In this paper, considering the Web service composition model described by DAG (Directed Acrylic Graph), a new Quality of Service ( QoS) computing method for the composition service based on topological sequence reduction (QCMTSR) was proposed. Based on the basic structures and their QoS computing formulas of iterative reduction method two kinds of basic structures (I. E. serial reduction structure and parallel reduction structure) were defined in graph DAG, and their QoS calculation formulas were also given. During accessing each node step by step in the topology sequence for DAG. Repeating this process until the last node in this queue, then the QoS measure results of the last node were the computing results of the composition service. It has been proved that the algorithm can be applied to all the composition services described by DAG, and the experimental results show that the algorithm QCMTSR is more accurate in the measurement of reliability and availability.%考虑有向无环图(DAG)描述的组合服务模型,提出了一种新的组合服务QoS度量方法——基于拓扑序列归约的Web服务QoS度量方法(QCMTSR).其借鉴迭代归约度量方法中的基本结构及Q6计算公式,定义了DAG图中的两类基本结构,串归约结构和并归约结构,并给出了两种基本结构的QoS属性计算公式；通过逐步归约DAG图拓扑序列中的每个节点,直至最后一个节点的QoS属性值就是组合服务的各QoS属性的度量结果.从理论上证明了QCMTSR算法适用于所有DAG描述的组合服务,并实验证明QCMTSR算法对可靠性和可用性能够更准确的度量.
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