Asynchronous messaging styles are useful for environments in which a service, such as a loan processor, can take a long time to process a client request. Asynchronous services also provide a more reliable fault-tolerant and scalable architecture than synchronous services.

This section introduces asynchronous web service invocation with a company called United Loan. United Loan publishes an asynchronous web service that processes a client's loan application request and then returns a loan offer. This use case discusses how to integrate a BPEL process service component with this asynchronous loan application approver web service.

This use case illustrates the key design concepts for requesting information from an asynchronous service, and then receiving the response. The asynchronous United Loan service in this example is another BPEL process service component. However, the same BPEL call can interact with any properly designed web service. The target web service WSDL file contains the information necessary to request and receive the necessary information.

For the asynchronous web service, the following actions take place (in order of priority):

1. An assign activity prepares the loan application.

2. An invoke activity initiates the loan request. The contents of this request are put into a request variable. This request variable is sent to the asynchronous loan processor web service.

   When the loan request is initiated, a correlation ID unique to the client and partner link initiating the request is also sent to the loan processor web service. The correlation ID ensures that the correct loan offer response is returned to the corresponding loan application requester.

3. The loan processor web service then sends the correct response to the receive activity, which has been tracked by the correlation ID.

4. An assign activity reads the loan application offer.

Subsequent sections in this chapter provide specific details about the asynchronous functionality.