Enabled database backend to properly require a cache backend that works in cross-process environments. This adds an optional setting CONSTANCE_DATABASE_CACHE_BACKEND.

README.rst

@@ -82,8 +82,17 @@ configuration values:
 
     pip install django-picklefield
 
-  The database backend will automatically cache the config values in memory
-  and clear them when when saving occurs.
+  The database backend has the ability to automatically cache the config
+  values and clear them when saving. You need to set the following setting
+  to enable this feature::
+
+    CONSTANCE_DATABASE_CACHE_BACKEND = 'memcached://127.0.0.1:11211/'
+
+  .. note::
+
+    This won't work with a cache backend that doesn't support
+    cross-process caching, because correct cache invalidation
+    can't be guaranteed.
 
 .. _django-picklefield: http://pypi.python.org/pypi/django-picklefield/
 

constance/backends/database/__init__.py

@@ -1,37 +1,59 @@
 from django.core.exceptions import ImproperlyConfigured
 from django.db.models.signals import post_save
 from django.utils.functional import memoize
+from django.core.cache import get_cache
+from django.core.cache.backends.locmem import CacheClass as LocMemCacheClass
 
 from constance.backends import Backend
+from constance import settings
 
-db_cache = {}
+db_cache = None
+if settings.DATABASE_CACHE_BACKEND:
+    db_cache = get_cache(settings.DATABASE_CACHE_BACKEND)
+    if isinstance(db_cache, LocMemCacheClass):
+        raise ImproperlyConfigured(
+            "The CONSTANCE_DATABASE_CACHE_BACKEND setting refers to a "
+            "subclass of Django's local-memory backend (%r). Please set "
+            "it to a backend that supports cross-process caching."
+            % settings.DATABASE_CACHE_BACKEND)
 
 class DatabaseBackend(Backend):
-
     def __init__(self):
         from constance.backends.database.models import Constance
-        if not Constance._meta.installed:
-            raise ImproperlyConfigured(
-                "The constance.contrib.database app isn't installed correctly. "
-                "Make sure it's listed in the INSTALLED_APPS setting.")
         self._model = Constance
+        if not self._model._meta.installed:
+            raise ImproperlyConfigured(
+                "The constance.backends.database app isn't installed "
+                "correctly. Make sure it's in your INSTALLED_APPS setting.")
         # Clear simple cache.
         post_save.connect(self.clear, sender=self._model)
 
-    def _get(self, key):
-        try:
-            value = self._model._default_manager.get(key=key).value
-        except self._model.DoesNotExist:
-            return None
+    def mget(self, keys):
+        for const in self._model._default_manager.filter(key__in=keys):
+            yield const.key, const.value
+
+    def get(self, key):
+        value = None
+        if db_cache:
+            value = db_cache.get(key)
+        if value is None:
+            try:
+                value = self._model._default_manager.get(key=key).value
+            except self._model.DoesNotExist:
+                pass
+            else:
+                if db_cache:
+                    db_cache.add(key, value)
         return value
-    get = memoize(_get, db_cache, 2)
 
     def set(self, key, value):
-        constance, created = self._model._default_manager.get_or_create(key=key, defaults={'value': value})
+        constance, created = self._model._default_manager.get_or_create(
+            key=key, defaults={'value': value}
+        )
         if not created:
             constance.value = value
             constance.save()
 
     def clear(self, sender, instance, created, **kwargs):
-        if not created:
+        if db_cache and not created:
             db_cache.clear()

constance/settings.py

@@ -17,3 +17,6 @@ CONNECTION_CLASS = getattr(settings, 'CONSTANCE_REDIS_CONNECTION_CLASS',
 
 REDIS_CONNECTION = getattr(settings, 'CONSTANCE_REDIS_CONNECTION',
                    getattr(settings, 'CONSTANCE_CONNECTION', {}))
+
+DATABASE_CACHE_BACKEND = getattr(settings, 'CONSTANCE_DATABASE_CACHE_BACKEND',
+                                 None)