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Objects model phenomena and a phenomenon is usually a component. Information characterizing a component is encapsulated and accessible only by its methods. The relations between components are modeled explicitly by means of associations or references. A relation is also a phenomenon and objects can model this type of phenomena too. Components are usually related conceptually in diverse and subtle ways: Some relations are implicitly given and some are local to other more basic relations. Such kinds of relations are important for understanding the organization and cooperation of objects and may be supported in object-oriented analysis, design, and programming: An implicit association describes a relation between an object and objects local to this enclosing object, and a complex association describes an explicit relation between local objects in different enclosing objects. Such associations are described by classes and the objects have the usual properties including methods and attributes.

In object-oriented systems, an object may recursively reference any number of other objects. The references, however, do not capture any special relationships between objects. An important semantic relationship which may be superimposed on a reference is the IS-PART-OF relationship between a pair of objects. A set of objects related by the IS-PART-OF relationship is collectively called a composite object. An earlier paper [KIM87b] presented a model of composite objects which has been implemented in the ORION object-oriented database system at MCC. Although the composite-object feature has been found quite useful, the model suffers from a number of serious shortcomings, primarily because it overloads a number of orthogonal semantics on the references. In this paper, first we present a more general model of composite objects which does not suffer from these shortcomings. Further, [KIM87b] made an important contribution by exploring the use of composite objects as a unit for versions, physical clustering, and concurrency control. The extended model of composite objects necessitates non-trivial changes to the results of [KIM87b]. This paper describes the new results on the use of composite objects as a unit of not only versions, physical clustering and concurrency control, but also authorization.

Our objective is to have a logical framework for natural representation and manipulation of complex objects. We start with an analysis of semantic modeling of complex objects, and attempt to understand what are the fundamental aspects which need to be captured. A logic, called C-logic, is then presented which provides direct support for what we believe to be basic features of complex objects, including object identity, multi-valued labels and a dynamic notion of types. C-logic has a simple first-order semantics, but it also allows natural specification of complex objects and gives us a framework for exploring efficient logic deduction over complex objects.

The development of PRADIS was started in early 1968 as a vehicle to implement and evaluate new schemes of man-machine-interaction, data base organization, and hidden-line detection which were being developed in our laboratory. With time it evolved into a rather elaborate programming system for generation, construction, manipulation, and perspective display of arbitrary three-dimensional objects. All relevant object types such as polyhedrons or groups of polyhedrons with convex or concave structure, objects that are described by analytical expression, as well as the most general case of objects that are bounded by arbitrarily curved surfaces can be handled. Different modes of defining and inputting objects and different-hidden-line suppression algorithms have been individually developed and implemented for these various types. Man-machine-communication has been facilitated by defining a simple command language for the user. The execution of any command can be initiated by pointing to the respective instruction in a command menu. Additionally, a keyboard (or the console typewriter) can be used for specification of parameters or the description of objects by analytical equations. Subroutines for scaling, translation, and rotation are part of the system as well as program modules which enable the user to construct three-dimensional objects by drawing two-dimensional views of them.