Partner Links Are Services and References in Java

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Partner Links Are Services and References
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So, what is the difference between a client and a service provider With bidirectional interfaces, both roles provide services that can be used by the other role The only difference is that the client sends the rst message of the conversation The direction of that rst message provides the asymmetry, and then after that, there are no constraints on the messages that might be sent between the two roles In BPEL, even that distinction is not made It is theoretically possible that a partner link type could be created where either role could initiate the conversation In that case, there would be no asymmetry at all However, although that may make the model more elegant and theoretically more powerful, in fact, almost every conversational service is designed where it is known in advance which role will send the rst message The distinction between a client and a service provider exists in the mind of the developer, so it makes sense to recognize it in the programming model Static Control Flow Analysis with SCA BPEL When you use a BPEL process de nition as the implementation of a component, SCA needs to be able to tell which of the partner links should be represented as services and which should be represented as references If the partner link type has only one role, the choice is easy: The partner links with the one role as myRole are services and the partner links with the one role as partnerRole are references When the partner link type has two roles (that is, it is bidirectional), SCA determines which are services and which are references by depending on the fundamental asymmetry between clients and service providers described previously gure out which role will send the rst message of the conversation The SCA BPEL speci cation says that this is accomplished by static control ow analysis of the BPEL process Partner links are used for either inbound or outbound communication Inbound communication occurs as either a receive, onMessage, or pick activity Outbound communication is done with either an invoke or a reply activity The order in which the activities can occur is constrained by the sequence of activities they
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SCA determines which are services and which are references by depending on the fundamental asymmetry between clients and service providers
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are in and by any links present in the process The SCA processor can analyze each use of a partner link and determine whether the rst activity for the partner link is inbound or outbound communication If it is inbound (for example, receive), the partner link is turned into a service If it is outbound (for example, invoke), it is turned into a reference According to SCA, the static analysis is not allowed to try to guess the path taken through any condition (Although in most cases, it would be impossible anyway, but the speci cation disallows it so that different processors don t come up with different answers) In the rare case that it can t be determined which will occur rst, the partner link is turned into a reference This can happen when a receive and an invoke activity for the same partner link are in the same ow, and there is no link that causes one of the two to occur before the other Usually, the programmer does not need to think about any of this The partner links that the developer thinks of as services become services, and the partner links that the developer thinks of as references become references Partner Link Types as Interfaces In SCA, a bidirectional interface is de ned in Java with an @Callback annotation, as we saw back in 3, ServiceBased Development Using Java (see Listing 102)
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