Consistency Maintenance for ABT

One of the most powerful techniques for solving centralized constraint satisfaction problems (CSPs) consists of maintaining local consistency during backtrack search (e.g. [11]). Yet, no work has been reported on such a combination in asynchronous settings. The difficulty in this case is that, in the usual algorithms, the instantiation and consistency enforcement steps must alternate sequentially. When brought to a distributed setting, a similar approach forces the search algorithm to be synchronous in order to benefit from consistency maintenance. Asynchronism [24, 14] is highly desirable since it increases flexibility and parallelism, and makes the solving process robust against timing variations. One of the most well-known asynchronous search algorithms is Asynchronous Backtracking (ABT). This paper shows how an algorithm for maintaining consistency during distributed asynchronous search can be designed upon ABT. The proposed algorithm is complete and has polynomial-space complexity. Since the consistency propagation is optional, this algorithms generalizes forward checking as well as chronological backtracking. An additional advance over existing centralized algorithms is that it can exploit available backtracking-nogoods for increasing the strength of the maintained consistency. The experimental evaluation shows that it can bring substantial gains in computational power compared with existing asynchronous algorithms.