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Reliable Topic

The Reliable Topic data structure has been introduced with the release of Hazelcast 3.5. The Reliable Topic uses the same ITopic interface as a regular topic. The main difference is that Reliable Topic is backed up by the RingBuffer (also introduced with Hazelcast 3.5) data structure. The following are the advantages of this approach:

  • Events are not lost since the RingBuffer is configured with 1 synchronous backup by default.
  • Each Reliable ITopic gets its own RingBuffer; if there is a topic with a very fast producer, it will not lead to problems at the topic that runs at a slower pace.
  • Since the event system behind a regular ITopic is shared with other data structures (e.g. collection listeners), you can run into isolation problems. This does not happen with the Reliable ITopic.

Sample Reliable ITopic Code

import com.hazelcast.core.Topic;
import com.hazelcast.core.Hazelcast;
import com.hazelcast.core.MessageListener;

public class Sample implements MessageListener<MyEvent> {

  public static void main( String[] args ) {
    Sample sample = new Sample();
    HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance();
    ITopic topic = hazelcastInstance.getReliableTopic( "default" );
    topic.addMessageListener( sample );
    topic.publish( new MyEvent() );
  }

  public void onMessage( Message<MyEvent> message ) {
    MyEvent myEvent = message.getMessageObject();
    System.out.println( "Message received = " + myEvent.toString() );
  }
}

You can configure the Reliable ITopic using its RingBuffer. If there is a Reliable Topic with the name Foo, then you can configure this topic by adding a ReliableTopicConfig for a RingBuffer with the name Foo. By default, a RingBuffer does not have any TTL (time to live) and it has a limited capacity; you may want to change that configuration.

By default, the Reliable ITopic uses a shared thread pool. If you need better isolation, you can configure a custom executor on the ReliableTopicConfig.

Because the reads on a RingBuffer are not destructive, it is easy to apply batching. ITopic uses read batching and reads 10 items at a time (if available) by default.

Slow Consumers

The Reliable ITopic provides control and a way to deal with slow consumers. It is unwise to keep events for a slow consumer in memory indefinitely since you do not know when it is going to catch up. You can control the size of the Ringbuffer by using its capacity. For the cases when a Ringbuffer runs out of its capacity, you can specify the following policies for the TopicOverloadPolicy configuration:

  • DISCARD_OLDEST: Overwrite the oldest item, no matter if a TTL is set. In this case the fast producer supersedes a slow consumer
  • DISCARD_NEWEST: Discard the newest item.
  • BLOCK: Wait until the items are expired in the Ringbuffer.
  • FAIL: Immediately throw TopicOverloadException if there is no space in the Ringbuffer.

RELATED INFORMATION

Please refer to the Reliable Topic Configuration section for more information on configuring the Reliable Topic.