Hi,
We're using SQLAlchemy (see below for versions) on Flask running on uWSGI
in gevent mode, with psycopg2 to PostgreSQL 9.4. We recently switched from
regular prefork+threaded to gevent mode, and with it applied all gevent
monkey patches and the psycogreen package for the psycopg2 gevent wait
callback.
After this switch, we're seeing sporadic ObjectDeletedError exceptions
illustrated by the following simple lines of code:
@newsletters.route('/create/', methods=['GET', 'POST'])
@login_required
def create():
newsletter = Newsletter()
# db is Flask-SQLAlchemy's SQLAlchemy object instance
db.session.add(newsletter)
# Commit explicitly to ensure we have a mid to redirect to.
db.session.commit()
return redirect(url_for('.set_recipients', id=newsletter.mid))
Newsletter is a regular SQLAlchemy ORM Base, where mid is a plain Integer
column as a primary key (i.e. a serial in the database).
Accessing newsletter.mid there throws an exception once in every 50-ish
executions or some such: Instance '<Newsletter at 0x7fbba0571908>' has been
deleted, or its row is otherwise not present.
The database row is surely not deleted.
This is a multi-tenant application, whereby pretty much the first thing we
do in every request is db.session.execute('SET search_path TO {.username},
public'.format(current_user)).
I've set _use_threadlocal = True on the connection pool, as my thinking
initially was that we could be getting a different connection object back
from the pool after the commit. Could this still be the case? And/or the
search_path not being the same after the commit?
Even so, if the search_path was reset to public, I would presume the row
would still be found -- at least that is the case in a vanilla psql shell,
as all tables are present in the public schema and inherited in each
tenant's schema (thus making PostgreSQL search all inherited tables across
all schemas).
No master/slave is involved, just a single database server. The SQLAlchemy
pool is the default QueuePool with only default options (apart from
_use_threadlocal as mentioned above).
Any guidance to figuring this out would be much appreciated. And thanks for
an awesome library!
Relevant package versions:
SQLAlchemy==1.1.11
Flask-SQLAlchemy==2.1
psycopg2==2.6.2
uWSGI==2.0.15
gevent==1.2.2
greenlet==0.4.12
Best,
Anders
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