  void createCallback(v8::Isolate* isolate, v8::Handle<v8::Function> callback) {
    WRITE_LOCKER(writeLocker, _callbacksLock);

    auto it = _callbacks.find(isolate);

    if (it == _callbacks.end()) {
      _callbacks[isolate].Reset(isolate, callback);
    } else {
      LOG_TOPIC("982a6", ERR, Logger::V8)
          << "cannot recreate callback for '" << _url << "'";
    }
  }
  void createCallback(v8::Isolate* isolate, v8::Handle<v8::Function> callback) {
    WRITE_LOCKER(writeLocker, _callbacksLock);

    auto it = _callbacks.find(isolate);

    if (it == _callbacks.end()) {
      _callbacks[isolate].Reset(isolate, callback);
    } else {
      LOG_TOPIC("982a6", ERR, Logger::V8)
          << "cannot recreate callback for '" << _url << "'";
    }
  }