[Django] #26463: Allowing Callbacks/Handlers to be called on Cache entry Expiration
Django
------------------------------+------------------------
Reporter: oneTimePad | Owner: oneTimePad
Type: New feature | Status: new
Component: Core (Other) | Version: master
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 1
Easy pickings: 0 | UI/UX: 0
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I read a suggestion about this here:
[http://stackoverflow.com/questions/20866460/django-cache-backend-how-to-
implement-callback-when-cache-timeout] .
I have written up an implementation of it with some tests for
FileBasedCache (filebased.py) and want to add it to locmem.py too (not
done yet).
Here are code snippets from filebased.py that have been tested:
The edits are in set and _is_expired
{{{
def set(self, key, value, timeout=DEFAULT_TIMEOUT,
version=None,handler=None):
self._createdir() # Cache dir can be deleted at any time.
fname = self._key_to_file(key, version)
self._cull() # make some room if necessary
fd, tmp_path = tempfile.mkstemp(dir=self._dir)
renamed = False
try:
with io.open(fd, 'wb') as f:
expiry = self.get_backend_timeout(timeout)
f.write(pickle.dumps(expiry, pickle.HIGHEST_PROTOCOL))
#check if handler exists
if handler:
#write handler to pickle
f.write(pickle.dumps(handler,pickle.HIGHEST_PROTOCOL))
f.write(zlib.compress(pickle.dumps(value,
pickle.HIGHEST_PROTOCOL)))
file_move_safe(tmp_path, fname, allow_overwrite=True)
renamed = True
finally:
if not renamed:
os.remove(tmp_path)
def _is_expired(self, f):
"""
Takes an open cache file and determines if it has expired,
deletes the file if it is has passed its expiry time.
"""
exp = pickle.load(f)
if exp is not None and exp < time.time():
#if there is a handler, call it
try:
handler = pickle.load(f)
handler(exp)
except pickle.UnpicklingError:
#handler not added(no handler specified), so pickling error
occurs
pass
f.close() # On Windows a file has to be closed before
deleting
self._delete(f.name)
return True
return False
}}}
The handler has the following format
{{{
class Handler(object):
def __call__(self,exp):
...
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/26463>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
Django
Reporter: oneTimePad | Owner:
| oneTimePad
Type: New feature | Status: new
Severity: Normal | Resolution:
Keywords: | Triage Stage:
| Unreviewed
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
Changes (by oneTimePad):
* needs_better_patch: => 0
* component: Core (Other) => Core (Cache system)
* needs_tests: => 0
* needs_docs: => 0
--
Ticket URL: <https://code.djangoproject.com/ticket/26463#comment:1>
Django
Reporter: oneTimePad | Owner:
| oneTimePad
Type: New feature | Status: new
Severity: Normal | Resolution:
| Unreviewed
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Comment (by timgraham):
I'm not sure about this feature. It won't work for memcached where cache
entries may be evicted without Django's knowledge, correct?
--
Ticket URL: <https://code.djangoproject.com/ticket/26463#comment:2>
Django
Reporter: oneTimePad | Owner:
| oneTimePad
Type: New feature | Status: new
Severity: Normal | Resolution:
| Unreviewed
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Comment (by oneTimePad):
Replying to [comment:2 timgraham]:
> I'm not sure about this feature. It won't work for memcached where cache
entries may be evicted without Django's knowledge, correct?
I'm not sure about memcached, but I believe for the db cache it won't
either. It seems to work nicely for filebased though, and possibly for
locmem
--
Ticket URL: <https://code.djangoproject.com/ticket/26463#comment:3>
Django
Reporter: oneTimePad | Owner:
| oneTimePad
Type: New feature | Status: closed
Component: Core (Cache system) | Version: master
Keywords: | Triage Stage:
| Unreviewed
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Changes (by timgraham):
* status: new => closed
* resolution: => wontfix
Comment:
I think the feature is too specialized for inclusion in Django. You could
ask on the DevelopersMailingList to see if anyone else feels it's an
appropriate use of a cache, but I'm skeptical. There are probably better
ways to solve the problem in the linked ticket than this idea. Also
requiring file-based or local memory caching in production is a bit
impractical.
--
Ticket URL: <https://code.djangoproject.com/ticket/26463#comment:4>