Availability analysis of transaction processing systems based on user-perceived performance

Transaction processing systems are judged by users to be correctly functioning not only if their transactions are executed correctly, but also if most of them are completed within an acceptable time limit. Therefore, we propose a definition of availability for systems for whom there is a notion of system failure due to frequent violation of response time constraints. We define the system to be available at a certain time if at that time the fraction of transactions meeting a deadline is above a certain user requirement. This definition lends to very different estimates of availability measures such as system downtimes as compared with more traditional measures. We conclude that for transaction processing systems, where the user's perception is important, our definition more correctly quantifies the availability of the system.