18.10. Listening for Feature Events

The GeoTools API includes a mechanism to fire off a FeatureEvent object each time that there is an “event,” which occurs when data is added, changed, or deleted in a SimpleFeatureSource. A client may implement a FeatureListener, which has a single method called changed() that is invoked each time that each FeatureEvent is fired.

Three types of messages are produced by a GeoMesa Kafka producer. Each message will cause a FeatureEvent to be fired when read by a GeoMesa Kafka consumer. All feature event classes extend org.locationtech.geomesa.kafka.utils.KafkaFeatureEvent and are contained in the companion object of the same name.

Message read	Class of event fired	FeatureEvent.Type	Filter
CreateOrUpdate	KafkaFeatureChanged	CHANGED	IN (<id>)
A single feature with a given id has been added; this may be a new feature or an update of an existing feature
Delete	KafkaFeatureRemoved	REMOVED	IN (<id>)
The feature with the given id has been removed
Clear	KafkaFeatureCleared	REMOVED	Filter.INCLUDE
All features have been removed

In addition to the normal information in a FeatureEvent, CreateOrUpdate messages expose the relevant SimpleFeature with the method feature(). Delete messages expose the feature ID with the method id(), and also include the SimpleFeature if it is available (it may be null). All events expose the original Kafka timestamp with the method time().

To register a FeatureListener, create the SimpleFeatureSource from a GeoMesa Kafka consumer data store, and use the addFeatureListener() method. For example, the following listener simply prints out the events it receives:

import org.geotools.data.FeatureEvent;
import org.geotools.data.FeatureListener;
import org.locationtech.geomesa.kafka.utils.KafkaFeatureEvent.KafkaFeatureChanged;
import org.locationtech.geomesa.kafka.utils.KafkaFeatureEvent.KafkaFeatureRemoved;
import org.locationtech.geomesa.kafka.utils.KafkaFeatureEvent.KafkaFeatureCleared;

// unless specified, the consumer will only read data written after its instantiation
SimpleFeatureSource source = ds.getFeatureSource(sftName);
FeatureListener listener = new FeatureListener() {
  @Override
  public void changed(FeatureEvent featureEvent) {
    if (featureEvent instanceof KafkaFeatureChanged) {
      KafkaFeatureChanged event = ((KafkaFeatureChanged) featureEvent);
      System.out.println("Received add/update for " + event.feature() +
                         " at " + new java.util.Date(event.time()));
    } else if (featureEvent instanceof KafkaFeatureRemoved) {
      KafkaFeatureRemoved event = ((KafkaFeatureRemoved) featureEvent);
      System.out.println("Received delete for " + event.id() + " " + event.feature() +
                         " at " + new java.util.Date(event.time()));
    } else if (featureEvent instanceof KafkaFeatureCleared) {
      KafkaFeatureCleared event = ((KafkaFeatureCleared) featureEvent);
      System.out.println("Received clear at " + new java.util.Date(event.time()));
    }
  }
};
store.addFeatureListener(listener);

At cleanup time, it is important to unregister the feature listener with removeFeatureListener(). For example, for code run in a bean in GeoServer, the javax.annotation.PreDestroy annotation may be used to mark the method that does the deregistration:

@PreDestroy
public void dispose() throws Exception {
    store.removeFeatureListener(listener);
    // other cleanup
}