RAM cache

Introduction

The RAM cache is a Zope facility to create custom in-process caches.

Using memcached backend

By default, Zope uses an in-process memory cache. It is possible to replace this with memcached.

Advantages:

  • All front-end clients share the cache.
  • Cache survives over a client restart.

Memoizers

Memoize's RAM cache can be replaced with a memcached backend with the following snippet.

See the set-up for the http://plone.org/ site as an example:

RAM Cache

The RAM cache is used e.g. as a rendered template cache backend.

You can add MemcachedManager to your Zope setup, and replace the RamCache instance in the ZMI with a new instance of MemcachedManager (keep the id the same).

Using custom RAM cache

You want to use a custom cache if you think cache size or saturation will pose problems.

The following advanced example shows how to enhance existing content type text and description accessors by performing HTML transformations and caching the result in a custom RAM cache.

Example:

import logging

import lxml.html

from zope.app.cache import ram

from Products.feedfeeder.content.item import FeedFeederItem
from gomobile.xhtmlmp.transformers.xhtmlmp_safe import clean_xhtml_mp

logger = logging.getLogger("GoMobile")

logger.info("Running in feedfeeder monkey-patches")

# Cache storing transformed XHTML
xhtml_cache = ram.RAMCache()
xhtml_cache.update(maxAge=86400, maxEntries=1000)

# Dummy object to mark missing values from cache
_marker = object()

def cache(name):
    """ Special cache decorator which generates cache key based on context object and cache name """
    def decorator(fun):
        def replacement(context):
            key = str(context.UID()) + "." + name

            cached_value = xhtml_cache.query(key, default=_marker)
            if cached_value is _marker:
                cached_value = fun(context)
                xhtml_cache.set(cached_value, key)
            return cached_value
        return replacement
    return decorator


def flush_cache(name, context):
    """ Clear entry in RAMCache

    global_key is function specific key, key is context specific key.

    """
    key = context.UID() + "." + name
    xhtml_cache.invalidate(key)


#
# Modify existing body text and description accessors so that
# 1) HTML is cleaned
# 2) The result cleaned HTML is cached in RAM
#
# We do not persistently want to store cleaned HTML,
# since our cleaner might be b0rked and we want to easily
# regenerate cleaned HTML when needed.
#

# Run in monkey patching
FeedFeederItem._old_getText = FeedFeederItem.getText
FeedFeederItem._old_setText = FeedFeederItem.setText
FeedFeederItem._old_Description = FeedFeederItem.Description
FeedFeederItem._old_setDescription = FeedFeederItem.setDescription

@cache("text")
def _getText(self):
    """ Body text accessor """
    text = FeedFeederItem._old_getText(self)

    if text:
        # can be None
        clean = clean_xhtml_mp(text)
        print "Cleaned text:" + clean
        return clean

    return text

def _setText(self, value):
    FeedFeederItem._old_setText(self, value)
    flush_cache("text", self)

@cache("description")
def _Description(self):
    """ Description accessor """
    text = FeedFeederItem._old_Description(self)

    #print "Accessing description:" + str(text)

    # Remove any HTML formatting in the description
    if text:
        parsed = lxml.html.fromstring(text.decode("utf-8"))
        clean = lxml.html.tostring(parsed, encoding="utf-8", method="text").decode("utf-8")
        #print "Cleaned decsription:" + clean
        return clean

    return text

def _setDescription(self, value):
    FeedFeederItem._old_setDescription(self, value)
    flush_cache("description", self)

FeedFeederItem.getText = _getText
FeedFeederItem.setText = _setText
FeedFeederItem.Description = _Description
FeedFeederItem.setDescription = _setDescription

ZCacheable

ZCacheable is an ancient Zope design pattern for caching. It allows persistent objects that are subclasses of OFS.Cacheable to have the cache backend configured externally.

The cache type (cache id) in use is stored persistently per cache user object, but the cache can be created at runtime (RAM cache) or externally (memcached) depending on the situation.

Note

Do not use ZCacheable in new code.

It takes optional backends which must be explicitly set: 

def enableCaching():
    pas=getPAS()
    if pas.ZCacheable_getManager() is None:
        pas.ZCacheable_setManagerId(manager_id="RAMCache")
    getLDAPPlugin().ZCacheable_setManagerId(manager_id="RAMCache")

The RAMCache above is per thread. You cannot clear this cache for all ZEO clients easily.

Some hints:

It is enabled per persistent object:

>>> app.test2.acl_users.ZCacheable_isCachingEnabled()
<Products.StandardCacheManagers.RAMCacheManager.RAMCache instance at 0x10a064cf8>

>>> app.test2.acl_users.ZCacheable_enabled()
1

Get known cache backends:

>>> app.test2.acl_users.ZCacheable_getManagerIds()
({'id': 'caching_policy_manager', 'title': ''}, {'id': 'HTTPCache', 'title': ''}, {'id': 'RAMCache', 'title': ''}, {'id': 'ResourceRegistryCache', 'title': 'Cache for saved ResourceRegistry files'})

Disabling it (persistent change):

>>> app.test2.acl_users.ZCacheable_setManagerId(None)
>>> app.test2.acl_users.ZCacheable_enabled()
1
>>> app.test2.acl_users.ZCacheable_getManagerIds()
({'id': 'caching_policy_manager', 'title': ''}, {'id': 'HTTPCache', 'title': ''}, {'id': 'RAMCache', 'title': ''}, {'id': 'ResourceRegistryCache', 'title': 'Cache for saved ResourceRegistry files'})
>>> app.test2.acl_users.ZCacheable_isCachingEnabled()
>>> app.test2.acl_users.ZCacheable_setEnabled(False)

More info: