[ZODB-Dev] transaction "Attempts" class

[Jim Fulton]

On Wed, Mar 28, 2012 at 6:37 PM, Chris McDonough <chrism at plope.com> wrote:
> 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...

It could be written that way, although I would
use a much simpler implementation.

The attempt design was a reach to overcome
the limitations of the with statement.  I'm not at all happy
with it, although I couldn't think of anything better at the
time.  I hate to build on it.

>> 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.

Yup.

Jim

-- 
Jim Fulton
http://www.linkedin.com/in/jimfulton