Dummy code for my thoughts described in https://groups.google.com/forum/#!topic/django-cms-developers/bFbhE1a-rTs

newcmscache.py

"""
Dummy code for my thoughts described in https://groups.google.com/forum/#!topic/django-cms-developers/bFbhE1a-rTs

Note that this pseudo-implementation has problems where between `cache.get(...)`
in `get_cached` and `cache.set(...)` the value *could* have changed in the cache,
so some kind of better way to prevent overwriting changes done by others should
be implemented.
"""
# the local python in-memory cache
LOCAL_CACHE = {}

def get_cached(key, calculator):
    """
    Returns the value returned by `calculator`.
    
    Does so ideally by getting a pre-calculated value using `key` in the
    `LOCAL_CACHE` dictionary.
    """
    # get the list of PIDs or an empty list from the django cache (eg: memcached)
    cached = cache.get(key, [])
    # if our PID is in that list
    if os.getpid() in cached:
        # check if we already did the computation of the value
        if key in LOCAL_CACHE:
            # if so, return it
            return LOCAL_CACHE[key]
        else:
            # else, aquire a write-lock for this process to prevent threading issues
            with Lock():
                # fill the local cache
                LOCAL_CACHE[key] = calculator()
                # add this PID to the list in the django cache
                cached.append(os.getpid())
                cache.set(key, cached)
            # return the value
            return LOCAL_CACHE[key]
    else:
        # if we're not in that list yet, aquire a write-lock for this process to prevent threading issues
        with Lock():
            # fill the local cache
            LOCAL_CACHE[key] = calculator()
        # return the value
        return LOCAL_CACHE[key]

def invalidate_cache(key):
    """
    Invalidate the local cache for `key` and signal other processes using the
    django cache that they should invalidate too.
    """
    # delete the django (memcached?) cache 
    cache.delete(key)
    # get a write lock, again for threading issues
    with Lock():
        # delete the key from the local cache    
        del LOCAL_CACHE[key]