This video will help you immensely: http://sites.google.com/site/io/under-the-covers-of-the-google-app-engine-datastore
You can skip to the end if you just want to grok the transactions, but the rest of it is extraordinarily helpful as well. You are correct, if you use transactions, you do not need to manage a timestamp. This is because GAE has its own timestamp on each entity group. When you load an entity in a transaction, GAE checks this timestamp. When you commit, it checks the timestamp again - and if something changed, you get a ConcurrentModificationException. By running your transaction in a retry loop and checking for ConcurrentModificationException, you can safely ensure that non-stale data is written. Note that this isn't too far off from a description of what happens when you do a non-transactional put(). It's just that the transaction and retry loop are performed within the datastore itself. Creating an explicit transaction just provides the opportunity for your business logic to get "in the loop" so to speak. Jeff On Thu, Apr 7, 2011 at 7:09 AM, Peter Backx <peter.ba...@gmail.com> wrote: > Hi Ikai, > > Could you expand your answer a little? Do you mean that I don't need to > manage a timestap/version if I use transactions? Or do you mean I should use > transactions to combine a read (to check for a changed timestamp/version) > with a write? > > If I don't manage the timestamp myself, how does AppEngine know another > process change the data since the last read? part of the key? > > Sorry for those beginner questions, I started using the datastore via the > JDO API with a relational background, but it looks it isn't always the best > fit for typical datastore issues. Objectify looks much better as it is > specifically designed for it. > > > Thanks every one for your answers > > Peter > > -- > You received this message because you are subscribed to the Google Groups > "Google App Engine for Java" group. > To post to this group, send email to google-appengine-java@googlegroups.com. > To unsubscribe from this group, send email to > google-appengine-java+unsubscr...@googlegroups.com. > For more options, visit this group at > http://groups.google.com/group/google-appengine-java?hl=en. > -- You received this message because you are subscribed to the Google Groups "Google App Engine for Java" group. To post to this group, send email to google-appengine-java@googlegroups.com. To unsubscribe from this group, send email to google-appengine-java+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine-java?hl=en.
