Transaction processing systems are judged by users to be correctlyfunctioning not only if their transactions are executed correctly, butalso if most of them are completed within an acceptable time limit.Therefore, we propose a definition of availability for systems for whomthere is a notion of system failure due to frequent violation ofresponse time constraints. We define the system to be available at acertain time if at that time the fraction of transactions meeting adeadline is above a certain user requirement. This definition lends tovery different estimates of availability measures such as systemdowntimes as compared with more traditional measures. We conclude thatfor transaction processing systems, where the user's perception isimportant, our definition more correctly quantifies the availability ofthe system
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