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When the object's relation is changed in the Backendless Real-Time Database, it instantly notifies connected clients about the change using the relation change event. Relations of objects can be modified either using the Backendless Data API or in Backendless Console. A listener for the relation change event can be notified about relation changes of any objects in the database or only those that match specific criteria such as a unique object identifiers(objectIds).

An event listener itself is an object which receives a callback when an event is triggered by the real-time database. To add an event listener, it is necessary to obtain a handler for a database table first. The handler provides methods for registering an event listener.

Backendless provides three types of relation change event listeners that can be triggered in the following scenarios when : a new relation is added, an existing relation is deleted and  an existing relation is set.