[PERF-344] Add versioning of cached course assets to allow graceful cache invalidation

When releasing the versioned assets work, we stumbled on a problem with old pickled
versions of the StaticContent objects residing in cache, which triggered a bug in the
code. Not wanting to blow away all cached items, we ended up having to revert and add
in some backwards-compatible helper code to ease the transition.

With this, we'll now utilize the version argument that Django's caching interface
allows, in conjunction with a constant value that can be modified when breaking changes
are being made, to let us gracefully ignore older cached course assets.

cms/djangoapps/contentstore/tests/test_core_caching.py

-from cache_toolbox.core import get_cached_content, set_cached_content, del_cached_content
+"""
+Tests core caching facilities.
+"""
+from contentserver.caching import get_cached_content, set_cached_content, del_cached_content
 from opaque_keys.edx.locations import Location
 from django.test import TestCase
 

cms/djangoapps/contentstore/views/assets.py

@@ -10,7 +10,7 @@ from django.views.decorators.http import require_http_methods, require_POST
 from django.conf import settings
 
 from edxmako.shortcuts import render_to_response
-from cache_toolbox.core import del_cached_content
+from contentserver.caching import del_cached_content
 
 from contentstore.utils import reverse_course_url
 from xmodule.contentstore.django import contentstore

cms/djangoapps/contentstore/views/tests/test_transcripts.py

@@ -13,7 +13,7 @@ from django.test.utils import override_settings
 from django.conf import settings
 
 from contentstore.tests.utils import CourseTestCase, mock_requests_get
-from cache_toolbox.core import del_cached_content
+from contentserver.caching import del_cached_content
 from xmodule.modulestore.django import modulestore
 from xmodule.contentstore.django import contentstore
 from xmodule.contentstore.content import StaticContent

common/djangoapps/cache_toolbox/core.py

@@ -107,30 +107,3 @@ def instance_key(model, instance_or_pk):
         model._meta.model_name,
         getattr(instance_or_pk, 'pk', instance_or_pk),
     )
-
-
-def set_cached_content(content):
-    cache.set(unicode(content.location).encode("utf-8"), content)
-
-
-def get_cached_content(location):
-    return cache.get(unicode(location).encode("utf-8"))
-
-
-def del_cached_content(location):
-    """
-    delete content for the given location, as well as for content with run=None.
-    it's possible that the content could have been cached without knowing the
-    course_key - and so without having the run.
-    """
-    def location_str(loc):
-        return unicode(loc).encode("utf-8")
-
-    locations = [location_str(location)]
-    try:
-        locations.append(location_str(location.replace(run=None)))
-    except InvalidKeyError:
-        # although deprecated keys allowed run=None, new keys don't if there is no version.
-        pass
-
-    cache.delete_many(locations)

common/djangoapps/contentserver/__init__.py

+"""
+Serves course assets to end users.
+"""
+
+CONTENTSERVER_VERSION = 1

common/djangoapps/contentserver/caching.py

+"""
+Helper functions for caching course assets.
+"""
+from django.core.cache import caches
+from django.core.cache.backends.base import InvalidCacheBackendError
+from opaque_keys import InvalidKeyError
+from . import CONTENTSERVER_VERSION
+
+# See if there's a "course_assets" cache configured, and if not, fallback to the default cache.
+CONTENT_CACHE = caches['default']
+try:
+    CONTENT_CACHE = caches['course_assets']
+except InvalidCacheBackendError:
+    pass
+
+
+def set_cached_content(content):
+    """
+    Stores the given piece of content in the cache, using its location as the key.
+    """
+    CONTENT_CACHE.set(unicode(content.location).encode("utf-8"), content, version=CONTENTSERVER_VERSION)
+
+
+def get_cached_content(location):
+    """
+    Retrieves the given piece of content by its location if cached.
+    """
+    return CONTENT_CACHE.get(unicode(location).encode("utf-8"), version=CONTENTSERVER_VERSION)
+
+
+def del_cached_content(location):
+    """
+    Delete content for the given location, as well versions of the content without a run.
+
+    It's possible that the content could have been cached without knowing the course_key,
+    and so without having the run.
+    """
+    def location_str(loc):
+        """Force the location to a Unicode string."""
+        return unicode(loc).encode("utf-8")
+
+    locations = [location_str(location)]
+    try:
+        locations.append(location_str(location.replace(run=None)))
+    except InvalidKeyError:
+        # although deprecated keys allowed run=None, new keys don't if there is no version.
+        pass
+
+    CONTENT_CACHE.delete_many(locations, version=CONTENTSERVER_VERSION)

common/djangoapps/contentserver/middleware.py

@@ -17,7 +17,7 @@ from xmodule.contentstore.content import StaticContent, XASSET_LOCATION_TAG
 from xmodule.modulestore import InvalidLocationError
 from opaque_keys import InvalidKeyError
 from opaque_keys.edx.locator import AssetLocator
-from cache_toolbox.core import get_cached_content, set_cached_content
+from .caching import get_cached_content, set_cached_content
 from xmodule.modulestore.exceptions import ItemNotFoundError
 from xmodule.exceptions import NotFoundError
 

lms/djangoapps/courseware/tests/test_video_handlers.py

@@ -20,7 +20,7 @@ from xmodule.modulestore import ModuleStoreEnum
 from xmodule.x_module import STUDENT_VIEW
 from . import BaseTestXmodule
 from .test_video_xml import SOURCE_XML
-from cache_toolbox.core import del_cached_content
+from contentserver.caching import del_cached_content
 from xmodule.exceptions import NotFoundError
 
 from xmodule.video_module.transcripts_utils import (