Hazelcast is an open-source, distributed in-memory data grid solution that provides high-performance, scalable caching and storage capabilities. One of the powerful features of Hazelcast is its support for distributed event journals, which allow capturing and processing of events across a cluster of nodes. In this blog post, we will explore how to work with Hazelcast distributed event journals in a Java application.

What is a Distributed Event Journal?

A distributed event journal is a replicated log of events that can be shared and processed by multiple distributed systems. It provides a reliable and ordered stream of events that can be used for various purposes, such as auditing, data synchronization, and event sourcing.

Setting Up Hazelcast in Java

To get started, we need to add the Hazelcast dependency to our Java project. We can do this by including the following Maven dependency in our pom.xml file:

<dependency>
    <groupId>com.hazelcast</groupId>
    <artifactId>hazelcast</artifactId>
    <version>4.2.2</version>
</dependency>

Next, we can create a Hazelcast instance by configuring and initializing the HazelcastInstance. Here’s an example:

Config config = new Config();
HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance(config);

Publishing Events to a Distributed Event Journal

Now that we have set up Hazelcast, let’s publish some events to a distributed event journal. To do this, we first need to get a reference to the event journal from the Hazelcast instance:

EventJournal<Object> eventJournal = hazelcastInstance.getEventJournal("myJournal");

We can then publish an event to the journal using the publish method:

eventJournal.publish("MyEvent");

Subscribing to Events from a Distributed Event Journal

After publishing events, we can subscribe to them and process them in our application. We can use the EventJournalReader to read events from the journal.

EventJournalReader<Object> eventJournalReader = eventJournal.newEventJournalReader(0, false);

The newEventJournalReader method takes two parameters: the startSequence (initial sequence to start reading from) and the includeValue flag (whether to include event values in the returned journal events).

We can then read events from the journal using the read method:

EventJournalEvent<Object> event = eventJournalReader.read();

We can access the event details, such as the sequence number, event value, and timestamp, using the appropriate methods available on the EventJournalEvent object.

Conclusion

In this blog post, we have explored how to work with Hazelcast distributed event journals in a Java application. We learned how to set up Hazelcast, publish events to a distributed event journal, and subscribe to events for further processing. Hazelcast’s distributed event journals provide a powerful mechanism for capturing and processing events in a distributed system. They can be used for a wide range of use cases, including event sourcing, auditing, and data synchronization.

#Java #Hazelcast