Actually sophisticated banking systems use append only datastores and bi-temporal timestamps, whereby you record when an event actually occurred and when we found out about the event occurring.
This way, when we only get told at 12:15 that you took $200 out of an ATM at 12:05, we simply append this info to the event log and can do sophisticated things later such as realise that as of 12:06, you had withdrawn cash, but when we checked your balance for an automated payment at 12:10, it APPEARED that you had the funds to cover it. That is, we can spot the difference between what we thought your balance was at the time, and what we now think it was at the time. It means we don't actually store "your current balance" because the concept is not precise enough, but we can tell you at any time what we think your current balance is. And because the datastore is append only, we avoid all sorts of transactional bottlenecks, and keep a much more accurate audit trail that includes being able to work out things like "we erroneously charged you for a service at time T1, and refunded the charge at time T2 to be effective from time T1 - between T1 and T2 we thought your balance was less and therefore charged you overdraft fees, but we can now see that you weren't overdrawn during those times, so we should also append an event that indicates that we now recognise the overdraft fee shouldn't have charged". This sort of bitemporal store is very handy for getting rid of all sorts of event ordering and transactional deadlock complexities, but it does tend to mean that some queries can take a bit more legwork :) If you're doing lots of writes, and relatively fewer reads, and given the performance profile of the datastore, it MAY be worthwhile for looking at such an approach for some people. http://en.wikipedia.org/wiki/Temporal_database -- 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.
