This is a major red flag issue for many types of application even looking beyond basic datastore timestamps and auto-date/time functionality. What's worse, a feature request may not carry much weight as it pertains to Google's global server infrastructure.
At the very least, we should have some firm guarantees on clock skew. I don't know much about Google's server infrastructure but I think 100mS is comfortably achievable globally with NTP. This would require individual servers to be proactively taken off the network if clock skew exceeds the guaranteed limits. Of course, absolute time accuracy is not usually a serious issue but clock skew is - and this problem cannot be eliminated even with the most accurate networks. As a simple case in point, an accounting application might have good reasons for requiring that invoice datetime stamps always increase with invoice number. Therefore another approach would be to allow individual applications to use their own (fault-tolerant) centralised reference clock. While, the accuracy of this reference clock will be impaired by things like round trip delays, processor load and context switching, the clock access "protocol" could be designed to at least preserve operation time sequence. Each clock access would be atomic in the sense that that a thread can request the current time, receive the current time (albeit after an undefined delay) and process the current time without worrying about a conflict with some other thread. I realise that this is not as simple as it sounds and people more qualified than me have probably trashed this out before but I really do think that the GAE team should at least flag this issue to receive serious consideration in the near future. Rgds, rvjcallanan On Dec 22, 6:59 pm, Marzia Niccolai <ma...@google.com> wrote: > Hi, > > Please see this > thread:http://groups.google.com/group/google-appengine/browse_thread/thread/... > > Generally one would expect the clocks to be synced within a few seconds, but > there are not any guarantees of any kind. > > -Marzia > > On Mon, Dec 22, 2008 at 4:20 AM, rvjcallanan <vinc...@callanan.ie> wrote: > > > I recall reading somewhere that, due to global time differences across > > servers, we cannot rely on timestamps to draw certain conclusions > > about GAE data e.g. operation sequence, etc. > > > Time synchronisation is a critical issue in any distributed app yet I > > have been unable to get hard facts and figures on how well Google > > synchronises its servers globally. > > > I assume some form of NTP syncing is performed so I would expect that > > steady state drift will be of the order of milliseconds across the > > globe. But what if a server is rebooted or added to the Google > > network? Will this server be disabled until it has reached a certain > > sync level. > > > In a nutshell, a few questions... > > > 1. How accurate are Google server clocks re: some global standard? > > 2. What is the maximum drift between servers? > > 3. Is tighter synchronisation guaranteed between servers in a local > > data centre servicing entity groups > > > TIA, > > > rvjcallanan On Dec 22, 6:59 pm, Marzia Niccolai <ma...@google.com> wrote: > Hi, > > Please see this > thread:http://groups.google.com/group/google-appengine/browse_thread/thread/... > > Generally one would expect the clocks to be synced within a few seconds, but > there are not any guarantees of any kind. > > -Marzia > > On Mon, Dec 22, 2008 at 4:20 AM, rvjcallanan <vinc...@callanan.ie> wrote: > > > I recall reading somewhere that, due to global time differences across > > servers, we cannot rely on timestamps to draw certain conclusions > > about GAE data e.g. operation sequence, etc. > > > Time synchronisation is a critical issue in any distributed app yet I > > have been unable to get hard facts and figures on how well Google > > synchronises its servers globally. > > > I assume some form of NTP syncing is performed so I would expect that > > steady state drift will be of the order of milliseconds across the > > globe. But what if a server is rebooted or added to the Google > > network? Will this server be disabled until it has reached a certain > > sync level. > > > In a nutshell, a few questions... > > > 1. How accurate are Google server clocks re: some global standard? > > 2. What is the maximum drift between servers? > > 3. Is tighter synchronisation guaranteed between servers in a local > > data centre servicing entity groups > > > TIA, > > > rvjcallanan --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Google App Engine" group. To post to this group, send email to google-appengine@googlegroups.com To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en -~----------~----~----~----~------~----~------~--~---
