alt Hacker News> the relations between events happening in a distributed system, is more fundamental than their absolute ordering
The important thing in most distributed systems is having an order. Having a single observer serialize events as it receives them is so much more tractable than trying to use absolute order. Using absolute order requires very precise time synchronization which is hard; using absolute order requires knowing when you have received all the reports of events that already happened which is hard. Determining a designated observer isn't typically easy, but having it determine the order it observes events is easy. If two events happen at a similar time it's typically not a big deal which one is considered first as long as all nodes will agree on the result --- let the designated observer just pick the first one it sees works pretty well. If your report takes an unexpectedly long time to make it to the designated observer, then it won't be first and you'll deal.
Much better than trying to figure out unknowable questions of relativity. :P
Replies
> absolute order requires very precise time synchronization which is hard
Presumably relativity is the reason precise time synchronization (and thus absolute ordering) is hard.
yes, it can be easier to have a central serializer for events, but that certainly makes things problematic for fault tolerance and basically excludes large-scale solutions. using an agreed-upon post-hoc ordering based on timestamps is certainly another way, but really only if you're working in a paradigm that lets you impose it (like mvcc) or doesn't care (like crdt).
personally I find when you view events in a distributed system as a partial order, its more liberating than confusing. its not unusual to assume that there is some kind of canonical event ordering that we have to preserve, when its often just not semantically important. so its a useless constraint that can impose complexity and limit the solution space. the partial ordering exposes the real causal constraints.