In today’s episode of the Twilight Zone, a young William Shatner stumbles into a time machine and travels back into the past. Cornered in a dark alley, he is threatened by a teenage hooligan waving a loaded pistol. A tussle ensues, and in trying to wrest the gun from his assailant, Shatner fires, killing him dead. Examining the contents of the dead youth’s wallet, Bill comes to a shocking conclusion: he has just killed his own grandfather. Tight focus: Shatner howling soundlessly as he stares at his own hand flickering in and out of view.
Shatner? Or Not(Shatner)? Having now changed history, he could not have been born, meaning he could not have traveled back in time and changed history, meaning he was indeed born, meaning…?
You see where this goes. It’s the old grandfather paradox, a hoary chestnut of SciFi and AI. Personally I side with Captain Kirk: I don’t like mysteries. They give me a bellyache. But whether or not you think a discussion of “p if Not(p)” is news that’s fit to print, it is something to avoid in your software. This is particularly tricky in distributed programming, where multiple machines have different clock settings, and those clocks may even turn backward on occasion. The theory of Distributed Systems is built on the notion of Causality, which enables programmers and programs to avoid doing unusual things like executing instructions in orders that could not have been specified by the program that generated them. Causality is established by distributed clock protocols. These protocols are often used to enforce causal orderings–i.e. to make machines wait for messages. And waiting for messages, as we know, is bad.
So I’m here to tell you today that Causality is overrated, and we can often skip the wait. To hell with distributed clocks: time travel can be fine. In many cases it’s even fine to change history. Here’s the thing: Casuality is Required Only to control Non-monotonicity. I call this the CRON principle.