Are your transactions idempotent? It is possible that the transaction is being run (and succeeding) twice in this case. What other request is colliding with first? You are not using any non-ancestor queries or setting read_policy=EVENTUAL on any reads correct?
On Fri, Aug 19, 2011 at 12:42 PM, Tom Phillips <[email protected]> wrote: > I'm seeing the same since moving to HR last week. It happens rarely, > and the only clue is a ConcurrentModification in the logs (java in my > case). > > Pure speculation, but it looks to me like some sort of background > transaction retry might overwrite the entity with stale data, rather > than a rollback. > > Scenario for me is like: > pre) Entity bob has property height=70 > 1) thread 1, transaction X, height=75->commit() [appears to succeed] > 2) Meanwhile (within a second or so) thread 2, transaction Y height=80- > >commit() [ConcurrentModificationException] -> I pause 500ms and retry > -> commit() [appears to succeed this time] > 3) For a while (up to a minute or so, but possibly much longer) all > get-by-key on bob show height==80 (ok) > 4) Another while later all get-by-key on bob suddenly show height==75, > as per transaction X (not good!) > > My speculation is that the ConcurrentModification could sometimes > indicate there was disruption of BOTH transaction X and Y, even though > reported for Y. Perhaps X had gotten past commit() call but hadn't > yet reached milestone A of > http://code.google.com/appengine/articles/transaction_isolation.html, > and was also (temporarily) aborted due to the contention. > > Then some sort of background retry on X sometimes (rarely) re-inserts > it into the transaction queue BEHIND my explicit retry on Y, and > eventually overwrites with the whole entity state from X in 1) > > And it appears that sometimes the background retry of X may not even > happen till a good while later. > > Any chance something like this is happening? > > /Tom > > On Aug 16, 10:11 pm, Greg <[email protected]> wrote: > > Please check your logs for a warning "Transaction collision. > > Retying...". > > > > Something very similar is happening on my app, where DB put()s > > silently fail (equivalent to the entity being rolled back) very > > occasionally. This has only started happening after moving to HR. > > > > In my app, I get this warning very consistently (every time) at > > exactly the time the entity is supposed to be stored. I would be very > > interested to hear if you find this warning too. If so, I think it > > points to a bug in the transaction collision handler in put(). Please > > let me know! > > > > See my earlier post here: > http://groups.google.com/group/google-appengine/browse_thread/thread/... > > > > Cheers > > Greg. > > > > On Aug 14, 10:21 pm, "Raymond C." <[email protected]> wrote: > > > > > I have recently ran into a problem after migrating to HRD: > > > > > My application is a social online game which I have recently migrated > from > > > M/S to HR Datastore around 3 weeks ago. Since 2 weeks ago I have > started > > > receiving reports from players which their game progress are rolled > back > > > suddenly while playing, which progress made in the recent few days are > > > missing. I have verified the problem through data on other entities > (in > > > different entity group) that the reports are actually legit and at > least > > > several days of progress are actually rolled back (with updates to the > > > entities in the last few days are all missing). > > > > > Player's data in the game are retrieved through id ( > > > Player.get_by_id(player_id) ) and because the gap is so large (days) I > > > believe it is not a problem on my code (nowhere in my code cache > player's > > > data). > > > > > It has never happened before for nearly 1 year so I am guessing if it > is > > > related to HRD. I remember there was a thread here before which > reported > > > data being rolled back on HRD but I can not find it anymore. > > > > > As you know with AppEngine datastore's distributed nature, it is so > hard to > > > monitor this kind of problem to ensure the problem exist. I would like > to > > > ask if anyone has ran into this problem as well or suspect that you > have had > > > this problem before with your HRD application? > > > > > > -- > 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. > > -- 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.
