Getting Started with Hazelcast IQueue as a Messaging Service

Hazelcast is an open source in-memory data grid based on Java. We can use Hazelcast’s in- memory shared queue as a message broker without the need for an external application such as Kafka, MQ etc. Note, the hazelcast queue has a scaling limitation, for additional information read here

Step one:

Paste the following dependency to your pom.xml:

<dependency>
  <groupId>com.hazelcast</groupId>
  <artifactId>hazelcast</artifactId>
  <version>3.8.1</version>
</dependency>
<dependency>
  <groupId>com.hazelcast</groupId>
  <artifactId>hazelcast-client</artifactId>
  <version>3.8.1</version>
</dependency>

If you are working in AWS you will need to add the following as well:

<dependency>
  <groupId>com.hazelcast</groupId>
  <artifactId>hazelcast-cloud</artifactId>
  <version>3.6.1</version>
</dependency>

Step two:

Create a hazelcast instance:

private def createInstance(groupName : String): HazelcastInstance ={
  val conf: Config = new Config
  conf.getGroupConfig.setName(groupName)
  Hazelcast.newHazelcastInstance(conf)
}

If you are working in an AWS env you will need to configure the hazelcast instance differently using ClientAwsConfig, but this will not be covered here.

Step three:

Create the Message case class:

case class MyMessage(messageData : String)

Step four:

Create/Get the message queue:

val QUEUE_NAME : String = “MyQueue”
private val queue : IQueue[MyMessage] = hazelcastInstance.getQueue[MyMessage](QUEUE_NAME)

Step five:

Add a message to the queue, with a timeout of 30 seconds:

val message = MyMessage(“my data”)
val wasSuccessful = queue.offer(message, 30, TimeUnit.SECONDS)

You can then handle unsuccessful additions to the queue.

Step six:

Create a message listener and register it to the the queue:

Class MessageListener() extends ItemListener[MyMessage]{
  override def itemAdded(p1: ItemEvent[MyMessage]): Unit = {
     val messageFromQueue : MyMessage = queue.take()
     //handle message here
  }
  override def itemRemoved(p1: ItemEvent[BuildDataRefreshTask]): Unit = {
   …
  }
}

Register the message listener to the queue

val messageListener = new MessageListener()
queue.addItemListener(messageListener,false)

Summary

Hazelcast distributed queue can be used as a light weight messaging broker without the need for an external messaging app. Use with caution if scaling is an issue.

Photo by Frederick Tubiermont on Unsplash