The Coordination Criterion

Abstract: When is coordination intrinsically required by a distributed specification, rather than imposed by a particular protocol or implementation strategy? We give a general answer using minimal assumptions. In an asynchronous message-passing model, we show that a specification admits a coordination-free implementation if and only if it is monotone with respect to history extension under an appropriate order on observable outcomes. This Coordination Criterion is stated directly over Lamport histories -- partially ordered executions under happens-before -- and specification-defined observable outcomes, without assuming any particular programming language, object implementation, or protocol structure. It yields a sharp boundary between specifications that can be implemented without coordination and those for which coordination is unavoidable. The criterion provides a uniform explanation for a range of classical results, including CAP-style impossibility, CALM-style coordination-freedom, agreement and snapshot tasks, transactional isolation levels, and invariant confluence -- all instances of the same underlying semantic phenomenon.