The arrival rate is one of the most fundamental aspects of expressing what is occurring in a system-whether it's Oracle, a river, or an expressway. As you'll see in the following sections, when combined with other metrics, the arrival rate can be used to convey part of the performance situation and also provides clues about how our proposed performance solutions will affect the system.

If I were standing in front of you right now, I would have in my hands an empty glass and a pitcher of water. I would hold out the empty glass and say over and over, "capacity." Then I would hold out the pitcher and say repeatedly, "requirements." Then I would ask you, "Is the water going to fit in the glass? Are the requirements going to exceed the capacity?" In IT, what usually occurs is the water is poured in the glass, and we all look away, hoping it will fit. After a while, we start feeling the water dripping down our arm, and we have a mess. That mess is the result of the requirements exceeding the available capacity. When this occurs, we have a performance firefighting situation.

The performance management trick is to ensure the requirements will fit into the available capacity. In fact, if we can mathematically express the requirements and capacity-injecting alterations such of politics, budget, purchases, timing, and new and changing workloads-we have a much better chance of anticipating change. But if we guess at the requirements or the capacity, then everyone is just plain lucky if the solution works.