Collection Management

It is easy to build, starting from this model, a document model in which complex documents, composed of various, eventually nested subparts, are represented as chains of Information Objects linked via appropriate relationships. For instance, an HTML document that includes a number of images may be modelled as a complex object that provides references to Information Objects (containing the images). The positioning attribute present in the information-object model helps in representing an aggregate object made up of parts that have to be fitted together in a certain order. A dedicated component in the Information Organization family, the Content Management Service (cf. Section 6.4), exposes the document model to other services. In a similar way, specific, complex metadata (like indexes, multimedia features) can be represented as separate Information Objects that are associated to the object they describe via appropriate relationships. For instance, a reference type may be “indexes” with a role name that gives additional information, like “full-text index”. The same representation mechanisms are also used to instantiate a concept of collection. Collections are the basic data structure used to organize information inside the Information Organization Services. Each collection is characterized by a collection identifier, labelled with a number of specific properties, and contains a number of documents. More specifically, a document can only exist as part of a given collection. Collections can in turn be nested, i.e. a collection can appear as member of another collection. A collection can be static (or materialized), that is contain a statically defined number of objects, that are added to it or deleted from it explicitly, or be virtual. The content of a virtual collection is not determined statically, but rather specified through declarative membership predicates that define which objects currently present in the gCube information-objects space are part of the collection. Its contents are thus determined dynamically at the moment when the collection is accessed by evaluating the membership predicates. For example, it is possible to define the collection of all objects having a certain MIME type (e.g. pdf).