We study the problem of determining which message ordering specifications can be implemented in a distributed system. Further, if a specification can be implemented, we give a technique to determine whether it can be implemented by tagging information with user messages or if it requires control messages. To specify the message ordering, we use a novel method called forbidden predicates. All existing message ordering guarantees such as FIFO, flush channels, causal ordering, and logically synchronous ordering, (as well as many new message orderings) can be concisely specified using forbidden predicates. We then present an algorithm that determines from the forbidden predicate the type of protocol needed to implement that specification.

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The paper studies the characteristics of synchronous ordering of messages. Synchronous ordering of messages defines synchronous communication based on the causality rather than time. We present the sufficient conditions, based on the causality relations, for any algorithm to provide synchronous ordering. We also propose an algorithm using acknowledgment messages to implement the sufficient conditions. The algorithm is deadlock-free, and provides a higher degree of concurrency than existing algorithms.>