Of all of the metrics that are generated by a service at all layers, end-user experience is a single metric that serves as an overall indicator of service health. If the end-user experience falls below a given threshold as mandated by an SLA, there is not sufficient capacity in the service to deliver the required SLA, and capacity must be added.

This approach monitors the service and uses this measure as a trigger for the scaling out and scaling back of dynamic infrastructure. Whenever the end-user experience falls below a threshold, capacity is added, and as the measure increases above the threshold, capacity is decreased. Increasing and decreasing capacity are equally important. You do not want to over-spend on infrastructure to provide a service that exceeds your SLA beyond where it provides business value based on the cost.

The drawback of using only an end-user monitoring approach is that it only indicates that there is a problem with the performance for end users. It does not provide the system any information as to where the problem is or what is causing the problem. To create a truly intelligent dynamic IaaS service, you must consider end-user experience as the key performance indicator (KPI), but infrastructure and application metrics provide the causal analysis data.