@Override
    public void postProcessRequest(RestRequest restRequest, Callback<Void> callback) {
      if (!isOpen) {
        throw new IllegalStateException("SecurityService closed");
      }
      completeOperation(callback, mode == Mode.PostProcessRequest);
    }
Defines the API in which {@link #exceptionToThrow} and {@link #exceptionToReturn} will work.
ambry-frontend/src/test/java/com/github/ambry/frontend/FrontendRestRequestServiceTest.java