Managing transactions using multiple databases

[Leandro Boscariol]

Hello,

During the design of a project, I came across a situation where I will have 2 databases.

Not a big deal, it is very well documented in Django docs.

Dealing with transactions even when having multiple databases is also trivial, simply select which one the transaction is about.

At a given point, I'll have to interact with both databases in the same view.

Consider the snippet below as an example (not tested, nor this is what I intend to do):

@transaction.commit_manually()  # using='db1' or using='db2' or none??

def my_view(request):

    try:

        obj1 = MyObject1()

        obj1.data = request.POST.get('data')

        obj1.save(using='db1')

        obj2 = MyObject2()

        obj2.obj1_id = obj1id

        obj2.save(using='db2')

    except:

        transaction.rollback()

    else:

        transaction.commit()

    ...

I didn't find any mention in the docs or in this list related to this, so here is my question:

How can I do a transaction management in both databases at the same time? Is that even possible?

Thanks in advance for your help.

Leandro.

[Christophe Pettus]

On Jul 6, 2013, at 2:54 PM, Leandro Boscariol wrote:

> How can I do a transaction management in both databases at the same time? Is that even possible?

You can certainly nest the transaction context managers (atomic() in 1.6+, or you can use xact() in 1.5 and earlier):

with xact(using=DATABASE1):
with xact(using=DATABASE2):
things

If an exception occurs, it will rollback both; if it exits normally, it will commit both. Note that this isn't true two-phase commit; it's perfectly possible for the inner transaction to successfully commit but the outer one to fail. If you want proper two-phase commit, you'll (at the moment, at least) have to roll your own.

--
-- Christophe Pettus
x...@thebuild.com

[Leandro Boscariol]

Well, even though without true two-phase commit, for now that will do, thanks.

And also thanks for mentioning xact, I wasn't aware of that!

Cheers,

Leandro

[Christophe Pettus]

On Jul 6, 2013, at 8:58 PM, Leandro Boscariol wrote:
> And also thanks for mentioning xact, I wasn't aware of that!

Full disclosure: I wrote it. :) In 1.6, the new atomic() decorator / context manager is the way to go, but xact() works fine for now!

[Leandro Boscariol]

Amazing work Christophe, congrats!

Lot's of new cool stuff being added to 1.6, looking forward to it.

Regarding the nested xact context managers, let's say something that I don't like happened and I want to abort everything:

with xact(using=db1):

    with xact(using=db2):

        obj1 = Ob1.save(using=db1)

        obj2 = Obj2.save(using=db2)

        if not_cool:

            raise Rollback

I'll raise Rollback since this is the way to force a rollback, as described in the README.

But, this will roll back only whatever happened in the inner context manager, thus obj1 will still exist, right?

Is there a way to make the Rollback reach the outer xact?

You wrote in a previous email that an exception will abort both, so something like this should be the solution? (I have not tested):

try:

    with xact(using=db1):

        with xact(using=db2):

            obj1 = Ob1.save(using=db1)

            obj2 = Obj2.save(using=db2)

            if not_cool:

                raise Exception

except Exception:

    return False

return True

Cheers,

Leandro

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