Abstract: System failures of the component-based software systems are mainly caused by component failures and component interface failures. The two kinds of failures and their repairs were seldom considered simultaneously in the previous studies of software system availability. System availability is analyzed. The system states are described by the states of the components, the failures and their repairs by component state transitions, and the system state transitions by continuous time Markov chain(CTMC). Thus the two kinds of failures and their repairs are considered at the same time. Finally a case study is discussed to compare the effects of the two kinds of failure rates o the software system availability.

Key words: component-based software system, availability, continuous time Markov chain