Active Database Systems

• active database 3

http://cs.ulb.ac.be/public/_media/teaching/infoh415/p63-paton.pdf

Active database systems respond automatically to events that are taking place either inside or outside the database system itself.

traditional database systems have been viewed as repositories that store the information required by an application, and that are accessed by user program or through interactive interface.

• passive — commands are executed by the database (e.g., query, update) as and when requested by the user or application program.

… towards the functionality required by an application being supported within the database system itself … for modeling both the structural and the behavioral aspects of an application

• active database: move the reactive behavior from the application (or polling mechanism) into the DBMS. The reactive semantics is both centralized and handled in a timely manner.

  A rule is known as an event-condition-action or ECA rule. ECA rules … support integrity constraints, materialized views, coordination of distributed computation.

  ECA rules … directly support the semantics of the application

4 Execution model

determines how a set of rules behave at runtime, specifies how a set of rules is treated at runtime.

8 Developing active applications

experience in the application of active database often indicates that although such facilities are suitable for …, they are not straightforward to use.

• what performance penalty is likely to result from the use of rules
• the functionality of a large rule base may be difficult to understand, with rules interacting in complex ways and no single description of how control flows through an application
• debugging of active rules

Rule analysis

• termination Can-Trigger relationship — the triggering graph is conservative: potential nontermination will be detected even when in practice the rules will always terminate

• confluence Is the result of rule processing independent of the order in which simultaneously triggered rules are selected for processing … confluence can be understood by considering that firing of a rule in one database state may lead to the creation of a new database state.

Unlike traditional programming lang, where sequential control is specified both explicitly and statically by the programmer, active rules are fired dynamically by the system based on the previous flow of events.