CalcService.java

public interface CalcService extends RespondingService {

	void add(final CallbackMessageController<CalcProto.Results> messageController, final CalcProto.Add add);

}

DefaultCalcService.java

public class DefaultCalcService implements CalcService {

	public void add(final CallbackMessageController<CalcProto.Results> messageController, final CalcProto.Add add) {
		if (add.hasFirst() && add.hasSecond()) {
			messageController.complete(CalcProto.Results.newBuilder().setValue(add.getFirst() + add.getSecond()).build());
			return;
		}
		messageController.fail(ServiceProto.Error.newBuilder().setCode(CalcErrors.MISSING_PARAMETERS.code()).build());
	}

}

FooThingDoer.java

public class FooThingDoer {

	public void stuff(final ServiceRef serviceRef) {
		final CalcService calcService = serviceRegistry.getService(serviceRef);
		final Callback<CalcProto.Results> callback = createCallback();
		final CallbackMessageController<CalcProto.Results> messageController = createMessageController();
		calcService.add(messageController, CalcProto.Add.newBuilder().setFirst(1).setSecond(1).build());
		try {
			if (messageController.isFinished() && messageController.hasCallbackValue()) {
				final CalcProto.Results results = messageController.getValue();
				// ... do stuff with results
			}
		} catch (final Exception e) {
			// ...
		}
	}

}

RespondingService.java

public interface RespondingService {

	ServiceProto.ServiceRef getServiceRef();

	List<Integer> getFlags();

	void setFlag(final int flag);

	void removeFlag(final int flag);

}