Min-domain ordering for asynchronous backtracking

Ordering heuristics are a powerful tool in CSP search algorithms. Among the most successful ordering heuristics are heuristics which enforce a fail first strategy by using the min-domain property [10,4,20,6]. Ordering heuristics have been introduced recently to Asynchronous backtracking (ABT), for distributed constraints satisfaction (DisCSP) [27], However, the pioneering study of dynamically ordered ABT, ABT_DO, has shown that a straightforward implementation of the min-domain heuristic does not produce the expected improvement over a static ordering. The best ordering heuristic for asynchronous backtracking was found to be the Nogood-triggered heuristic. The present paper proposes an asynchronous dynamic ordering which does not follow the standard restrictions on the position of reordered agents in ABT_DO. Agents can be moved to a position that is higher than that of the target of the backtrack {culprit). Combining the Nogood-triggered heuristic and the min-domain property in this new class of heuristics results in the best performing version of ABT_DO. The new version of retroactively ordered ABT is faster by a large factor than the best form of ABT.