We will now add an event handler to our Book Warehousing BPEL process. Event handlers allow a BPEL process to execute and still listen to the events and handle them whenever they occur. The event can be a message or an alarm event. If the corresponding events occur, an event handler is invoked concurrently with the BPEL process instance. We can specify event handlers for the whole BPEL process as well as for each scope.