BPEL's communication activities (invoke, receive, reply, and onMessage) receive and send messages through specified message variables. These default activities permit one variable to operate in each direction. For example, the invoke activity has inputVariable and outputVariable attributes. You can specify one variable for each of the two attributes. This is enough if the particular operation involved uses only one payload message in each direction.

However, WSDL supports multiple messages in an operation. In the case of SOAP, multiple messages can be sent along the main payload message as SOAP headers. However, BPEL's default communication activities cannot accommodate the additional header messages.

Oracle BPEL Process Manager solves this problem by extending the default BPEL communication activities with the bpelx:headerVariable extension. The extension syntax is as shown in the following example:

<invoke bpelx:inputHeaderVariable="inHeader1 inHeader2 ..."
  bpelx:outputHeaderVariable="outHeader1 outHeader2 ..."
  .../>

<receive bpelx:headerVariable="inHeader1 inHeader2 ..." .../>
<onMessage bpelx:headerVariable="inHeader1 inHeader2 ..." .../>
<reply bpelx:headerVariable="inHeader1 inHeader2 ..." .../>