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.
