[ZODB-Dev] transaction "Attempts" class

[Chris McDonough]

On Wed, 2012-03-28 at 17:06 -0400, Jim Fulton wrote:
> On Wed, Mar 28, 2012 at 4:37 PM, Chris McDonough <chrism at plope.com> wrote:
> > On Wed, 2012-03-28 at 14:21 -0400, Jim Fulton wrote:
> ...
> > A decorator for running some code in the context of a txn and retrying
> > retryable exceptions would be nice higher level behavior. Â I'd be
> > willing to do this work over this weekend.
> 
> Cool.  Don't forget the transaction note part. :)
> Too many transactions in our apps don't have
> notes, especially transactions that happen
> outside of web requests.
> 
> > In the meantime, I think the existing attempts context manager still
> > needs the small fix I proposed in my original message. Â Can you confirm
> > that my understanding of the its intent seems roughly correct?
> 
> I didn't see a statement of intent.  I think your fix us good,
> but the code (that I wrote and your fix) makes my head hurt. :)
> 
> I'd say, make some test cases for the bug and make it pass.

OK.  Once I fix this "Attempts" bug, I think the decorator code is just
a higher level interface that uses it:

class job(object):
    def __init__(self, attempts=1, note=None, manager=None):
        self.attempts = attempts
        self.note = note
        if manager is None:
            manager = transaction.manager
        self.manager = manager

    def __call__(self, wrapped):
        note = self.note
        if note is None:
            note = getattr(wrapped, '__name__', None)
        def inner(*arg, **kw):
            for attempt in self.manager.attempts(self.attempts):
                with attempt as t:
                    t.note(note)
                    return wrapped(*arg, **kw)

.. or something like that...
                    
> I'll admit, I didn't understand "Either that or it needs to not try to
> do a commit itself, and leave it up to the caller."

"The caller" in this case is the body of the context manager which
"calls" __exit__ by raising an exception or returning.  I just meant
instead of:

   for attempt in transaction.attempts(3):
       with attempt as t:
           .. do stuff, rely on context manager __exit__ to commit ..

You might change its contract to:

   for attempt in transaction.attempts(3):
       with attempt as t:
           .. do stuff ..
           t.commit() # you must commit yourself, __exit_ catches 
                      # errors only and aborts on exception

But this would break existing code as well as probably going against the
spirit of intent.



- C