Ticket #6791: cached_db_with_documentation.diff

django/contrib/sessions/backends/cached_db.py

@@ +1 @@
+from django.conf import settings
+from django.contrib.sessions.models import Session
+from django.contrib.sessions.backends.base import SessionBase
+from django.core.exceptions import SuspiciousOperation
+from django.core.cache import cache
+import datetime
+
+class SessionStore(SessionBase):
+    """
+    Implements cached database session store
+    """
+    def __init__(self, session_key=None):
+        super(SessionStore, self).__init__(session_key)
+
+    def _get_cache_key(self):
+        return 'django_session_backend_cache_%s' % (self.session_key)
+    
+    def load(self):
+        try:
+            data = cache.get(self._get_cache_key(), None)
+            if data is None:
+                s = Session.objects.get(
+                    session_key = self.session_key, 
+                    expire_date__gt=datetime.datetime.now()
+                    )
+                data = self.decode(s.session_data)
+                cache.set(self._get_cache_key(), data, settings.SESSION_COOKIE_AGE)
+            return data
+        except (Session.DoesNotExist, SuspiciousOperation):
+            # Create a new session_key for extra security.
+            self.session_key = self._get_new_session_key()
+            self._session_cache = {}
+
+            # Save immediately to minimize collision
+            self.save()
+            return {}
+            
+    def exists(self, session_key):
+        try:
+            Session.objects.get(session_key=session_key)
+        except Session.DoesNotExist:
+            return False
+        return True
+            
+    def save(self):
+        cache.set(self._get_cache_key(), self._session, settings.SESSION_COOKIE_AGE)
+        Session.objects.create(
+            session_key = self.session_key,
+            session_data = self.encode(self._session),
+            expire_date = datetime.datetime.now() + datetime.timedelta(seconds=settings.SESSION_COOKIE_AGE))
+    
+    def delete(self, session_key):
+        try:
+            cache.delete(self._get_cache_key())
+            Session.objects.get(session_key=session_key).delete()
+        except Session.DoesNotExist:
+            pass

docs/sessions.txt

@@ -56 +56 @@
 Using cache-based sessions
 --------------------------
 
-To store session data using Django's cache system, set ``SESSION_ENGINE``
-to ``"django.contrib.sessions.backends.cache"``. You'll want to make sure
-you've configured your cache; see the `cache documentation`_ for details.
+There are two cache-based session stores to choose from.  To directly
+store session data using Django's cache system, set ``SESSION_ENGINE``
+to ``"django.contrib.sessions.backends.cache"``.  To store sessions in
+the database but cache read operations using Django's cache system, set
+``SESSION_ENGINE`` to ``"django.contrib.sessions.backend.db_cache"``.
+This session store is useful if your cache is reset or flushed frequently
+and combines efficient read operations with persistent storage. You'll
+want to make sure you've configured your cache; see the 
+`cache documentation`_ for details.
 
 .. _cache documentation: ../cache/