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Interaction with Provider

Once the provider has been selected the requester should initiate the transaction with it. To support the interaction the provider has to make public its own interaction protocol declaring what information it expects from the requester, in what order, the format of such information and binding information that specify the ports the provider and the requester will use during the transaction. Still, the declaration of the sequence of messages is not enough for the provider. Rather it needs also to declare the material consequences of each step in the protocol.

Current Web Services standards address different parts of the problem. Protocols like XLang [15], WSFL [10] and more recently BPEL4WS [3] address the problem of describing the temporal part of the interaction protocol by specifying workflow models that describe the sequence of messages to be exchanged. WSDL [2] maps descriptions of abstract information to be exchanged by Web Services into message formats and binding that specify where the message is delivered and the transmission protocol.

The specification of the interact protocol is not really enough to allow a successful transaction, rather the requester should derive from the interaction protocol what the provider does with the information it receives. The discussion about commitments in the previous section provides an argument in favor of this requirement: in order to compute its commitments the requester should understand what are the consequences of the steps in the protocol, so the requester will avoid expensive commitments that it does not need. Expressing the consequences of the transaction is not only a problem for parallel interaction with multiple Web Services, rather it is a problem even when there is a single provider and a single requester. For example, a requester will want to make sure that the goods will be delivered after it provides payment information, or that the money will be refunded. These types of inferences are impossible unless the provider specifies also what are the consequences of the message exchange with the requester.

next up previous
Next: Implementing Web Services with Up: Composition Cycle Previous: Selection of Provider
Massimo Paolucci 2003-02-28