A Generic Persistence Model for (C)LP Systems (and Two Useful Implementations)

Abstract

This paper describes a model of persistence in (C)LP languages and two different and practically very useful ways to implement this model in current systems. The fundamental idea is that persistence is a characteristic of certain dynamic predicates (i.e., those which encapsulate state). The main effect of declaring a predicate persistent is that the dynamic changes made to such predicates persist from one execution to the next one. After proposing a syntax for declaring persistent predicates, a simple, file-based implementation of the concept is presented and some examples shown. An additional implementation is presented which stores persistent predicates in an external database. The abstraction of the concept of persistence from its implementation allows developing applications which can store their persistent predicates alternatively in files or databases with only a few simple changes to a declaration stating the location and modality used for persistent storage. The paper presents the model, the implementation approach in both the cases of using files and relational databases, a number of optimizations of the process (using information obtained from static global analysis and goal clustering), and performance results from an implementation of these ideas.

This work has been supported in part by the European Union IST program under contract IST-2001-34717 “Amos” and IST-2001-38059 “ASAP” and by MCYT projects TIC 2002-0055 “CUBICO” and HI2000-0043 “ADELA.” M. Hermenegildo is also supported by the Prince of Asturias Chair in Information Science and Technology at UNM. J. Correas is supported by a grant from Madrid Regional Government. The authors would like to thank I. Caballero, J.F. Morales, S. Genaim, and C. Taboch for their collaboration in some implementation aspects and for feedback and discussions on the system, and the anonymous reviewers for their suggestions.