An actual SCG（service candidate graph） model-based optimal service selection approach was proposed.Firstly

the approach created transactional constraint relationships among candidates as an actual SCG model with several build-ing rules

whose correctness had been also proved;Then

an optimal QoS-aware service selection algorithm on the basis of BFS（breadth first search） was designed

where a relaxing and pruming method was applied to keep the computation scale in polynomial time.Finally

simulation experiments were conducted with real-world QoS dataset and random data-set

whose results demonstrated the beneficial performance on global QoS utility and outstanding successful selecting ra-tio over other related work.Meanwhile

the correctnesss has also been proved in practice by implementing a transactional automation.