Question

I would like to implement a decorator that provides per-request caching to any method, not just views. Here is an example use case.

I have a custom tag that determines if
a record in a long list of records is
a “favorite”. In order to check if an
item is a favorite, you have to query
the database. Ideally, you would
perform one query to get all the
favorites, and then just check that
cached list against each record.

One solution is to get all the
favorites in the view, and then pass
that set into the template, and then
into each tag call.

Alternatively, the tag itself could
perform the query itself, but only the
first time it’s called. Then the
results could be cached for subsequent
calls. The upside is that you can use
this tag from any template, on any
view, without alerting the view.

In the existing caching mechanism, you
could just cache the result for 50ms,
and assume that would correlate to the
current request. I want to make that
correlation reliable.

Here is an example of the tag I currently have.

@register.filter()
def is_favorite(record, request):

    if "get_favorites" in request.POST:
        favorites = request.POST["get_favorites"]
    else:

        favorites = get_favorites(request.user)

        post = request.POST.copy()
        post["get_favorites"] = favorites
        request.POST = post

    return record in favorites

Is there a way to get the current request object from Django, w/o passing it around? From a tag, I could just pass in request, which will always exist. But I would like to use this decorator from other functions.

Is there an existing implementation of a per-request cache?

Answer 1

Using a custom middleware you can get a Django cache instance guaranteed to be cleared for each request.

This is what I used in a project:

from threading import currentThread
from django.core.cache.backends.locmem import LocMemCache

_request_cache = {}
_installed_middleware = False

def get_request_cache():
    assert _installed_middleware, 'RequestCacheMiddleware not loaded'
    return _request_cache[currentThread()]

# LocMemCache is a threadsafe local memory cache
class RequestCache(LocMemCache):
    def __init__(self):
        name = 'locmemcache@%i' % hash(currentThread())
        params = dict()
        super(RequestCache, self).__init__(name, params)

class RequestCacheMiddleware(object):
    def __init__(self):
        global _installed_middleware
        _installed_middleware = True

    def process_request(self, request):
        cache = _request_cache.get(currentThread()) or RequestCache()
        _request_cache[currentThread()] = cache

        cache.clear()

To use the middleware register it in settings.py, e.g:

MIDDLEWARE_CLASSES = (
    ...
    'myapp.request_cache.RequestCacheMiddleware'
)

You may then use the cache as follows:

from myapp.request_cache import get_request_cache

cache = get_request_cache()

Refer to the django low level cache api doc for more information:

Django Low-Level Cache API

It should be easy to modify a memoize decorator to use the request cache. Have a look at the Python Decorator Library for a good example of a memoize decorator:

Python Decorator Library