Robustness

                Robustness is the ability of software systems to react appropriately to abnormal conditions.Robustness complements correctness.Correctness addresses the behavior of a system incases covered by its specification; robustness characterizes what happens outside of that specification.As reflected by the wording of its definition, robustness is by nature a more fuzzy notion than correctness. 

                       Since we are concerned here with cases not covered by the specification, it is not possible to say, as with correctness, that the system should “perform its tasks” in such a case; were these tasks known, the abnormal case would become part of the specification and we would be back in the province of correctness. This definition of “abnormal case” will be useful again when we study exception handling.

                   It implies that the notions of normal and abnormal case are always relative to a certain specification; an abnormal case is simply a case that is not covered by the specification. If you widen the specification, cases that used to be abnormal become normal even if they correspond to events such as erroneous user input that you would prefer not to happen. “Normal” in this sense does not mean “desirable”, but simply “planned for in the design of the software”.

                Although it may seem paradoxical at first that erroneous input should be called a normal case, any other approach would have to rely on subjective criteria, and so would be useless.There will always be cases that the specification does not explicitly address. The role of the robustness requirement is to make sure that if such cases do arise, the system does not cause catastrophic events; it should produce appropriate error messages, terminate its execution cleanly, or enter a so-called “graceful degradation” mode.