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High-availability assurance of cloud service is a critical and challenging issue for cloud service providers. To quantify the availability of cloud services from both architectural and operational points of views, availability modeling and evaluation are essential. This paper presents a component-based availability modeling framework, named Candy, which constructs a comprehensive availability model semi-automatically from system specifications described by Systems Modeling Language (SysML). SysML diagrams are translated into components of availability model and the components are assembled together to form the entire availability model in Stochastic Reward Nets (SRNs). In order to incorporate the maintenance operations of cloud services in availability models, Candy defines the translation rules from Activity diagram to SRN and synchronizes the related SRNs according to SysML allocation notations. The feasibility of the proposed modeling and availability evaluation process is studied by an illustrative example of a web application service hosted on a cloud infrastructure having multiple failure isolation zones and automatic scale-up function.