You can check request headers to see if this is a first invocation of a task ( http://code.google.com/appengine/docs/java/taskqueue/overview.html#Task_Request_Headers), so you can detect a second invocation of a task.
Maybe if you combine this with specific catch blocks for mail exceptions? On Mon, Jul 12, 2010 at 10:07 PM, Marcus Brody <mhrab...@gmail.com> 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 <pieter.cou...@onthoo.com> 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 <mhrab...@gmail.com> > 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 > > > google-appengine-j...@googlegroups.com. > > > To unsubscribe from this group, send email to > > > google-appengine-java+unsubscr...@googlegroups.com<google-appengine-java%2bunsubscr...@googlegroups.com> > <google-appengine-java%2bunsubscr...@googlegroups.com<google-appengine-java%252bunsubscr...@googlegroups.com> > > > > > . > > > 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 > google-appengine-j...@googlegroups.com. > To unsubscribe from this group, send email to > google-appengine-java+unsubscr...@googlegroups.com<google-appengine-java%2bunsubscr...@googlegroups.com> > . > For more options, visit this group at > http://groups.google.com/group/google-appengine-java?hl=en. > > -- Pieter Coucke Onthoo BVBA http://www.onthoo.com http://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 google-appengine-j...@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.