Thank you very much for the guidance Jonathan and Mike. I've implemented
nesting counting on my context manager and turned off autocommit and
subtransactions. It looks like it's working well!
On Wednesday, August 9, 2017 at 5:14:09 PM UTC+1, al...@withplum.com wrote:
> I'd like some help regarding nested session usage please.
> I'm working on an application that has an API layer but also has a lot of
> cron jobs (via Celery) and scripts. I'm trying to design the app in a way
> that my "business" logic is contained and re-usable by any of these
> The SQLAlchemy session scope is request/task-wide (i.e requests and tasks
> remove the scoped session at the end) but I am doing explicit commits
> instead of committing on request end because I sometimes have to deal with
> complicated logic like creating/submitting transactions to payment
> processors etc.
> To start off, I use a context manager, much like the docs, which commits
> or rollbacks as necessary. I then have a layer of actions, which are
> considered "top-level" functions that can do a simple operation e.g update
> something or a collection of operations i.e create and submit a
> transaction. These actions use the context manager above to persist stuff
> and I've opted to keep all session "usage" in these actions alone and
> nowhere else in the code. Pretty soon, the need to use some of the simpler
> actions inside other, bigger actions arose which, after reading the docs,
> led me to turn autocommit=True and use session.begin(subtransactions=True).
> Note that I don't want to use savepoints, I just want to be able to use my
> actions inside other actions. The docs recommend that expire_on_commit is
> set to False with autocommit, which I've done but that led to a couple of
> situations where I was operating on out-of-date data hence I want to turn
> expire_on_commit to True again.
> My questions:
> (1) Does my application layout make sense from a SQLAlchemy perspective?
> (2) What is the problem with expire_on_commit=True and autocommit=True?
> (3) I feel that, even with the context manager, the transaction boundaries
> are still blurry because the developer does not know what will actually get
> committed in the database. For example, if a previous part of the code
> changed something, then called an action that commits the session, the
> previous change will get committed as well. I've searched around and found
> this: https://github.com/mitsuhiko/flask-sqlalchemy/pull/447 which
> basically issues a rollback on entering the context manager to ensure that
> only what is within the context manager will get committed. What do you
> think of it? I can immediately see a problem where if I query for an object
> before passing it to an action, then use the context manager, all the work
> done on querying is lost since the object state is expired on rollback.
> I'd appreciate any advice/input.
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