What protocol is used by a BPEL server to coordinate interactions with asynchronous Web services, how is the asynchronous call carried out and how is it supported by WSDL?
A BPEL server implements the BPEL (1.1) specification which includes support for invoking Web services asynchronously as well as receiving such calls from Web services that are external to the process. WSDL supports asynchronous calls by declaring a partnerLinkType that defines one or two roles, each with a portType that supports the interaction. Normally, one portType will be for the service provider while the other will be a callback on the client to the service.
Support for such asynchronous exchanges of messages (aka multi-step conversations), between a BPEL process and a Web service peer (aka partner), requires the use of message correlation. Correlation can be either implicit - by using WS-Addressing header information (i.e. reply-to address and correlation ID) in the SOAP message, or explicit - by using a set of pre-defined correlator fields in the request as well as response messages.
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