Modularity in Databases

model, independent of any specific information model, which supports modules, called contexts. They specify basic rules for defining an information model with modules. An example is the rule stating that elements belonging to several modules should be allowed to have a specific name local to each module. Another rule states that whenever two modules share some elements, they should agree on the propagation of their updates. The Mads approach was defined independently of that work, but its principles are very much in line with the work and its results confirm and refine the ideas in [MMP95, MP00]. However, Mads has been designed and implemented as a data model specifically targeted to support spatio-temporal databases. This emphasized the quest for orthogonality between the structure, space, time, and 34 Christine Parent, Stefano Spaccapietra, Esteban Zimányi perception modeling dimensions, which certainly influenced the way the perception mechanism has been defined. Let us now turn our attention to ontologies and compare Mads multi-perception databases with modular ontologies. The closest to Mads approach in the ontology world is Cyc’s microtheories concept and mechanism. The comparison between the two has been discussed in Chapter 1 of this book, and is not repeated here. The two have almost identical goals. The overall goal of Mads is the creation of a new database composed of modules – the Mads perceptions. The goal of Cyc in this respect is to create an ontology composed of microtheories. These goals are quite different from the goals of the other approaches to modular ontologies that have been investigated and whose major representatives are described in Parts II and III of this book. These other approaches are: ontology partitioning, module extraction, interconnection of existing ontologies. The latter looks at reusing a set of existing ontologies as modules of a broader ontology that is built by inter-connecting the existing ones. The other two approaches propose different ways to create modules from an existing ontology. Partitioning is meant to split an ontology into several modules according to some splitting criteria, while extraction targets creating a module by extracting information from the ontology (similarly as in view materialization). The creation of a new module (perception) from a running database is also possible in Mads. The definition of an additional simple perception can be done anytime through a schema modification process. First, an identifier has to be specified for the new perception, and second, the definition of the database schema has to be revisited to add the new perception identifier to the set of perceptions associated to the elements (schema and instances) the database administrator wants to see in the new perception. This extensional process has to be validated by checking the consistency rules that enforce a perception to obey the modeling constraints of a normal database. The other mechanism that dynamically creates new perceptions is the composition of existing perceptions into a composite perception. This intensional process is prompted anytime a transaction uses the openDatabase command with a composite perception. The new composite perception remains a virtual one. It is not materialized. Nevertheless, the database administrator can anytime decide to materialize a composite perception, if required. Let us now compare the constructs supported by the Mads data model for perceptions to the ones supported by approaches that connect existing ontologies. Mads support three kinds of links between perceptions: 1. An implicit link defined by the fact that the different perceptions are integrated into a multiperception definition of an object (or relationship) type with several representations. These representations are, by definition, related to each other, whatever their dissimilarities. The descriptions (attributes, keys) and the populations for each perception may be the same, different, or even disjoint. 2. Two different object types belonging to two different perceptions, but representing at least partially the same real-world entities, may be linked by an interperception multi-instantiation link (is-a or overlap link). 3. Two different object types belonging to two different perceptions may be linked by an interperception relationship type. The first and second kinds of interperception links, in the case where the populations of the linked types are one included in or equal to the other, are similar to the bridge rules of C-OWL, which allow relating two concepts that, in any interpretation, describe two sets of entities that are linked by an inclusion [BGv04]. A 5 Modularity in Databases 35 difference between the Mads links and the bridge rules is that Mads’ first mechanism works even if the populations are disjoint, and Mads’ second mechanism works with included or overlapping populations. On the other hand, bridge rules are intended for two concepts related by an inclusion (or an equality). Mads’ third mechanism is similar to the link property of E-connections that allows relating two classes from disjoint modules (i.e., modules that describe disjoint parts of the world) by an intermodule role, called link property [CPS08]. The main difference between Mads and approaches that connect existing ontologies is that Mads allows representing the same real-world phenomenon with representations that are quite dissimilar from each other. Two representations may have disjoint populations and still be two representations of the same object type. For instance, a perception of the Wine object type may describe only European wines while another perception may describe only American wines. As another example, in Fig. 5.4 the attribute barrels has two representations that are quite dissimilar, and still in Mads these two representations are related: Users querying the barrels attribute with the composite perception (Pm + Pe) get for each wine two values, one for each perception. This possibility of stating that several object types, several relationship types, or several attributes describe the same phenomenon, even if they are totally different, is – as far as we know – peculiar to Mads.