We have a large high-performance software system which consists of multiple interacting Java processes (not EJBs). Each process can be on the same machine or on a different machine.
Certain events are generated in one process, and are then propagated in different ways to other processes for further processing and so on.
For benchmarking purposes, we need to create a log of when each event passed through a "checkpoint", eventually combine these logs to obtain a timeline of how each event propagated through the system and with what latency (of course, process switching and IPC adds latency, which is ok).
The problem, of course, is clock synchronization. So here are my questions:
1) If all processes are on the same machine, is it guaranteed that currentTimeMilis would be accurate at the time of call? Is there some bound on the errors of ITP?
2) If some processes may be on different machines, is there an off-the-shelf solution (that is also free or open-source) for clock synchronization? I am preferably looking for a solution that may bypass the operating system (Windows or Linux) and work straight from Java. I am also ideally looking for something that can operate at microsecond accuracy. I've thought about NTP, but I'm not sure if it's available via Java rather than through the OS, and I am not sure about its complexity.
3) Is there a way to determine the margin of error in using NTP in a particular configuration (or of any solution I end up using) so that I can give a margin of error on our calculation of the latency?
Thanks!
With distributed programming, clock synchronisation is often not enough. you might want to build a logical time framework (such as the Lamport or vector clocks or Singhal-Kshemkalyani methods ... and there are loads more to keep causality in sync across machines). Which you choose often depends on the application and required causality between events.
Clocks are sync'd to ensure concurrent events are kept in the right sequential order. There are other ways to do this than keeping the system clock synchronized ... which unless they share a common physical clock ... is quite tricky.
In terms of NTP error margin, there are solutions:
my recommendation:
Read: Distributed Computing: Principles, Algorithms and Systems
Especially: Chapter 3, logical time
Edit
Further to Cheeso's post, I found
http://www.uniforum.org/publications/ufm/apr96/opengroup.html
http://sourceforge.net/projects/freedce
There maybe DCE Java bindings out there.