Added database cache autofill feature.

This refs #47 and #45.

README.rst

@@ -125,17 +125,9 @@ you need to install this library, too. E.g.::
 Alternatively follow the backend specific installation instructions above.
 
 The database backend has the ability to automatically cache the config
-values and clear them when saving. You need to set the following setting
+values and clear them when saving. Assuming you have a ``CACHES`` setting set
-to enable this feature::
+you need to set the the ``CONSTANCE_DATABASE_CACHE_BACKEND`` 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.
-
-Starting in Django 1.3 you can alternatively use the name of an entry of
-the ``CACHES`` setting. E.g.::
 
     CACHES = {
         'default': {
@@ -145,6 +137,14 @@ the ``CACHES`` setting. E.g.::
     }
     CONSTANCE_DATABASE_CACHE_BACKEND = 'default'
 
+.. note:: This won't work with a cache backend that doesn't support
+   cross-process caching, because correct cache invalidation
+   can't be guaranteed.
+
+.. note:: By default Constance will autofill the cache on startup and after
+   saving any of the config values. If you want to disable the cache simply
+   set the ``CONSTANCE_DATABASE_CACHE_AUTOFILL_TIMEOUT`` setting to ``None``.
+
 Just like the Redis backend you can set an optional prefix that is used during
 database interactions. To keep backward compatibility it defaults to ``''``
 (an empty string). To use something else do this::
@@ -229,6 +229,13 @@ Screenshots
 Changelog
 ---------
 
+v1.0 (unreleased)
+~~~~~~~~~~~~~~~~~
+
+* Added new autofill feature for the database backend cache which is enabled
+  by default.
+
+
 v0.6 (2013/04/12)
 ~~~~~~~~~~~~~~~~~
 

constance/__init__.py

@@ -1,7 +1,7 @@
 from constance.config import Config
 
 try:
-    from django.apps import AppConfig
+    from django.apps import AppConfig  # noqa
 except ImportError:
     config = Config()
 else:

constance/apps.py

@@ -2,6 +2,7 @@ from django.apps import AppConfig
 from constance.config import Config
 from django.utils.translation import ugettext_lazy as _
 
+
 class ConstanceConfig(AppConfig):
     name = 'constance'
     verbose_name = _('Constance')

constance/backends/database/__init__.py

@@ -10,32 +10,49 @@ except ImportError:
 from constance.backends import Backend
 from constance import settings
 
-db_cache = None
-if settings.DATABASE_CACHE_BACKEND:
-    db_cache = get_cache(settings.DATABASE_CACHE_BACKEND)
-    if isinstance(db_cache, LocMemCache):
-        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
         self._model = Constance
         self._prefix = settings.DATABASE_PREFIX
+        self._autofill_timeout = settings.DATABASE_CACHE_AUTOFILL_TIMEOUT
+        self._autofill_cachekey = 'autofilled'
+
         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.")
+
+        if settings.DATABASE_CACHE_BACKEND:
+            self._cache = get_cache(settings.DATABASE_CACHE_BACKEND)
+            if isinstance(self._cache, LocMemCache):
+                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)
+        else:
+            self._cache = None
+        self.autofill()
         # Clear simple cache.
         post_save.connect(self.clear, sender=self._model)
 
     def add_prefix(self, key):
         return "%s%s" % (self._prefix, key)
 
+    def autofill(self):
+        if not self._autofill_timeout or not self._cache:
+            return
+        full_cachekey = self.add_prefix(self._autofill_cachekey)
+        if self._cache.get(full_cachekey):
+            return
+        autofill_values = {}
+        autofill_values[full_cachekey] = 1
+        for key, value in self.mget(settings.CONFIG.keys()):
+            autofill_values[self.add_prefix(key)] = value
+        self._cache.set_many(autofill_values, timeout=self._autofill_timeout)
+
     def mget(self, keys):
         if not keys:
             return
@@ -46,17 +63,18 @@ class DatabaseBackend(Backend):
 
     def get(self, key):
         key = self.add_prefix(key)
-        value = None
+        if self._cache:
-        if db_cache:
+            value = self._cache.get(key)
-            value = db_cache.get(key)
+        else:
+            value = None
         if value is None:
             try:
                 value = self._model._default_manager.get(key=key).value
             except self._model.DoesNotExist:
                 pass
             else:
-                if db_cache:
+                if self._cache:
-                    db_cache.add(key, value)
+                    self._cache.add(key, value)
         return value
 
     def set(self, key, value):
@@ -68,5 +86,9 @@ class DatabaseBackend(Backend):
             constance.save()
 
     def clear(self, sender, instance, created, **kwargs):
-        if db_cache and not created:
+        if self._cache and not created:
-            db_cache.delete_many(self.add_prefix(k) for k in settings.CONFIG.keys())
+            keys = [self.add_prefix(k)
+                    for k in settings.CONFIG.keys()]
+            keys.append(self.add_prefix(self._autofill_cachekey))
+            self._cache.delete_many(keys)
+            self.autofill()

constance/settings.py

@@ -22,6 +22,10 @@ REDIS_CONNECTION = getattr(settings, 'CONSTANCE_REDIS_CONNECTION',
 DATABASE_CACHE_BACKEND = getattr(settings, 'CONSTANCE_DATABASE_CACHE_BACKEND',
                                  None)
 
+DATABASE_CACHE_AUTOFILL_TIMEOUT = getattr(settings,
+                                          'CONSTANCE_DATABASE_CACHE_AUTOFILL_TIMEOUT',
+                                          60 * 60 * 24)
+
 DATABASE_PREFIX = getattr(settings, 'CONSTANCE_DATABASE_PREFIX', '')
 
 SUPERUSER_ONLY = getattr(settings, 'CONSTANCE_SUPERUSER_ONLY', True)

example/cheeseshop/settings.py

@@ -116,3 +116,12 @@ CONSTANCE_CONFIG = {
 }
 
 CONSTANCE_BACKEND = 'constance.backends.database.DatabaseBackend'
+
+
+CACHES = {
+    'default': {
+        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
+        'LOCATION': '127.0.0.1:11211',
+    }
+}
+CONSTANCE_DATABASE_CACHE_BACKEND = 'default'