Research into knowledge representation within the artificial intelligence (AI) community has led to the development of AI tools that use frames to structure knowledge. Concurrent research in databases has led to the development of semantic data models and object-oriented databases. These two types of system seem to have much in common --they are structurally object-oriented, support inheritance, and store programs with the objects to which they relate. What then are the differences between frame systems and object-oriented databases? The paper compares a frame system called CRL with an object-oriented database called ADAM to identify common ground and differences between the systems and the philosophies underlying them. What emerges from the comparison is that while the systems have many superficial similarities, the different rationales that led to their development have resulted in significant practical differences in certain fundamental constructs.
object-oriented, object-oriented databases, frame-based systems, artificial intelligence, data modelling
The object-oriented paradigm is characterized in a general sense by a grouping of information with the concept or entity to which it relates. This paradigm has recently become both fashionable and widely used. One of the reasons for the recent interest in the objectoriented approach is that it permits many concepts from the real world to be modelled in a direct and natural way. In addition, from an engineering point of view, the object-oriented paradigm brings such benefits as reusability and extensibility.
It is widely recognised that systems which are in some sense object-oriented have been developed by researchers working on programming languages, artificial intelligence (AI), and databases. Systems developed by researchers from these different communities often share similar terminology, but are based on fundamentally different notions of objects, inheritance, instantiation, etc. These differences derive from the different motivations driving the various strands of research.
Database researchers have been seeking to solve the impedance mismatch between database systems and the languages used to implement data-intensive applications, and to develop systems that support powerful datamodelling constructs absent from the relational data model. The first motivation led to the development of database programming languages ~, while the second fuelled interest in semantic data modelling 2. By contrast, AI research aims to find powerful knowledge representation schemes that are able to model complex reasoning techniques (such as default and common-sense reasoning) used by humans in solving difficult problems. Frame-based systems have been widely used in AI, when attempting to model stereotypical situations in a way that humans seem to do 3. Finally, research into programming languages has stressed the need to improve modifiability and reusability. Abstract data types, encapsulating data structures and operations, have been proposed as a means of addressing these goals 4.
This paper aims to identify how the different motivations underlying the development of systems that are in some sense object-oriented have affected the way in which the paradigm is supported within the database and AI communities. The approach taken has been to implement an application using the object-oriented database (OODB) ADAM and the frame-based system CRL as a means of illustrating the similarities and differences in their support for the paradigm.
ADAM 5 is an implementation in Prolog of an OODB system (OODBS). ADAM is both structurally and behaviourally object-oriented as described by Dittrich 6. Many of the behavioural features of ADAM come from the same school as LOOPS 7, but a number of its structural characteristics have roots in earlier work on functional data models 8.
Knowledge Craft is an expert-system development environment built at Carnegie-Mellon University 9. Knowledge Craft includes the programming languages OPS-5, Prolog, CRL, and Common Lisp, the latter forming the core of the system. As a good example of a frame-based system, in this paper the focus is on the CRL component of Knowledge Craft.