An event handler is an extension to the engine that is configured to run when a specified event occurs. You can add event handlers for several purposes and you can configure which events you want to catch. We strongly recommend that you add only appropriate handlers and carefully code the handler filters to handle only those events that you are interested in.
An event is a change to any object in the database (user, activity, processDefinition,... ). You can create an event handler to track any change to any object in the database and take the appropriate action. For example: