Hi Carl,
> Den 10/02/2015 kl. 18.34 skrev Carl Meyer <c...@oddbird.net
> <mailto:c...@oddbird.net>>:
>
> I assume you're using the 'db' cache backend? Otherwise, it wouldn't
> make sense to expect transactions to affect cache calls at all.
Yes, I'm using the db backend.
> The difference between the snippets is that you are doing a conditional
> branch based on the return value of the cache add, but your set_val
> helper method always returns None, it doesn't return the return value of
> cache.add().
Wow, *that* was embarrassing :-) You see what you want to see, I guess...
I'm still having issues with this, though. I reduced my own code to the
following. The intent of the code is to use the cache to implement
multiprocess/multiserver locking:
from django.db import transaction
from django.core.cache import cache
class CacheError(Exception):
pass
cache.delete('foo')
while True:
print('Trying to cache foo')
try:
with transaction.atomic():
if cache.get('foo'):
raise CacheError()
print('adding foo to cache')
assert cache.add(key='foo', value='bar')
print('foo cached')
time.sleep(random.random())
with transaction.atomic():
if cache.get('foo'):
cache.delete('foo')
except CacheError:
print('Failed to cache foo')
time.sleep(random.random())
Running this in parallel in two processes on the same machine returns this
after a while:
Process A:
Trying to cache foo
2015-02-10 21:02:25,781 DEBUG (0.001) SELECT cache_key, value, expires FROM
"dispatch_cache" WHERE cache_key = 'foo'; args=['foo']
adding foo to cache
2015-02-10 21:02:25,782 DEBUG (0.000) SELECT COUNT(*) FROM "dispatch_cache";
args=None
2015-02-10 21:02:25,782 DEBUG (0.000) SAVEPOINT "s140735261451008_x1";
args=None
2015-02-10 21:02:25,783 DEBUG (0.000) SELECT cache_key, expires FROM
"dispatch_cache" WHERE cache_key = 'foo'; args=['foo']
2015-02-10 21:02:25,784 DEBUG (0.000) INSERT INTO "dispatch_cache"
(cache_key, value, expires) VALUES ('foo', 'gASVBwAAAAAAAACMA2JhcpQu',
'9999-12-31 23:59:59'); args=['foo', 'gASVBwAAAAAAAACMA2JhcpQu', '9999-12-31
23:59:59']
2015-02-10 21:02:25,784 DEBUG (0.000) RELEASE SAVEPOINT
"s140735261451008_x1"; args=None
foo cached
2015-02-10 21:02:26,771 DEBUG (0.000) SELECT cache_key, value, expires FROM
"dispatch_cache" WHERE cache_key = 'foo'; args=['foo']
2015-02-10 21:02:26,772 DEBUG (0.000) DELETE FROM "dispatch_cache" WHERE
cache_key = 'foo'; args=['foo']
Process B:
Trying to cache foo
2015-02-10 21:02:25,782 DEBUG (0.000) SELECT cache_key, value, expires FROM
"dispatch_cache" WHERE cache_key = 'foo'; args=['foo']
adding foo to cache
2015-02-10 21:02:25,783 DEBUG (0.000) SELECT COUNT(*) FROM "dispatch_cache";
args=None
2015-02-10 21:02:25,784 DEBUG (0.000) SAVEPOINT "s140735261451008_x1";
args=None
2015-02-10 21:02:25,784 DEBUG (0.000) SELECT cache_key, expires FROM
"dispatch_cache" WHERE cache_key = 'foo'; args=['foo']
2015-02-10 21:02:25,791 DEBUG (0.007) INSERT INTO "dispatch_cache"
(cache_key, value, expires) VALUES ('foo', 'gASVBwAAAAAAAACMA2JhcpQu',
'9999-12-31 23:59:59'); args=['foo', 'gASVBwAAAAAAAACMA2JhcpQu', '9999-12-31
23:59:59']
2015-02-10 21:02:25,792 DEBUG (0.000) ROLLBACK TO SAVEPOINT
"s140735261451008_x1"; args=None
Traceback (most recent call last):
File "tmp.py", line 30, in <module>
assert cache.add(key='foo', value='bar')
AssertionError
I don't see how this is possible when I'm using transaction.atomic().
Thanks,
Erik
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