Typically they use either of two mechanisms:
- periodically poll the database
- embed or encode event detection and related action execution in the application code.
The rules that govern the operation are typically formulated as Event-Condition-Action (ECA) where occurence of various types of events (e.g. dtabase transitions, time events, external signals) trigger the evaluation of a condition and if and only if the condition evaluates to true, the action is carried out.
"The Active Database Systems Manifesto: A rulebase of ADBMS Features" edited by Klaus R. Dittrich, Stella Gatziu, Andreas Geppert (Institut fur Informatik, Universitat Zurich, Switzerland)
is an attempt to clarify the notion of this type of system. The
editors distinquish mandatory features as well as desirable features.
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