This paper proposes a semantic model for web service based on Dynamic Description Logic (DDL).In this paper

service composition is divided into two stages:logic plan stage and grounding stage.Therefore

a service composition problem is reduced to an action planning problem at the planning stage and a QoS-aware service selection problem at the grounding stage.This paper mainly focuses on the QoS model and service selection.A composite service is divided to several smaller services

which are then solved separately by a recursive branch-and-bound algorithm.Experiments show that the proposed approach is efficient and scalable for runtime service selection

and better handles complicated structure of service composition than existing approaches.