Hi James, On Fri, Jun 11, 2010 at 3:46 PM, james lesorg <[email protected]>wrote:
> I really enjoyed this talk. Would make a great article. > > I wonder whether i understand the motivation behind "sequence" numbers > correctly: we cannot just timestamp worker entities and then churn > through them chronologically because instance times can be wildly out > of sync? If that's the case, how can i trust the eta parameter when > creating task objects? > Timestamps will generally not be 'wildly' out of sync due to NTP, but even a few hundred milliseconds is enough to cause concurrency/synchronization issues. In order to maintain a consistent view of data, we need a true global clock, and using a wall-clock is not a practical solution in a distributed system. As far as ETAs go, the ETA only specifies the earliest time a task should run - the task queue may run your task after the ETA. > > Is the possibility of a worker / task race condition really greater > than the risk of the memcache lock disappearing? Is it at all possible > that the memcache lock will disappear in this scenario, and if so how > can we survive this edge case? > The memcache locks on the fan-in control batching, so the worst case if the locks vanish should be poor fan-in - it shouldn't affect data consistency. -Nick Johnson > > Sorry if this is a duplicate post - It seems the question i asked > earlier got lost in google groups weirdness. > > -- > You received this message because you are subscribed to the Google Groups > "Google App Engine" group. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to > [email protected]<google-appengine%[email protected]> > . > For more options, visit this group at > http://groups.google.com/group/google-appengine?hl=en. > > -- Nick Johnson, Developer Programs Engineer, App Engine Google Ireland Ltd. :: Registered in Dublin, Ireland, Registration Number: 368047 Google Ireland Ltd. :: Registered in Dublin, Ireland, Registration Number: 368047 -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
