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

Taking this approach, we can monitor the service and use this measure as a trigger for the scaling out and back of our dynamic infrastructure. Whenever the end-user experience falls below a threshold, capacity is added, and as the measure increases above our threshold we decrease capacity. Increasing and decreasing capacity are equally important. We do not want to over-spend on infrastructure to provide a service that exceeds our SLA beyond where it provides business value based on the cost.

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