Hi,
Here's some thoughts on possible problems with Aymeric's plan. I like
the look of the suggested transaction API, I'm raising these to help
make sure it can be realized.
On Sunday 03 March 2013, Aymeric Augustin wrote:
> On 1 mars 2013, at 13:48, Aymeric Augustin
> <aymeric.augus...@polytechnique.org> wrote:
>
> I'd like to add an @atomic decorator that:
> - Really guarantees atomicity, by raising an exception if you attempt
> commit within an atomic block,
Fragility alert: databases commit implicitly. Most notably, Oracle and MySql
for every DDL command.
It is true that the commands that trigger this are not usually found in
normally-executed application
code. However, since the proposal is all about running in autocommit mode
unless a transaction
was started explicitly, it should be noted that once such a command is
executed, it does not
just break the transaction in the middle, it renders everything after it
non-transactional.
Most DDL commands executed by Django apps, 1.6 and on, should be done through
the upcoming
schema alteration mechanisms -- code under core's control, so things can be set
up correctly
(that is, when such a command is executed under @atomic with a
non-DDL-transactional backend,
it should raise an exception). They just need care. Other than that, strong
documentation warnings
should be given around the use of raw sql in transactions.
> - Supports nesting within an existing transaction, with savepoints.
The use of savepoints in Django apps so far has been very little, as far as I
know. One point
I'm unsure of is the interaction of savepoints with cursors, since querysets
are lazy; so the scenario
I'm worrirf about is, generally speaking,
@atomic
def main():
for obj in query_with_more_than_100_objects:
try:
handle(obj)
except Bad:
pass
@atomic
def handle(obj):
if good(obj):
mark_good(obj)
obj.save()
else:
raise Bad(obj)
Will the next (database) fetch after an exception is raised get the right
objects?
My reading of the Postgresql documentation is that it will do the right thing,
not so sure about
the other backends.
>
> I'm reasonably confident that this plan can work.
My concerns about autocommit notwithstanding, I hope you're right.
Shai.
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