Locking will not help. You may need to be able to pass a unique identifier to the mail api to prevent sending mails twice (see http://groups.google.com/group/google-appengine/browse_thread/thread/c64caf9539a45051/00d157921c2fde92)
On 12 Jul., 22:07, Marcus Brody <[email protected]> wrote: > Pieter, > > although this looks easy its not that easy, I am aware that I can > queue a mail sending task, > problem is that task has to be idempotent to achiev desired behaviour, > > a queue task can be executed for example twice, since those are > specifications > and this would result in sending mail twice, > a naive aproach would be to update some db lock to prevent sending > mail more than 1 time, > but it is not easy, this is much more complicated, > in other words operation is either atomic or it isnt, and without very > precise mechanism you > cannot prove that task will be runned only one. > > My own tests proved that due to email quota being reached (task fail > and has be resheduled) some of the mails > has been send twice. > > If Mail API would allowed to send mail only in case of some db > transaction success it would be very easy to garant > that mail will be send exactly 1 time. (although if it fails for some > other reasons like recipient not found or so thats whole another > story :) > > Basically this can be solved by > 1) adding new API for sending mail (dont know if it so easy or whole > problem isnt much much bigger than i think - distributed > transactions ?) > 2) implementing locking mechanism > > On Jul 12, 9:13 pm, Pieter Coucke <[email protected]> wrote: > > > > > Create a task for sending e-mail instead of calling the mailservice > > directly. > > > use this in your task (a task can be part of a transaction) > > > try{ > > ... > > > } catch (Throwable thr) { > > log.error(...); > > } > > > and all exceptions will be catched. I think a task will be re-executed only > > if it fails (throws an exception). > > > On Sat, Jul 10, 2010 at 6:43 PM, Marcus Brody <[email protected]> wrote: > > > Hello, > > > > I would like to ask, if there is some way how to send mail only in > > > case that some db transaction will be successful. > > > I am not very experienced with gae, but I did study it now for about 2 > > > weeks. > > > > Desired output: > > > > 1) prepare email > > > 2) db transaction.begin() > > > 3) do some db operation > > > 4) bind sending mail with current transaction (similar to > > > how can you can attach Task to queue) > > > 5) db transaction.commit() > > > > I am aware that I can queue mail sending task to task queue, but i > > > cannot find the way how to do that mail sending task Idempotent > > > without (i think) complicated locking mechanism . > > > Most simple solution would be to put mail sending task to task queue, > > > but this means mail could be send twice in some rare situation. (this > > > is maybe not that bad but if there is some way how to avoid it, I > > > would like to know) > > > > Thank you, > > > > Marcus > > > > -- > > > 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 > > > [email protected]. > > > To unsubscribe from this group, send email to > > > [email protected]<google-appengine-java%2B > > > [email protected]> > > > . > > > For more options, visit this group at > > >http://groups.google.com/group/google-appengine-java?hl=en. > > > -- > > Pieter Coucke > > Onthoo BVBAhttp://www.onthoo.comhttp://www.koopjeszoeker.be -- 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 [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-java?hl=en.
