Web Service Composition with O'Grape and Osiris

摘要

Services are well known building, blocks in modern information systems. Technologies and standards like XML, SOAP (Simple Object Access Protocol) [8], and WSDL (Web Service Description Language) [10] provide a simple means to describe these services and to make them accessible to a large community in a distributed environment. Yet, the full potential of web services becomes only apparent if we can combine several service invocations in a well-defined order and with well-designed execution guarantees to establish even more powerful composite services. Among others, processes provide a simple mechanism to compose services [1]. A process defines the logical dependencies between independent services by specifying an invocation order (control flow) as well as rules for the transfer of data items between different invocations (data flow). In addition and following the model of trans-actional processes [7], we can define the transactional behavior and execution guarantees to ensure a correct execution of processes in case of concurrency and failures. An infrastructure for transactional processes has to support all these run-time features. Furthermore, a graphical process modeling tool should support the specification of all these features. An important aspect is that such a modeling tool is transparently integrated into the process management environment. In addition to the transactional semantics of service composition, several other run-time aspects are crucial. Usually, several semantically equivalent web services are available at different places. An infrastructure for process execution should equally distribute the load over all web service providers. Similarly, process executions should take costs and expected execution times into account to optimize response times. To this end, the infrastructure should bind services dynamically at run time instead of at hard-coding this binding at process definition time. This decoupling provides a high degree of flexibility since new providers and services are seamlessly integrated (following the ideas of autonomic computing [9]). With the Osiris (Open Service Infrastructure for Reliable and Integrated process Support) prototype infrastructure for the execution of transactional processes and the O'Grape (Osiris GRAphical Process Editor) modeling tool, the aspects of modelling and execution of transactional processes can be seamlessly combined. O'Grape allows to graphically specify processes by combining web services. This includes both control and data flow aspects as well as transactional semantics. An application developer can then export new/revised processes from O'Grape to the Osiris system, where they can be executed. Osiris combines the advantages of several infrastructures, namely: ⅰ.) discovery and invocation of web services from frameworks like .NET [3], ⅱ.) process support and execution guarantees from workflow management systems, ⅲ.) late binding of service calls and load balancing from grid infrastructures [2], and, finally, ⅳ.) Peer-to-Peer systems. With the peer-to-peer based execution of processes, navigation costs can be accumulated only on nodes that are directly involved in the execution of a process. In the following, we first give a brief overview of Osiris in Section 2. Section 3 then describes O'Grape, our modeling tool. Finally, Section 4 enumerates the subjects of our demonstration.