id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
30601	Extend documentation about transaction rollbacks to mention global state mutations such as caching	Sebastian Wagner	Lufafa Joshua	"ATOMIC_REQUESTS=True essentially creates nested transactions. And nested transactions might not play well with custom caching.

A small example of how to use nested transactions and caching to get inconsistent data:

{{{
my_mem_cache_a = ...
my_mem_cache_b = ...

def set_a(x):
    with transaction.atomic():
        a = ...
        a.x = x
        a.save()
    my_mem_cache_a = x 

def set_a_b(x, y):
    with transaction.atomic():
        set_a(x)
        b = ...
        b.y = y
        raise Exception()
        # at this point, my_mem_cache_a is out of sync with the database. 
        b.save()
    my_mem_cache_b = x 

set_a_b(...)
}}}

(this example also works without transaction.atomic() )

Would be nice to add a warning to 

https://docs.djangoproject.com/en/2.2/topics/db/transactions/#tying-transactions-to-http-requests

that ATOMIC_REQUESTS=True might be harmful, if code 

* uses caching
* proactively updates data in the cache
* makes the assumption that values are updated in the database after a transaction is finished."	Cleanup/optimization	closed	Documentation	dev	Normal	fixed			Ready for checkin	1	0	0	0	0	0