Active Database, Active Database (Management) System
An active database (aDB) or active database (management) system (aDBS/aDBMS) is a database (management) system that supports reactive behavior through ECA rules.
In addition, the groups within the ACT-NET consortium (A European research network of Excellence on active databases 1993–1996) reached a consensus on what constitutes an active database management system with the publication of the Active Database System Manifesto .
Most of the active databases are monolithic and assume a centralized environment; consequently, the majority of the prototype implementations do not consider distributed issues. Initial work on how active databases are affected by distributed issues is reported in .
Periodically polling the database
Embedding or encoding event detection and related action execution in the application code
The first approach implies that the queries must be run exactly when the event occurs. The frequency of polling can be increased in order to detect such an event, but if the polling is too frequent, then the database is overloaded with queries and will most often fail. On the other hand, if the frequency is too low, the event will be missed.
The second approach implies that every application which updates the database needs to be augmented with condition checks in order to detect events. For example, an application may be extended with code to detect whether the quantity of certain items has fallen below a given level. From a software engineering point of view, this approach is inappropriate, since a change in a condition specification implies that every application that uses the modified condition needs to be updated.
Neither of the two previous approaches can satisfactorily support reactive behavior in a database context . An active database system avoids the previous disadvantages by moving the support for reactive behavior inside the database (management) system. Reactive behavior in an active database is supported by ECA rules that have the following semantics: when an event is detected, evaluate a condition, and if the condition is true, execute an action.
Similar to describing an object by its static features and dynamic features, an active database can be described by its knowledge model (static features) and execution model (dynamic features). Thus, by investigating the knowledge model and execution model of an active database, one can identify what type of ECA rules that can be defined and how the active database behave at run-time.
An aDB or aDBS/aDBMS is useful for any non-mission critical application that requires reactive behavior.
Looking back, the RIDS workshop marks the end of the active database period, since there are very few active database publications after 1997. However, the concept of ECA rules has resurfaced and has been picked up by other research communities such as Complex Event Processing and Semantic Web. In contrast to typical active database approaches that assume a centralized environment, the current research on ECA rules within Complex Event Processing and Semantic Web assumes that the environment is distributed and heterogeneous. Thus, as suggested within the REWERSE project , one cannot assume that the event, condition, and action parts of an ECA rule are defined in one single ECA rule language. For example, the event part of an ECA rule can be defined in one language (e.g., Snoop), whereas the condition part and action part are defined in a completely different rule language.
The popularity of using XML for manipulating data has also led to proposals of ECA rule markup languages. These ECA rule markup languages are used for storing information about ECA rules and facilitate exchange of ECA rules between different rule engines and applications.
One research question that remains from the active database period is how to model and develop applications that use ECA rules. Some research on modeling ECA rules has been carried out, but there is no widely agreed approach for modeling ECA rules explicitly in UML, or how to derive ECA rules from existing UML diagrams.
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