CS 501: Software Engineering

1. CS 501: Software Engineering Lecture 20 Reliability 2

2. Administration Projects Four weeks to the end of the semester. Leave time for system testing and to make small changes discovered when the complete system is assembled.

3. Quiz 3 Two online retailers, A and B, have agreed to merge. They have separate computer systems. Each has separately developed (i) a product database with information about products, suppliers and prices, (ii) a customer database of customer accounts, and (iii) an ordering system for online customers. You are a member of the team that will combine the two systems into a single system. The strategy chosen is to combine the two product databases into a single database, based on the software developed by retailer A, but to keep the customer databases separate. During Phase 1, the two ordering systems will be kept separate with minimal changes. In Phase 2, the two ordering systems will be replaced by a new ordering system that combines features from the systems provided by both A and B.

4. Quiz 3 Question 1 i Draw a UML interface diagram for the system architecture for Phase 1. ii Discuss each component in your system architecture. Is it: (a) an existing component from retailer A or B, or (b) an existing component with modifications, or (c) a new component? For existing components, what modifications will be needed? iii List the principal risks in developing Phase 1.

5. Quiz 3 Question 1 OrderA OrderB DBInt CustomerDbA ProductDb CustomerDbB

6. Quiz 3 Question 2 Changes may be needed to the product data base and the ordering systems for Phase 1. Careful design during Phase 1 will help the subsequent development of a new ordering system in Phase 2 i Select a design pattern that applies to this problem. Why is this design pattern appropriate? ii Draw a UML class diagram for Phase 1. iii If your class diagram uses any of the following, explain their functions: (a) abstract classes (b) inheritance (c) delegation

7. OrderBInterface prodRequestB() Adapter prodRequestB() ProdDbA prodRequestA() Quiz 3 Question 2 OrderA OrderB CustDbB CustDbA

8. OrderBInterface prodRequestB() Adapter prodRequestB() ProdDbA prodRequestA() Quiz 3 Question 2: Retailer A OrderA OrderB CustDbB CustDbA

9. ClientInterface request() Adapter request() LegacyClass existingRequest() Adapter Design Pattern: Class Diagram Client abstract class delegation inheritance

10. OrderBInterface prodRequestB() Adapter prodRequestB() ProdDbA prodRequestA() Quiz 3 Question 2:Adapter Design Pattern OrderA OrderB delegation CustDbB CustDbA

11. Static and Dynamic Verification Static verification: Techniques of verification that do not include execution of the software. • May be manual or use computer tools. Dynamic verification: • Testing the software with trial data. • Debugging to remove errors.

12. Static Validation & Verification Carried out throughout the software development process. Validation & verification Requirements specification Program Design REVIEWS

13. Static Verification: Program Inspections Formal program reviews whose objective is to detect faults • Code may be read or reviewed line by line. • 150 to 250 lines of code in 2 hour meeting. • Use checklist of common errors. • Requires team commitment, e.g., trained leaders So effective that it is claimed that it can replace unit testing

14. Inspection Checklist: Common Errors Data faults: Initialization, constants, array bounds, character strings Control faults: Conditions, loop termination, compound statements, case statements Input/output faults: All inputs used; all outputs assigned a value Interface faults: Parameter numbers, types, and order; structures and shared memory Storage management faults: Modification of links, allocation and de-allocation of memory Exceptions: Possible errors, error handlers

15. Static Analysis Tools Program analyzers scan the source of a program for possible faults and anomalies (e.g., Lint for C programs). • Control flow: loops with multiple exit or entry points • Data use: Undeclared or uninitialized variables, unused variables, multiple assignments, array bounds • Interface faults: Parameter mismatches, non-use of functions results, uncalled procedures • Storage management: Unassigned pointers, pointer arithmetic

16. Static Analysis Tools (continued) Static analysis tools • Cross-reference table: Shows every use of a variable, procedure, object, etc. • Information flow analysis: Identifies input variables on which an output depends. • Path analysis: Identifies all possible paths through the program.

17. Failures and Faults Failure: Software does not deliver the service expected by the user (e.g., mistake in requirements, confusing user interface) Fault (BUG): Programming or design error whereby the delivered system does not conform to specification (e.g., coding error, interface error)

18. Faults and Failures? Actual examples (a) A mathematical function loops for ever from rounding error. (b) A distributed system hangs because of a concurrency problem. (c) After a network is hit by lightning, it crashes on restart. (d) A program dies because the programmer typed: x = 1 instead of x == 1. (e) The head of an organization is paid $5 a month instead of $10,005 because the maximum salary allowed by the program is $10,000. (f) An operating system fails because of a page-boundary error in the firmware.

19. Terminology Fault avoidance Build systems with the objective of creating fault-free (bug-free) software Fault tolerance Build systems that continue to operate when faults (bugs) occur Fault detection (testing and validation) Detect faults (bugs) before the system is put into operation.

20. Fault Avoidance Software development process that aims to develop zero-defect software. • Formal specification • Incremental development with customer input • Constrained programming options • Static verification • Statistical testing It is always better to prevent defects than to remove them later. Example: The four color problem.

21. Defensive Programming Murphy's Law: If anything can go wrong, it will. Defensive Programming: • Redundant code is incorporated to check system state after modifications. • Implicit assumptions are tested explicitly. • Risky programming constructs are avoided.

22. Defensive Programming: Error Avoidance Risky programming constructs • Pointers • Dynamic memory allocation • Floating-point numbers • Parallelism • Recursion • Interrupts All are valuable in certain circumstances, but should be used with discretion

23. Defensive Programming Examples • Use boolean variable notinteger • Test i <= nnot i = = n • Assertion checking (e.g., validate parameters) • Build debugging code into program with a switch to display values at interfaces • Error checking codes in data (e.g., checksum or hash)

24. Maintenance Most production programs are maintained by people other than the programmers who originally wrote them. (a) What factors make a program easy for somebody else to maintain? (b) What factors make a program hard for somebody else to maintain?

25. Fault Tolerance General Approach: • Failure detection • Damage assessment • Fault recovery • Fault repair N-version programming -- Execute independent implementation in parallel, compare results, accept the most probable.

26. Fault Tolerance Basic Techniques: • After error continue with next transaction (e.g., drop packet) • Timers and timeout in networked systems • Error correcting codes in data • Bad block tables on disk drives • Forward and backward pointers in databases Report all errors for quality control

27. Fault Tolerance Backward Recovery: • Record system state at specific events (checkpoints). After failure, recreate state at last checkpoint. • Combine checkpoints with system log that allows transactions from last checkpoint to be repeated automatically. • Test the restore software!

28. Software Engineering for Real Time The special characteristics of real time computing require extra attention to good software engineering principles: • Requirements analysis and specification • Development of tools • Modular design • Exhaustive testing Heroic programming will fail!

29. Software Engineering for Real Time Testing and debugging need special tools and environments • Debuggers, etc., can not be used to test real time performance • Simulation of environment may be needed to test interfaces -- e.g., adjustable clock speed • General purpose tools may not be available

30. Some Notable Bugs • Built-in function in Fortran compiler (e0 = 0) • Japanese microcode for Honeywell DPS virtual memory • The microfilm plotter with the missing byte (1:1023) • The Sun 3 page fault that IBM paid to fix • Left handed rotation in the graphics package Good people work around problems. The best people track them down and fix them!