PRISM: A Parallel Inference System for Problem Solving

PRISM (a P a r a l l e l Inference System), f o r p a r a l l e l execution of problems, has been implemented on ZMOB (Rieger [1980 ] ) . PRISM is an experimental t o o l fo r the development of d i s t r i b u t e d AI problem solvers and is based on l og i c programming. F a m i l i a r i t y w i th log ic programming (Kowalski [1979]) is assumed. In conventional programming systems the log ic and con t ro l of an a lgor i thm are combined making it d i f f i c u l t to separate or to modify cont ro l wi thout a f f e c t i n g the l o g i c . Logic as the s p e c i f i c a t i o n language, is neut ra l w i th respect to con t ro l and spec i f i es only the problem semantics. The method of how the problem is to be solved is external to the log ic s p e c i f i c a t i o n . Thus, a primary issue in achieving a p a r a l l e l system is developing an e f f ec t i v e con t ro l s p e c i f i c a t i o n that exp lo i t s p a r a l l e l ism. PRISM permits us to speci fy the problem independently of the con t ro l and al lows us to experiment w i th a l t e r n a t i v e con t ro l p o s s i b i l i t i e s fo r the same problem. The basic tasks i d e n t i f i e d in the execution of a problem are : management of the search space; management and r e t r i e v a l of procedures (deductive axioms); management and r e t r i e v a l of asser t ions ( f a c t s , or knowledge); and i n t e r a c t i o n w i th the problem and con t ro l s p e c i f i e r (use r ) . PRISM consis ts of three f unc t i ona l l y d i s t i n c t sets of machines. The ln tens lona l database machines (IDB) u n i f i e s a goal w i th a l l procedure heads wi th the same name. The extensional database machines (EDB) func t ion as a d i s t r i b u t e d r e l a t i o n a l database machine and provide fas t access to func t i on f ree ground asser t ions . The problem so lv ing machines (PSM) manage the search space. The PSM is the major por t ion of PRISM and is where the cont r o l spec i f i ca t i on i s p r ima r i l y i n t e r p r e t e d . I n t h i s paper we describe the PSM and i t s support of the con t ro l f a c i l i t i e s provided in PRISM. (See Kas i f [1983] fo r f u r the r d e t a i l s and references on PRISM).