Abstract: A probe mechanism detects failed software components in a running software system. The probe mechanism is based on requesting service, or a certain level of service, from a set of functions, modules and/or subsystems and checking the response to the request. The probe is directed towards a service rendered by a collection of software modules and functions which is termed a target. The probe will then label a target as either healthy or failed. The objective is to localize the failure only up to the level of a target, however, and achieve a high degree of efficiency and confidence in the process. Targets are chosen such that they represent a collection of functions that can be defined by a service level input/output (I/O) specification. Targets can be identified at different levels or layers in the software. The choice of a level is based on the granularity of fault detection that is desired, taken in consideration with the level at which recovery can be implemented.