Consolidate caching black magic

Get the stuff used to retrieve and refresh the latest date values all in
the same place, and make it a bit more beautiful by refactoring it all
into a common set of methods.

Signed-off-by: Dan McGee <dan@archlinux.org>

4 files changed, 35 insertions, 54 deletions

diff --git a/feeds.py b/feeds.py
index 0be1253..269d0a3 100644
--- a/feeds.py
+++ b/feeds.py
@@ -2,15 +2,13 @@ import pytz
 
 from django.contrib.sites.models import Site
 from django.contrib.syndication.views import Feed
-from django.core.cache import cache
 from django.db.models import Q
 from django.utils.feedgenerator import Rss201rev2Feed
 from django.utils.hashcompat import md5_constructor
 from django.views.decorators.http import condition
 
+from main.utils import retrieve_latest
 from main.models import Arch, Repo, Package
-from main.utils import CACHE_TIMEOUT
-from main.utils import CACHE_PACKAGE_KEY, CACHE_NEWS_KEY
 from news.models import News
 
 def check_for_unique_id(f):
@@ -31,32 +29,14 @@ class GuidNotPermalinkFeed(Rss201rev2Feed):
         super(GuidNotPermalinkFeed, self).write_items(handler)
 
 
-def retrieve_package_latest():
-    # we could break this down based on the request url, but it would probably
-    # cost us more in query time to do so.
-    latest = cache.get(CACHE_PACKAGE_KEY)
-    if latest:
-        return latest
-    try:
-        latest = Package.objects.values('last_update').latest(
-                'last_update')['last_update']
-        # 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_PACKAGE_KEY, latest, CACHE_TIMEOUT)
-        return latest
-    except Package.DoesNotExist:
-        pass
-    return None
-
 def package_etag(request, *args, **kwargs):
-    latest = retrieve_package_latest()
+    latest = retrieve_latest(Package)
     if latest:
         return md5_constructor(str(kwargs) + str(latest)).hexdigest()
     return None
 
 def package_last_modified(request, *args, **kwargs):
-    return retrieve_package_latest()
+    return retrieve_latest(Package)
 
 class PackageFeed(Feed):
     feed_type = GuidNotPermalinkFeed
@@ -125,28 +105,14 @@ class PackageFeed(Feed):
         return (item.repo.name, item.arch.name)
 
 
-def retrieve_news_latest():
-    latest = cache.get(CACHE_NEWS_KEY)
-    if latest:
-        return latest
-    try:
-        latest = News.objects.values('last_modified').latest(
-                'last_modified')['last_modified']
-        # same thoughts apply as in retrieve_package_latest
-        cache.add(CACHE_NEWS_KEY, latest, CACHE_TIMEOUT)
-        return latest
-    except News.DoesNotExist:
-        pass
-    return None
-
 def news_etag(request, *args, **kwargs):
-    latest = retrieve_news_latest()
+    latest = retrieve_latest(News)
     if latest:
         return md5_constructor(str(latest)).hexdigest()
     return None
 
 def news_last_modified(request, *args, **kwargs):
-    return retrieve_news_latest()
+    return retrieve_latest(News)
 
 class NewsFeed(Feed):
     feed_type = GuidNotPermalinkFeed
diff --git a/main/models.py b/main/models.py
index 8d34731..3812013 100644
--- a/main/models.py
+++ b/main/models.py
@@ -128,8 +128,7 @@ class Package(models.Model):
     class Meta:
         db_table = 'packages'
         ordering = ('pkgname',)
-        #get_latest_by = 'last_update'
-        #ordering = ('-last_update',)
+        get_latest_by = 'last_update'
 
     def __unicode__(self):
         return self.pkgname
@@ -390,10 +389,10 @@ def set_todolist_fields(sender, **kwargs):
         todolist.date_added = datetime.utcnow()
 
 # connect signals needed to keep cache in line with reality
-from main.utils import refresh_package_latest
+from main.utils import refresh_latest
 from django.db.models.signals import pre_save, post_save
 
-post_save.connect(refresh_package_latest, sender=Package,
+post_save.connect(refresh_latest, sender=Package,
         dispatch_uid="main.models")
 pre_save.connect(set_todolist_fields, sender=Todolist,
         dispatch_uid="main.models")
diff --git a/main/utils.py b/main/utils.py
index d7681cb..12d1250 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:
diff --git a/news/models.py b/news/models.py
index 17d51de..5e46751 100644
--- a/news/models.py
+++ b/news/models.py
@@ -36,10 +36,10 @@ def set_news_fields(sender, **kwargs):
                 now.strftime('%Y-%m-%d'), news.get_absolute_url())
 
 # connect signals needed to keep cache in line with reality
-from main.utils import refresh_news_latest
+from main.utils import refresh_latest
 from django.db.models.signals import pre_save, post_save
 
-post_save.connect(refresh_news_latest, sender=News,
+post_save.connect(refresh_latest, sender=News,
         dispatch_uid="news.models")
 pre_save.connect(set_news_fields, sender=News,
         dispatch_uid="news.models")