diff options
Diffstat (limited to 'main/utils.py')
| -rw-r--r-- | main/utils.py | 34 |
1 files changed, 25 insertions, 9 deletions
diff --git a/main/utils.py b/main/utils.py index d7681cb6..12d12503 100644 --- a/main/utils.py +++ b/main/utils.py @@ -6,10 +6,8 @@ from django.core.cache import cache from django.utils.hashcompat import md5_constructor CACHE_TIMEOUT = 1800 -INVALIDATE_TIMEOUT = 15 - -CACHE_PACKAGE_KEY = 'cache_package_latest' -CACHE_NEWS_KEY = 'cache_news_latest' +INVALIDATE_TIMEOUT = 10 +CACHE_LATEST_PREFIX = 'cache_latest_' def cache_function_key(func, args, kwargs): raw = [func.__name__, func.__module__, args, kwargs] @@ -53,16 +51,34 @@ make_choice = lambda l: [(str(m), str(m)) for m in l] # and hoops otherwise. The only thing currently using these keys is the feed # caching stuff. -def refresh_package_latest(**kwargs): +def refresh_latest(**kwargs): + '''A post_save signal handler to clear out the cached latest value for a + given model.''' + cache_key = CACHE_LATEST_PREFIX + kwargs['sender'].__name__ # We could delete the value, but that could open a race condition # where the new data wouldn't have been committed yet by the calling # thread. Instead, explicitly set it to None for a short amount of time. # Hopefully by the time it expires we will have committed, and the cache # will be valid again. See "Scaling Django" by Mike Malone, slide 30. - cache.set(CACHE_PACKAGE_KEY, None, INVALIDATE_TIMEOUT) + cache.set(cache_key, None, INVALIDATE_TIMEOUT) -def refresh_news_latest(**kwargs): - # same thoughts apply as in refresh_package_latest - cache.set(CACHE_NEWS_KEY, None, INVALIDATE_TIMEOUT) +def retrieve_latest(sender): + # we could break this down based on the request url, but it would probably + # cost us more in query time to do so. + cache_key = CACHE_LATEST_PREFIX + sender.__name__ + latest = cache.get(cache_key) + if latest: + return latest + try: + latest_by = sender._meta.get_latest_by + latest = sender.objects.values(latest_by).latest()[latest_by] + # Using add means "don't overwrite anything in there". What could be in + # there is an explicit None value that our refresh signal set, which + # means we want to avoid race condition possibilities for a bit. + cache.add(cache_key, latest, CACHE_TIMEOUT) + return latest + except sender.DoesNotExist: + pass + return None # vim: set ts=4 sw=4 et: |