code: public StorageService getStorageService(ApplicationDefinition appDef) {
          checkServiceState();
          String ssName = getStorageServiceOption(appDef);
          StorageService storageService = DoradusServer.instance().findStorageService(ssName);
          Utils.require(storageService != null, "StorageService is unknown or hasn't been initialized: " + ssName);
          return storageService;
      }
docs: Examine the given application's StorageService option and return the corresponding
  {@link StorageService}. An error is thrown if the storage service is unknown or has
  not been initialized.
  
  @param   appDef  {@link ApplicationDefinition} of an application.
  @return          The application's assigned {@link StorageService}.