Event specifies what causes the rule to be triggered. Possible events that can be supported in an active database system are,
- Data modification/retrieval events – the event is raised by an operation (insert, update, delete, access) on some database object.
- Transaction event – the event is raised by some transaction command (e.g. begin, abort, commit etc.).
- Application-defined event – the application program may signal the occurrence of an event.
- Temporal events – events are raised at some point in time. Temporal events may be absolute (e.g., 25th December, 2002) or relative (e.g. 15 minutes after x occurs).
- External events – the event is occurring outside the database (e.g. the sensor recording temperature goes above 100 degrees Celsius).
Events can further be classified into primitive and composite events.
a. Primitive event – the event cannot be divided into subparts.
b. Composite event – the event is raised by some combination of primitive events.