SurrogateKeysCache now accepts several cache backends

- uses queue-async to parallelize the call to invalidate

lib/cartodb/cache/surrogate_keys_cache.js

@@ -1,9 +1,11 @@
+var queue = require('queue-async');
+
 /**
- * @param cacheBackend should respond to `invalidate(cacheObject, callback)` method
+ * @param {Array|Object} cacheBackends each backend backend should respond to `invalidate(cacheObject, callback)` method
  * @constructor
  */
-function SurrogateKeysCache(cacheBackend) {
-    this.cacheBackend = cacheBackend;
+function SurrogateKeysCache(cacheBackends) {
+    this.cacheBackends = Array.isArray(cacheBackends) ? cacheBackends : [cacheBackends];
 }
 
 module.exports = SurrogateKeysCache;
@@ -22,5 +24,18 @@ SurrogateKeysCache.prototype.tag = function(response, cacheObject) {
  * @param {Function} callback
  */
 SurrogateKeysCache.prototype.invalidate = function(cacheObject, callback) {
-    this.cacheBackend.invalidate(cacheObject, callback);
+    var invalidationQueue = queue(this.cacheBackends.length);
+
+    this.cacheBackends.forEach(function(cacheBackend) {
+        invalidationQueue.defer(function(cacheBackend, done) {
+            cacheBackend.invalidate(cacheObject, done);
+        }, cacheBackend);
+    });
+
+    invalidationQueue.awaitAll(function(err, result) {
+        if (err) {
+            return callback(err);
+        }
+        callback(null, result);
+    });
 };