When the BPEL process service component is on the client side of this transaction, it needs a scope activity containing the invoke activity to send the request, and a receive activity to accept the mandatory reply. The onMessage handler of the scope activity is set to accept the optional message and instructions on what to do if the optional message is received (for example, notify you that the product has been delayed). The client BPEL process service component waits to receive the mandatory reply. If the mandatory reply is received first, the BPEL process service component continues without waiting for the optional reply. As with all partner activities, the WSDL file defines the interaction.