Leveraging Fault Tree Analysis for Safety & Reliability in Software Projects

1. NASA OSMA SAS '03 Fault Tree Analysis Application for Safety and Reliability Massood Towhidnejad Embry-Riddle University Dolores Wallace & Al Gallo NASA Goddard, SATC

2. Overview • FTA Background • SFTA and the System Life Cycle • SFTA Examples and Diagrams • Modeling System Behavior • Advantages & Disadvantages of SFTA • Impact of Our Research • Summary

3. General Hazard Analysis technique started in the 1960s Attributes: Graphical Top Down Analytical Qualitative Quantitative Goal:To identify all conditions that put system in a Hazardous States FTA applied to software Little work has been done to date SFTA focuses on the code and requirements Generally applied to “small” projects (<2000 LOC) Observations: SFTA impractical at code level Should be applied to Systems at the early stages of life cycle Need to address the quantitative analysis Fault Tree Analysis

4. Requirements Phase Design Phase . . . Coding & Test Highlight requirements for Safety concerns & hazards Perform analysis on elements of the design (i.e., Activity, Sequence, and State diagram) Adjust design to eliminate /mitigate hazardous states . .. Increase reviews and walkthroughs effectiveness Applied only to critical code Adjust design to eliminate /mitigate hazardous states SFTA Applied to System Life Cycle

5. SFTA Road Map Use fault tree diagrams as a graphical communication vehicle for developers, testers, designers, managers and customers Requirements, Design, Code and Test Fault Tree Verify and Modify Verify/add missing Customer/Domain expert Software Engineer

6. User modifies profile Display security screen User enter securitydata Update priority: Update name: System validatePermission Update Address: Security access denied System validate entry System update profile Fault Tree Example(Activity Diagram) Yes Try > 3 Valid Access No No Reenter Yes Mod. Accept Yes Try <=3 No

7. Fault Tree Example(Activity Diagram)

8. Fault Tree Example(Sequence Diagram)

9. Fault Tree Example(Sequence Diagram)

10. Fault Tree Example(Sequence Diagram)

11. Fault Tree Example(State Diagram) Source: Smart Draw

12. Fault Tree Example(State Diagram)

13. Reliability S/W H/W t = release 0 Modeling System Behavior • Hardware • Large sample size • Large collections of historic data • Classification of failure types • Degradation (increase failure rate) • Software • Limited sample size (usually one) • Limited availability of software failure data • Classification of cause more relevant • Improvement!!!! (decrease failure rate) • Probability values not available, though subject of research efforts Time

14. Advantages: Easy to learn Graphical Representation Communication vehicle with customer Partial automatic conversion possible (but not desirable) Disadvantages: Conversion is labor intensive Automatic conversion is not attractive Lack of software reliability data Timing and Loops need special attention No dedicated SFTA tool Assessing SFTA

15. Impact of Our Research • Technology Transfer, Infusion, Recognition & Commercialization • SATC collaborating with commercial vendor of commercial FTA tool • Enhancing product to accommodate software uniquenesses • Planning to build in paradigm features • GSFC Center Director and SMA Director tracking through center’s Tech Transfer Office • SFTA activity and a safety-related tool • Assisting in the collaboration / licensing

16. Summary • Applied Fault Tree (FT) to Object Oriented design • In addition to fault detection, FT can serve as a communication medium with customer • Lack of reliability data reduces the quantitative analysis of the FT • Seeking alternative sources of data for quantitative analysis • Collaborating with a FT tool vendor to develop software fault tree tool