how to maintain State in Biztalk orchestration

 I have a scenerio where I have to maintain state in the Orchestration like if the client sends a request to change PO and it hass the long running transaction it can take a day or many to complete the request, and without completing the transaction the client again sends a request to change the same PO biztalk should tell him that the previous process is still running and not completed. I have to implement this logic how can I do that.

Thanks