Polled monitoring and stream monitoring are both approaches to monitoring the service state. The following figure shows a typical monitoring process.

 

The observable service state is critical in implementing an effective, dynamic IaaS architecture. Observability is related to the possibility of observing, through measurement, the state of the service. If we don’t have a way of understanding what the service is providing to the end users, we cannot dynamically react to that state. The fidelity of the monitoring is important, as monitoring provides the information that makes it possible for the system to respond.