IT systems have become increasingly complex and contain many thousands or even millions of interacting software and hardware components. Their reach is as encompassing as the air traffic control system for an entire country or an e-commerce system .
System designers and analysts often do not take into account QoS requirements when designing and/or analyzing IT systems. A primary reason for this is a simple lack of awareness about the issues and of the available techniques to consider performance related issues. This text helps to provide a framework to reason about performance.
In this chapter we introduced several properties and metrics used to assess the quality of IT systems. Such metrics include response time, throughput, availability, reliability, security, scalability, and extensibility. We also discussed the various phases of the life cycle of a computer system and showed the importance of addressing QoS issues early on in the design stage as opposed to after the system is deployed.
The remaining chapters of this book provide a model-based framework that can be used by system designers to reason about performance and scalability considerations during the various phases of the life cycle of an IT system. Part I of the book deals with the use of such models while Part II presents the underlying formulas and algorithms needed to solve the models. Most of the formulas and algorithms are provided as MS Excel workbooks that accompany the book. Therefore, those readers interested in how to use the models and not in the theory behind them can skip Part II entirely.