On Jan 25, 2013, at 5:35 PM, Pedro Werneck wrote:
>> If the script that is searching for status=0 is finding rows that are
>> committed, then the worker that is querying for those rows should be able to
>> see them, unless the worker has been holding open a long running
>> transaction.
>
> Exactly.
>
>> Long running transactions here are more of the antipattern. The worker
>> should ensure it responds to new messages from the status=0 script with a
>> brand new transaction to read the message in question.
>
>
> That's the point. What's the best way to do that, considering the worker is
> never updating anything, only reading? Should I commit in the end of every
> task then, even without anything to commit? Should I start a new session on
> every call? The commit does that automatically if I'm not using
> autocommit=True, right?
just do it like this:
def receive_some_request(args):
session = Session(some_engine) # "connect" to the database (in reality,
pulls a connection from a pool as soon as the Session is used to emit SQL)
try:
.. do things with session ...
session.commit() # if you have data to commit
finally:
session.close() # close what was opened above.
just like it were a plain database connection. that's per request received by
your worker.
>
>
> The worker should wait for a request from the script in a non-transactional
> state, without a Session. A request from the script comes in- the worker
> starts a new Session to respond to that request, hence new transaction.
> Thinking about transaction demarcation in reverse still seems to suggest that
> this worker is leaving a dormant connection open as it waits for new jobs.
>
> I'm pretty sure it does. I'm using Flask SQLAlchemy and Celery for the
> workers. The workers reach the global app for the session and are keeping the
> connection open, but they do have work almost all the time and never sleep
> for more than a few secs.
>
>
> ---
> Pedro Werneck
>
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