Hi,
Strange behaviour with sqa in multi-process environment... already posted
on StackOverflow for a web app but still missing some understanding so
posting here.
I've created an application where my "sqa calls" are encapsulated: My API's
methods always do the same kind of stuff:
1- request a session
2- do my stuff - mainly requesting the db through session.query(...)
3- eventually:
- modify some mapped object attributes
- and write to db (session.commit()).
Several consecutive calls to my API's methods return the same session
object as long as I'm in the same thread which is the required behaviour.
Some commits happen but the session is always the same (expire_on_commit =
False)
# this factory is thread safe: a session object is returned (always
the same) to the
# caller. If called from another thread, the returned session
object will be different
session_factory = sessionmaker(bind=engine, expire_on_commit=False)
session_maker = scoped_session(session_factory)
# to get a session:
session = session_maker()
If I fork another thread, using the API is still safe since the first step
(getting a session) will return a new session instance (due to
scoped_session)
Here's my problem:
I have two processes which such an architecture (based on this API). While
P1 is writing to DB (session.commit()), P2 is reading from it
(session.query(...)). The problem is that P2 does not see the DB change and
has to poll several times before getting it. I'm actually requesting the DB
through session.query(...) calls from P2 but nothing happens. *Sometimes it
works though! :-(* Don't know why (did not change the code) -> Over 10
times, it will work 3 times...
If I change my API to close the session after every commit, P2 sees the
change as if commiting was not actually writing to disk and closing the
session did it. I do not understand this behavour as the sqa documentation
encourages to create a global session in a thread a work with it (which is
done with the scoped_session mechanism)
My configuration is : Linux OS, SQA 0.8.4 and a ram based db
(/dev/shm/my_db.sqlite but the problem is still there with regular disk
based db on my ~/my_db.sqlite)
Thanks a lot for your time,
Pierre
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