Measurements of real workloads are referred to as passive measurements, because the act of measurement involves simply observing an application’s behavior under normal usage conditions and recording what happens. Active measurements, in contrast, are obtained by generating synthetic application traffic. For example, one might measure a system’s maximum capacity by emulating a mix of user actions and increasing the number of simulated users until a component of the system saturates. Note that the passive and active measurement approaches differ only in the way application traffic is generated—both still require mechanisms to measure how the system behaves in response to that traffic.
Passive measurements must capture the behavior and experience of real application users, while active measurements must do the same for synthetic users. So both approaches must deal with the same set of technical complications created by the need to measure Rich Internet Applications.
Although active measurements do impose extra traffic, they rarely distort a system’s behavior sufficiently to affect the validity of the results. For a typical e-business application, the number of additional active measurements required to sample the system and obtain useful data is usually insignificant compared with normal traffic volumes. Thus normal levels of application responsiveness and availability can be measured using either active or passive methods. Load testing on the other hand normally involves active measurement of a portion of the system that is isolated from real users for the purpose of the test.