diff --git a/README b/README
index 6343e3e024..3bebb11ee0 100644
--- a/README
+++ b/README
@@ -1538,6 +1538,18 @@ external: external links in notices. One of three values: 'sometimes',
     nofollowed on profile, notice, and favorites page. Default is
     'sometimes'.
 
+router
+------
+
+We use a router class for mapping URLs to code. This section controls
+how that router works.
+
+cache: whether to cache the router in memcache (or another caching
+    mechanism). Defaults to true, but may be set to false for
+    developers (who might be actively adding pages, so won't want the
+    router cached) or others who see strange behavior. You're unlikely
+    to need this unless you're a developer.
+
 Plugins
 =======
 
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 a91fa338fc..029dbb3906 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',
@@ -327,4 +328,6 @@ $default =
         array('ssl_cafile' => false, // To enable SSL cert validation, point to a CA bundle (eg '/usr/lib/ssl/certs/ca-certificates.crt')
               'curl' => false, // Use CURL backend for HTTP fetches if available. (If not, PHP's socket streams will be used.)
               ),
+	'router' =>
+	array('cache' => true), // whether to cache the router object. Defaults to true, turn off for devel
         );
diff --git a/lib/router.php b/lib/router.php
index 8670519ff4..eff88bb462 100644
--- a/lib/router.php
+++ b/lib/router.php
@@ -127,15 +127,19 @@ class Router
     function __construct()
     {
         if (empty($this->m)) {
-            $k = self::cacheKey();
-            $c = Cache::instance();
-            $m = $c->get($k);
-            if (!empty($m)) {
-                $this->m = $m;
-            } else {
+	    if (!common_config('router', 'cache')) {
                 $this->m = $this->initialize();
-                $c->set($k, $this->m);
-            }
+	    } else {
+		$k = self::cacheKey();
+		$c = Cache::instance();
+		$m = $c->get($k);
+		if (!empty($m)) {
+		    $this->m = $m;
+		} else {
+		    $this->m = $this->initialize();
+		    $c->set($k, $this->m);
+		}
+	    }
         }
     }
 
@@ -156,21 +160,7 @@ class Router
     
     static function cacheKey()
     {
-        $plugins     = StatusNet::getActivePlugins();
-        $names       = array();
-	
-        foreach ($plugins as $plugin) {
-            $names[] = $plugin[0];
-        }
-
-        $names = array_unique($names);
-        asort($names);
-
-        // Unique enough.
-	
-        $uniq = crc32(implode(',', $names));
-	
-        return Cache::key('router:'.STATUSNET_VERSION.':'.$uniq);
+        return Cache::codeKey('router');
     }
     
     function initialize()