1 Chapter 5 Software Reliability Software Engineering: Design, Reliability, and Management, International Std. Edition by Martin L. Shooman

2 Failure Modes Mode of Failure: The general reason for a failure, e.g., poor quality of fabrication, design error, overload of the component, wear due to old age (wear-out). Hardware Failure: It refers to a system failure traceable to some component malfunction. Software Failure: It refers to a system failure traceable to some error in the software. Failure Mechanism: The detailed cause of a failure. (See Table-5.1)

3 Reliability Theory

4

5

6

7 Failure Rate

8

9

10 General form of Failure Curves t1t1 t2t2 t1t1 t2t2 Time t f(t)

11 Software Reliability & Availability Software Reliability R(t): Software Reliability is the probability that the program performs successfully, according to specifications, for a given time period. Software Availability : Software Availability is the probability that the program is performing successfully, according to specifications, at a given point in time. The important difference is that reliability means no failure in the interval 0 to t, whereas availability means only that the system is up at time t.

12 New and Old Errors Previously Fixed(Old) Error: One which recurs in substantially the same form after the programmer has terminated the work on a code change believing that the error has been corrected. Generated(New) Error: One which does not exist until it is created as a by product of a code change made to correct another error.

13 Reliability Model A Bug-Proportional (Macro) Model: (Page No.: ) (Self)