#30601: ATOMIC_REQUESTS=True and caching might be dangerous
-------------------------------------+-------------------------------------
Reporter: | Owner: nobody
sebastian-philipp |
Type: | Status: new
Uncategorized |
Component: Database | Version: 2.2
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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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.
--
Ticket URL: <https://code.djangoproject.com/ticket/30601>
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