Phalcon\Cache\Backend\Memcache fails on high usage

[Plaputta]

Like mentioned in https://github.com/phalcon/docs/blob/master/en/api/Phalcon_Cache_Backend_Memcache.rst the Memcached cache backend stores all keys it manages in a key called _PHCM.

Memcached items are limited to 1 MB by default and the Memcached docs state that it is not safe to increase this value.

However, since Phalcon stores all keys in _PHCM, on high traffic applications this item grows quickly. On our application we're using the Memcached adapter to store user sessions and since we have some traffic on our site, within a few hours we have 60k sessions and the _PCHM item grows to about 4mb.

Phalcon should avoid this by not storing the keys in one item, giving up the ability to query all keys but being failsafe.

[temuri416]

Hm... This seems to be a valid concern.

[stamster]

This has been fixed in 2.0.9.
You can just omit the 'statsKey' parameter while instantiating new Libmemcached instance.

[stamster]

Uh-oh.
it is fixed only for SESSION libmemcached adapter, not for BACKEND libmemcached.

But, fortunately this works fine as override:
https://github.com/phalcon/cphalcon/blob/master/phalcon/cache/backend/libmemcached.zep#L321

While trying to query cached keys this exception is thrown:
Class:Phalcon\Cache\Exception#File:phalcon/cache/backend/libmemcached.zep#Line:322#

So just pass to the constructor: 'statsKey' => null
or:
'statsKey' => ''