diff --git a/lib/cache.php b/lib/cache.php
index ea0ff769d1..dc667654ab 100644
--- a/lib/cache.php
+++ b/lib/cache.php
@@ -86,6 +86,55 @@ class Cache
         return 'statusnet:' . $base_key . ':' . $extra;
     }
 
+    /**
+     * Create a cache key for data dependent on code
+     *
+     * For cache elements that are dependent on changes in code, this creates
+     * a more-or-less fingerprint of the current running code and adds it to
+     * the cache key. In the case of an upgrade of core, or addition or
+     * removal of plugins, a new unique fingerprint is generated and used.
+     * 
+     * There can still be problems with a) differences in versions of the 
+     * plugins and b) people running code between official versions. This is
+     * usually a problem only for experienced users like developers, who know
+     * how to clear their cache.
+     *
+     * For sites that run code between versions (like the status.net cloud),
+     * there's an additional build number configuration setting.
+     * 
+     * @param string $extra the real part of the key
+     *
+     * @return string full key
+     */
+    
+    static function codeKey($extra)
+    {
+        static $prefix = null;
+	
+        if (empty($prefix)) {
+	    
+            $plugins     = StatusNet::getActivePlugins();
+            $names       = array();
+	    
+            foreach ($plugins as $plugin) {
+                $names[] = $plugin[0];
+            }
+	    
+            $names = array_unique($names);
+            asort($names);
+	    
+            // Unique enough.
+	
+            $uniq = crc32(implode(',', $names));
+
+            $build = common_config('site', 'build');
+
+            $prefix = STATUSNET_VERSION.':'.$build.':'.$uniq;
+        }
+	
+        return Cache::key($prefix.':'.$extra);
+    }
+    
     /**
      * Make a string suitable for use as a key
      *
diff --git a/lib/default.php b/lib/default.php
index a19453fce4..6c8b039274 100644
--- a/lib/default.php
+++ b/lib/default.php
@@ -59,7 +59,8 @@ $default =
               'textlimit' => 140,
               'indent' => true,
               'use_x_sendfile' => false,
-              'notice' => null // site wide notice text
+              'notice' => null, // site wide notice text
+              'build' => 1, // build number, for code-dependent cache
               ),
         'db' =>
         array('database' => 'YOU HAVE TO SET THIS IN config.php',