Fault, Error and Failure

Failure = delivered service is no longer complies with specification (agreed description of the system expected function and service)

Error = part of the system state that is liable to lead to subsequent failure

Fault = adjudged or hypothesized cause of an error

Failures are detected by observers or users of system.  Failures are dependent upon the definition of agreed-upon correct operation of the system.  If there is no specification of what a system should do, there could not be a failure.  The same failure can be resulted from different errors.

Error is the incorrect system behanvior from which a faulure may occur.  Error can be categorized into 2 types.  Error that manifest as value error might be incorrect discrete values or incorrect system state.  Timing error can include total non-performance or race condition.  Errors can be detected before they become failures.  Error is a manifestion of fault.  Error is the way that we can look into the system to discover if fault is present. 

Fault is the defect that in system that can cause error.  Neither the software or observer are aware of the presence of fault until an error occurs.  Latent fault is fault that is lying dormant and has not cause any error.  A latent fault becomes an active fault when the circumstances arise.