protected function writeAppConfig($app_id) {

    $project_yml = $this->getConfigValue('blt.config-files.project');
    $this->say("Updating ${project_yml}...");
    $project_config = YamlMunge::parseFile($project_yml);
    $project_config['cloud']['appId'] = $app_id;
    try {
      YamlMunge::writeFile($project_yml, $project_config);
    }
    catch (\Exception $e) {
      throw new BltException("Unable to update $project_yml.");
    }
  }