   public void addItemsToMap() {}
   public void addItemsToMultiMap() {}
   public void addItemsToQueue() { }
   public void tryLock() {} 


public class Consumer {

  private HazelcastInstance hzInstance = null;

  public Consumer() {
     hzInstance = HazelcastClient.newHazelcastClient();

  public void shutdown() {

   public void readItemsFromMap() {}
   public void readItemsFromMultiMap() {}
   public void readItemsFromQueue() {}
   public void tryLock() {}


Main Application class to try out the above functions.

 public class App 
  public static void main( String[] args )
     if (args.length != 1) {
        System.out.println("use argument either -producer or -consumer");

    String argType = args[0];
    if ("-producer".equals(argType)) {
         Producer producer = new Producer();

         try {
        catch(Exception exp) {
  else if ("-consumer".equals(argType)) {
     Consumer consumer = new Consumer();

   try {
     catch (Exception exp) {

Execute the client in two terminals.

$ java -jar hazelcast-client.jar -producer
$ java -jar hazelcast-client.jar -consumer





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