The graph shown in Figure 1-3 is based on a queuing theory model.14 So it's not real; it's an abstraction. Service time is never perfectly horizontal, and queue time does not occur exactly as we plan in real systems. All those Oracle optimizations and messy workload issues muddy up things a bit. However, transactions and computing systems do behave in a queuing-like manner. They must-it's their nature. I've been asked how I know a system will respond this way. The answer is always the same. That's how transactions behave, and so do humans when we become a transaction and enter a queuing system, such as the one at McDonald's.

When you perform ORTA, you construct a response time curve graph, or simply numerically detail its components (service time and queue time), based on what occurred during an interval of time. For example, Figure 1-3 could represent Monday morning between 9 a.m. and 10 a.m.

So far, this discussion has been fairly academic. The following case study introduces how to perform an ORTA. I hope you will begin to see that this type of analysis is both straightforward and very useful.