Configuration Spaces

0 7 4 0 7 4 5 9 / 0 4 / $ 2 0 . 0 0 © 2 0 0 4 I E E E distributed systems is most often specified in terms of quality-of-service (QoS) requirements, which help define the acceptable levels of dependability with which capabilities such as processing capacity, data throughput, or service availability reach users. For longer-term properties such as scalability, maintainability, adaptability, and system security, we can similarly use persistent software attributes (PSAs) to specify how and to what degree such properties must remain intact as a network expands and evolves over time. In the past, a common approach to ensuring the long-term persistence of important service properties was simply to freeze all changes after a system’s properties had been sufficiently validated. However, for an increasing range of important QoS-intensive systems, this kind of development process no longer suffices. Today’s global information economy strongly encourages forms of development that bring together participants from across geographical locations, time zones, and business organizations. While such distributed development processes provide a powerful and economically important development model, they unfortunately also increase the churn rates in QoS-intensive software (see the “QoS and PSAs” sidebar). focus