Add caching

src/currencyservice/server.js

@@ -28,7 +28,9 @@ const shopProto = grpc.load(PROTO_PATH).hipstershop;
  * Helper function that gets currency data from an XML webpage
  * Uses public data from European Central Bank
  */
+let _data;
 function _getCurrencyData (callback) {
+  if (!_data) {
     request(DATA_URL, (err, res) => {
       if (err) {
         throw new Error(`Error getting data: ${err}`);
@@ -45,9 +47,13 @@ function _getCurrencyData (callback) {
           acc[x['currency']] = x['rate'];
           return acc;
         }, { 'EUR': '1.0' });
-      callback(results);
+        _data = results;
+        callback(_data);
       });
     });
+  } else {
+    callback(_data);
+  }
 }
 
 /**