This inspired me to add a little snippet to Objectify's BestPractices document:
http://code.google.com/p/objectify-appengine/wiki/BestPractices#Use_Pythonic_Transactions Basically, to run an automatically repeated transaction that updates a (non-sharded to make the example simple) counter, I would do this: DAOT.repeatInTransaction(new Transactable() { @Override public void run(DAOT daot) { Counter count = daot.ofy().find(Counter.class, COUNTER_ID); count.increment(); daot.ofy().put(count); } }); This will guarantee that updates to the counter are not lost. Jeff On Thu, Apr 7, 2011 at 10:08 AM, Jeff Schnitzer <j...@infohazard.org> wrote: > 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.
