On 8/11/15 9:51 PM, Jinghui Niu wrote:
In the Documentation -- Session Basics, I read this:
E.g. don’t do this:
### this is the **wrong way to do it** ###
class ThingOne(object):
def go(self):
session = Session()
try:
session.query(FooBar).update({"x": 5})
session.commit()
except:
session.rollback()
raise
class ThingTwo(object):
def go(self):
session = Session()
try:
session.query(Widget).update({"q": 18})
session.commit()
except:
session.rollback()
raise
def run_my_program():
ThingOne().go()
ThingTwo().go()\
I really can't think of any reasons/risks that the above way of doing
things would incur any bad consequences. Could someone shed a little
light here? Thanks a lot.
1. your program will incur many tiny little transactions that each do
just one SQL operation, and will lack atomicity
2. your program will make excessive use of multiple connections, often
simultaneously as function A, which has its own Session, calls function
B, which has its own Session, now you have two connections open in one
operation, both on their own transactions, wasting resources, not
sharing context, not atomic
3. the anti-pattern in #2 makes it extremely easy for deadlocks and race
conditions to occur
4. your code will be littered with redundant session and transaction
code which will quickly grow to be inconsistent and the code will be
unmaintainable until a vast refactoring sweep goes through and fixes it
(where by "fixes" it comes down to, making it look like the
documentation you refer to recommends).
This is not hypothetical. I work heavily with Openstack today and
large parts of Openstack suffer terribly from this antipattern and I'm
trying to get them to fix all their code. See
http://specs.openstack.org/openstack/oslo-specs/specs/kilo/make-enginefacade-a-facade.html
for a case study and proposal I wrote.
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