1. 1 CS 501 Spring 2004 CS 501: Software Engineering Lecture 20 Reliability 2

2. 2 CS 501 Spring 2004 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. 3 CS 501 Spring 2004 Reliability Reliability: Probability of a failure occurring in operational use. Perceived reliability: Depends upon: user behavior set of inputs pain of failure

4. 4 CS 501 Spring 2004 User Perception of Reliability 1. A personal computer that crashes frequently v. a machine that is out of service for two days. 2. A database system that crashes frequently but comes back quickly with no loss of data v. a system that fails once in three years but data has to be restored from backup. 3. A system that does not fail but has unpredictable periods when it runs very slowly.

5. 5 CS 501 Spring 2004 Reliability Metrics Traditional Measures Mean time between failures Availability (up time) Mean time to repair Market Measures Complaints Customer retention User Perception is Influenced by Distribution of failures Hypothetical example: Cars are less safe than airplanes in accidents per hour, but safer in accidents per mile.

6. 6 CS 501 Spring 2004 Requirements Specification of System Reliability Example: ATM card reader Failure class ExampleMetric Permanent System fails to operate1 per 1,000 days non-corrupting with any card -- reboot Transient System can not read1 in 1,000 transactions non-corrupting an undamaged card Corrupting A pattern ofNever transactions corrupts database

7. 7 CS 501 Spring 2004 Reliability Metrics for Distributed Systems Traditional metrics are hard to apply in multi-component systems: In a big network, at any given moment something will be giving trouble, but very few users will see it. A system that has excellent average reliability may give terrible service to certain users. There are so many components that system administrators rely on automatic reporting systems to identify problem areas.

8. 8 CS 501 Spring 2004 Cost of Improved Reliability $ Up time 99% 100% Will you spend your money on new functionality or improved reliability?

9. 9 CS 501 Spring 2004 Example: Dartmouth Time Sharing (1978) A central computer serves the entire campus. Any failure is serious. Step 1 Gather data on every failure 10 years of data in a simple data base Every failure analyzed: hardware software (default) environment (e.g., power, air conditioning) human (e.g., operator error)

10. 10 CS 501 Spring 2004 Example: Dartmouth Time Sharing (1978) Step 2 Analyze the data Weekly, monthly, and annual statistics Number of failures and interruptions Mean time to repair Graphs of trends by component, e.g., Failure rates of disk drives Hardware failures after power failures Crashes caused by software bugs in each module

11. 11 CS 501 Spring 2004 Example: Dartmouth Time Sharing (1978) Step 3 Invest resources where benefit will be maximum, e.g., Orderly shut down after power failure Priority order for software improvements Changed procedures for operators Replacement hardware

12. 12 CS 501 Spring 2004 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)

13. 13 CS 501 Spring 2004 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 President 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.

14. 14 CS 501 Spring 2004 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.

15. 15 CS 501 Spring 2004 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.

16. 16 CS 501 Spring 2004 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.

17. 17 CS 501 Spring 2004 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

18. 18 CS 501 Spring 2004 Defensive Programming Examples Use boolean variable not integer Test i <= n not 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)

19. 19 CS 501 Spring 2004 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?

20. 20 CS 501 Spring 2004 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.

21. 21 CS 501 Spring 2004 Static Validation & Verification Carried out throughout the software development process. Validation & verification Requirements specification Design Program REVIEWS

22. 22 CS 501 Spring 2004 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

23. 23 CS 501 Spring 2004 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

24. 24 CS 501 Spring 2004 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

25. 25 CS 501 Spring 2004 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.

26. 26 CS 501 Spring 2004 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.

27. 27 CS 501 Spring 2004 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 Report all errors for quality control

28. 28 CS 501 Spring 2004 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.

29. 29 CS 501 Spring 2004 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!

30. 30 CS 501 Spring 2004 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

31. 31 CS 501 Spring 2004 Some Notable Bugs Built-in function in Fortran compiler (e 0 = 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!