Allow configuring the memcached Client with CACHES 'OPTIONS'

[alex@…]

This appears to be an issue related to the fact that older releases of memcached insisted on a 1 MB value size limit.

Current releases of memcached have a user-configurable limit, however Django still uses a hard-wired 1 MB limit.

This (undocumented) 'feature' is currently causing errors, see the following two SO questions (one by me):

​http://stackoverflow.com/questions/18143159/apparent-bug-storing-large-keys-in-django-memcached
​http://stackoverflow.com/questions/11874377/django-caching-a-large-list

A 'solution' is proposed by the second SO question:

also edit this class in memcached.py located in /usr/lib/python2.7/dist-packages/django/core/cache/backends to look like this:

class MemcachedCache(BaseMemcachedCache):
"An implementation of a cache binding using python-memcached"
def __init__(self, server, params):
    import memcache
    memcache.SERVER_MAX_VALUE_LENGTH = 1024*1024*10 #added limit to accept 10mb
    super(MemcachedCache, self).__init__(server, params,
                                         library=memcache,
                                         value_not_found_exception=ValueError)

Perhaps this could be added to the config of the logger?

For example, an ITEM_SIZE_MAX key could be created for the dictionary, so the following code could be used:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
        'ITEM_SIZE_MAX': 10,
        'LOCATION': 'localhost:11211',
    }
}

Obviously, the user would also have to add the necessary -I parameter to their /etc/memcached.conf (system dependent). I suppose the best would be if we could automatically detect the item size from their current memcached setup. For example:

from subprocess import Popen, Pipe
memcached_call = Popen("echo stats settings | nc localhost 11211", stdout=PIPE, stderr=PIPE, shell=True)
out, err = memcached_call.communicate()
if err:
    log("Error when getting memcached stats to find item size max: "+err+"\n. Assuming max size is 1 MB")
    maxsize_int = 1024*1024
else:
    values = out.split("\n")
    maxsize_field = filter(lambda field: field.startswith("STAT item_size_max"), values)[0]
    maxsize_int = int(maxsize_field.strip()[maxsize_field.rindex(" "):])

[anonymous]

import memcache
memcache.SERVER_MAX_VALUE_LENGTH = 1024*1024*10 #added limit to accept 10mb

Apparently this solution does NOT actually work... the value from the module is not propagated to the client properly.
I've reported the issue to python-memcache: ​https://github.com/linsomniac/python-memcached/issues/13

[Aymeric Augustin]

As far as I can tell, Django itself doesn't contain any code to limit the size of cached items. (If it does, sorry, I missed it.)

Could you clarify your assertion that "Django still uses a hard-wired 1 MB limit."?

If the limitation is entirely handled by the memcache client library and server, I'm inclined to close this ticket as invalid.

[Shai Berger]

If I understand correctly, the ticket complains that there is no sane way to configure the max-item-size for use in Django; this leads to Django apps "always" using the default 1M limit. For the suggested change to work, the user also needs to take care of their server (at least if using a larger value), and if the anonymous user is right, also fix a bug in the client lib, but the Django part is missing either way.

I think that is valid.

[Tim Graham]

It's possible to accomplish this using a custom cache backend, but it may be nice to make this a bit easier:

import pickle

from django.core.cache.backends.memcached import MemcachedCache

MIN_COMPRESS_LENGTH = 1048576 # 1MB

class CustomMemcachedCache(MemcachedCache):

    @property
    def _cache(self):
        """
        Override to add pass some custom parameters to the python_memcached
        backend.
        """
        if getattr(self, '_client', None) is None:
            options = {}
            if self._options and 'SERVER_MAX_VALUE_LENGTH' in self._options:
                options['server_max_value_length'] = self._options['SERVER_MAX_VALUE_LENGTH']
            self._client = self._lib.Client(self._servers,
                pickleProtocol = pickle.HIGHEST_PROTOCOL, **options
            )
        return self._client

    def set(self, key, value, timeout=0, version=None):
        """
        Override to pass MIN_COMPRESS_LENGTH so large values are compressed.
        """
        key = self.make_key(key, version=version)
        self._cache.set(key, value, self._get_memcache_timeout(timeout), MIN_COMPRESS_LENGTH)

[anonymous]

""If I understand correctly, the ticket complains that there is no sane way to configure the max-item-size for use in Django""

Precisely.

Django does not deliberately limit the size, but it's dependent modules do, without having their configuration options passed through.

[anonymous]

Sorry guys, I just realised I really ought to make an account and stop posting anonymously... I posted this issue, and the anonymous comments are by me.

[AlexF124]

Ok, python-memcache has gone ahead and fixed the bug, so the

import memcache
memcache.SERVER_MAX_VALUE_LENGTH = 1024*1024*10 #added limit to accept 10mb

would work from python-memcache's next release and onwards.

[Aymeric Augustin]

I'm still unconvinced there's much value in channeling this configuration through Django's CACHE setting, but I'm not opposing it if another core dev wants it.

[Shai Berger]

I'm not sure if that was Aymeric's point, but in any case, there should be a "don't do that unless you know what you're doing" warning on the use of this feature; memcached recommends strongly against it. I think as part of this, the entry should be named "FORCE_ITEM_MAX_SIZE" or something like it.

BTW, I think the most common cause for wanting it -- the only case I've seen up close -- is pushing big objects into a cached session. Perhaps this is something we should specifically recommend against.

[Olav Morken]

Hi,

I ran into this issue now, and have a few comments:

• This problem only affects the django.core.cache.backends.memcached.MemcachedCache backend. A simple workaround may be to use the django.core.cache.backends.memcached.PyLibMCCache backend.
• To fix this, one doesn't actually need to write to memcache.SERVER_MAX_VALUE_LENGTH variable. It can also be passed as an option to the constructor: [...].Client(self._servers, pickleProtocol=pickle.HIGHEST_PROTOCOL, server_max_value_length=max_value_size)

As for reasons to want this: In my case, I am fetching a blob of data from a remote web service that is 1.2 MiB. Caching that data blob saves an expensive API call.

[Tim Graham]

Lets make this ticket a bit more generic after a request in #26301 to set another option (socket_timeout) in Client.__init__(). I'm thinking a key in 'OPTIONS' like CLIENT_OPTIONS which would be a dictionary passed to Client.__init__() as kwargs might work.

[Maciej Dziardziel]

PR: ​https://github.com/django/django/pull/6233

[Tim Graham]

Please uncheck "patch needs improvement" after you update the patch, thanks!

[Tim Graham <timograham@…>]

In 606a303:

Fixed #27124 -- Excluded cull-related cache configs from memcached tests.

Since the cull and zero_cull test cache configs set MAX_ENTRIES
and CULL_FREQUENCY in OPTIONS, which are only intended for use with
the locmem, filesystem, and database backends. This prevents test
failures once refs #20892 is fixed.

[Ed Morley]

I've opened a new PR, which takes a slightly different approach:

• The contents of OPTIONS is now passed directly to the client constructor
• For pylibmc, behaviors are now passed inside the behaviors key within OPTIONS, rather than as top-level properties of OPTIONS. The old behavior is still supported for now, but generates deprecation warnings.

Discussion of alternative approaches (on the old PR):
​https://github.com/django/django/pull/6233#issuecomment-192459369

The new PR:
​https://github.com/django/django/pull/7160

[Tim Graham <timograham@…>]

In fb8eea56:

[1.10.x] Fixed #27124 -- Excluded cull-related cache configs from memcached tests.

Since the cull and zero_cull test cache configs set MAX_ENTRIES
and CULL_FREQUENCY in OPTIONS, which are only intended for use with
the locmem, filesystem, and database backends. This prevents test
failures once refs #20892 is fixed.

Backport of 606a303856afee684563f9349c2a55578854f1ba from master

[Tim Graham <timograham@…>]

In 65ec8fa:

Fixed #20892 -- Allowed configuring memcached client using OPTIONS.

Previously, the MemcachedCache backend ignored OPTIONS and
PyLibMCCache used them to set pylibmc behaviors. Both backends now
pass OPTIONS as keyword arguments to the client constructors.

[Tim Graham <timograham@…>]

In e47b56d7:

Refs #20892 -- Removed support for passing pylibmc behavior settings as top-level attributes of CACHESOPTIONS.

Per deprecation timeline.