View the variables and sequence sections. Two areas of particular interest concern the invoke and receive activities:

• An invoke activity invokes a synchronous web service (as discussed in Invoking a Synchronous Web Service from a BPEL Process) or initiates an asynchronous service.

  The invoke activity includes the request global input variable defined in the variables section. The request global input variable is used by the loan application approver web service. This variable contains the contents of the initial loan application request document.

• A receive activity that waits for the asynchronous callback from the loan application approver web service. The receive activity includes the response global output variable defined in the variables section. This variable contains the loan offer response. The receive activity asynchronously waits for a callback message from a service. While the BPEL process service component is waiting, it is dehydrated, or compressed and stored, until the callback message arrives.

The following provides an example.

  <variables>
    <variable name="request"
               messageType="services:LoanServiceRequestMessage"/>
    <variable name="response"
               messageType="services:LoanServiceResultMessage"/>
  </variables>
<sequence>
    <!-- initialize the input of LoanService -->
    <assign>
    . . .
    . . .
    </assign>
    <!--  initiate the remote process -->
    <invoke name="invoke" partnerLink="LoanService"
        portType="services:LoanService"
        operation="initiate" inputVariable="request"/>
    <!--  receive the result of the remote process -->
    <receive name="receive_invoke" partnerLink="LoanService"
        portType="services:LoanServiceCallback"
        operation="onResult" variable="response"/>

When an asynchronous service is initiated with the invoke activity, a correlation ID unique to the client request is also sent, using Web Services Addressing (WS-Addressing) (described in Using WS-Addressing in an Asynchronous Service). Because multiple processes may be waiting for service callbacks, the server must know which BPEL process service component instance is waiting for a callback message from the loan application approver web service. The correlation ID enables the server to correlate the response with the appropriate requesting instance.