Traveler: The Apiary Observatory

Abstract

Observing and debugging concurrent actor programs on a distributed architecture such as the Apiary poses new problems not found in sequential systems. Since events are only partially ordered, the chronological order of events no longer corresponds to their causal ordering, so the execution trace of a computation must be more structured than a simple stream. Many events may execute concurrently, so a stepper must give the programmer control over the order in which events are stepped. Because of the arrival order nondeterminism of the actor model, different actors may have different views on the ordering of events. We conquer these problems by recording the activation ordering, the transaction pairing, and the arrival ordering of messages in the Apiary and displaying the resulting structures in Traveler’s window oriented interface under user control.