Reuse with PROTÉGÉ-II: Adapting Problem-Solving Methods with Mapping Relations

This paper describes the PROTÉGÉ-II architecture for the construction of knowledge-based systems from reusable components: problem-solving methods and knowledge bases. We argue that these components are easier to reuse when their terminologies are described as formal ontologies. We define declarative mapping relations that we use to connect pre-existing methods to new domains and knowledge bases. With PROTÉGÉ-II and a set of mapping relations, we are able to reuse the same problem-solving method with two disparate tasks: (1) configuring the parts of an elevator system and (2) identifying plausible configurations of helices in a ribosome molecular strand. 1. Reuse for Knowledge-Based Systems Software reuse is an appealing solution to the high cost of software construction and maintenance: If a library of reusable software components were available, then developers could use this library to greatly reduce software development time and effort. Since the goal of software reuse is to reduce development cost, it is valuable to view reuse from an economic perspective. Thus, the effort needed to build a software component library is the reuse investment cost, and the return on that investment is measured by the savings in effort achieved by exploiting reuse over the lifetime of each component. The benefit from a single instance of reuse is the difference between development costs with reuse and estimated development costs without reuse. Reuse is successful only when these benefits outweigh the investment costs. Barnes and Bollinger (1991) outline three ways to make reuse more cost-effective: (1) reduce the initial investment cost of constructing the component; (2) increase the number of times a component is reused; and (3) reduce the cost of selecting, adapting and reusing a component. In this paper, we focus on the third approach, and especially on the cost of adapting a pre-existing component. We present PROTÉGÉ-II, a development environment and methodology for the construction of knowledge-based systems with reusable components. This environment has been developed within the knowledge-acquisition research community. Thus, it is designed to help developers build systems that include both a knowledge base of domain 1. This paper is an edited version of Gennari, Altman and Musen, 1995. information, and a problem-solving method that operates on that knowledge base. For our purposes, these two types of components are the objects for reuse. In particular, we demonstrate the reuse of a problem-solving method across two domains: configuring the parts of an elevator system and finding plausible models for the positions of helices within a ribosome strand. The elevator-configuration task is a well-studied problem in the knowledge-acquisition research community, originally described and solved by Marcus, Stout, and McDermott (1988). The task is a constraint-satisfaction problem: given a set of building specifications and requirements such as elevator speed and capacity, and given a large body of knowledge about available elevator components and safety constraints, find a configuration of elevator components so that no constraints are violated. This task was chosen for the Sisyphus-2 project: a benchmark for comparing knowledge modeling efforts in the knowledge-acquisition research community. The PROTÉGÉ-II solution to this problem has been described in detail by Rothenfluh, Gennari, Eriksson, and Musen (1994). The ribosome topology task is another type of constraint-satisfaction problem, but in a very different domain. Given information about the secondary structure of components of the ribosome structure, and distance-constraint information among those components, the task is to locate the position and orientation of those components, relative to a set of known proteins, such that no distance constraints are violated. This problem has been described by Altman, Weiser, and Noller (1994). These two constraint-satisfaction problems are clearly very different in terminology, and notably different in the size of their search space. Thus, this pair of problems should be a good testbed for software reuse: if a solution can be constructed to solve one problem, it should be adaptable to solve the other. As we will show, PROTÉGÉ-II allows developers to minimize adaptation costs when reusing a problemsolving method. 2. PROTÉGÉ-II: An Environment for Reuse PROTÉGÉ-II (Puerta, Egar, Tu, & Musen, 1992) can be viewed as a software architecture (as in Krueger, 1992) for reuse: an architecture and methodology that make it easier for developers to access and use a library of pre-existing components. For PROTÉGÉ-II, there are two types of components: (1) declarative domain knowledge that may be reused Reuse with PROTÉGÉ-II: Adapting Problem-Solving Methods with Mapping Relations John H. Gennari, Russ B. Altman, and Mark A. Musen Section on Medical Informatics Knowledge Systems Laboratory Stanford University School of Medicine Stanford, CA 94305-5479, U.S.A URL: From: AAAI Technical Report FS-95-03. Compilation copyright ' 1995, AAAI (www.aaai.org). All rights reserved.