I'm working on an application where I want to use one and only one
connection, even in the face of errors.
Originally I used AssertionPool, but it seemed to misbehave sometimes in
the face of a disconnection.
Switching to QueuePool, I somewhat surprisingly got the same result.
This is how I perform my test. Using the code below, I start it with a URI
and wait for it to tell me to stop postgresql, which I then do.
I hit enter, and get an (expected) error as it fails to connect.
When I hit enter a second time, I get an *unexpected* traceback, a
different one for QueuePool as for AssertionPool, but unexpected
nonetheless.
If I up the pool_size to 2, everything works fine but it leaves open the
possibility of having more than one connection.
#! /usr/bin/python
import sys
import sqlalchemy as sa
def handle_checkout_event(dbapi_con, con_record, con_proxy):
try:
with dbapi_con.cursor() as cur:
cur.execute("SELECT 1")
cur.fetchone()
except Exception, e:
raise sa.exc.DisconnectionError()
def main():
uri = sys.argv[1]
#engine = sa.create_engine(uri, poolclass=sa.pool.AssertionPool)
engine = sa.create_engine(uri, pool_size=1, max_overflow=0,
pool_timeout=0, pool_recycle=3600)
sa.event.listen(engine, 'checkout', handle_checkout_event)
with engine.begin() as conn:
pass
print "Now stop PG."
raw_input('-=>')
# should get an operational error
try:
with engine.begin() as conn:
pass
raw_input('-=>')
except sa.exc.OperationalError, e:
print >>sys.stderr, "Got an (expected) error: ", e
raw_input('-=!')
# *should* get the same thing.
try:
with engine.begin() as conn:
pass
raw_input('-=>')
except sa.exc.OperationalError, e:
print >>sys.stderr, "Got an (expected) error: ", e
print "We never get here."
raw_input('-=!')
if __name__ == '__main__':
main()
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