CAU_Event - Event

An event is defined through the state transition of one or more object instances. The event defines potential state transitions by means of pre- and post condition, i.e. pre- and post-states. Events can be defined on the structure or property level as well as on the extent level.

An event is fired when the instance state before modification returns a true precondition and when the state of the instance after the modification returns a true post-condition. Pre- and post-conditions can be defined as functions, expressions or any other type of action. When not defining pre- and post-condition any modification on the instance is considered as event.

Special events are system events that are signaled by the database system and usually indicate a change in the state of the instance, which cannot be expressed in terms of pre- and post-conditions.

Event definitions are stored globally with a unique name that is constructed from the structure name, the extent or property name and the event name. This allows referring to an event from a far linked instance or registering instances for event observation.

When an event is fired it can be handled by an event handler defined in the causality rule, which is a function, expression or other action again.