What are Event Handling in Web Service?

Event handlers can be associated with a scope or a process for concurrent processing of events. BPEL defines two types of events:
Message event—Implements a request-response or one-way operation invoked by a business partner
Alarm event—Implements timer-driven behavior

When a scope is active, then event handlers attached to the scope are enabled to receive concurrent messages or alarm events. They are disabled when the processing of the scope ends. Events can only be processed when the corresponding event handler is enabled. When the event handlers for a scope become disabled, running event handlers are allowed to complete. The scope is terminated when all event handlers have completed processing.

During the processing of message events, the event handler remains enabled. Therefore, it can concurrently process multiple events of the same type. In addition to flow activities, this is the second place in BPEL where concurrency is possible. Note that event handlers for a process can't create new process instances.

If an alarm event handler is specified with a duration, then this duration starts when the event handler for the scope is enabled.