1 Introduction
Cloud workflow systems are a class of workflow management systems that are deployed in various cloud computing environments. Cloud workflow systems can be widely used as platform software (or middleware services) to facilitate the usage of cloud services. Every tenant can design, configure and run his/her business processes on it [1], [2]. With the increase of tenants’ applications, the underlying cloud workflow systems accumulate a large number of process descriptions (i.e., business process models) [3]. For example, there are more than 2,000 enterprise users (namely tenants) and 100,000 business processes (roughly 50 business processes per tenant) in the cloud workflow system of Ceramic Cloud Service Platform (CCSP for short, http://www.pasp.cn). So how to efficiently and exactly query large process model repositories in a cloud workflow system is challenging.