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CHAPTER 6

CHAPTER 6. TESTING. Overview. Quality issues Nonexecution-based testing Execution-based testing What should be tested? Testing versus correctness proofs Who should perform execution-based testing? When testing stops?. Testing.

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CHAPTER 6

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  1. CHAPTER 6 TESTING

  2. Overview • Quality issues • Nonexecution-based testing • Execution-based testing • What should be tested? • Testing versus correctness proofs • Who should perform execution-based testing? • When testing stops?

  3. Testing • Testing is an integral component of software process and an activity that must be carried out throughout the life cycle • Two types of testing • Execution-based testing • Nonexecution-based testing

  4. Testing (contd) • “V & V” • Verification • Determine if the phase was completed correctly • Performed at the end of each phase • Validation • Determine if the product as a whole satisfies its requirements • Performed just before the product is delivered to the client • In this book, testing simply denotes V & V

  5. Software Quality • Quality implies “excellence” of some sort • In S/W, we do not strive for “excellence” but merely getting the software to function correctly is enough • That is, does the software satisfy its specifications? • Read Just In Case You Wanted to Know on page 138 • Software Quality Assurance (SQA) • Ensures that the product is correct • Performs testing at the end of each phase as well as at the end of the product development • Managerial independence is important • Development group • SQA group

  6. Nonexecution-Based Testing • Underlying principles • One should not review his or her own work • Why not? • Nonexecution-based testing is usually referred to as review • There are two types of reviews • Walkthroughs • Inspections

  7. Walkthroughs • Should consist of 4-6 members • For example, a specification walkthrough should include • Specification team manager & member • Client representative • Member that will perform the next phase (design team member) • SQA team member (should chair the walkthrough) • The material to be reviewed should be distributed in advance to the participants • Each reviewer should study the material and prepare two lists: • items that the reviewer does not understand • items that the reviewer believes incorrect

  8. Walkthroughs (contd) • Walkthrough process • Usually no more than 2 hours • Detect and record faults – DO NOT correct • Two ways of conducting walkthroughs • Participant driven • Participants present their lists of unclear and incorrect items • The author must respond to each query • Document driven • The author walks the participants through the document • The reviewers interrupt whenever unclear or incorrect items are presented • The document-driven approach is usually more thorough (i.e., finds more faults)

  9. Inspections • More detailed than walkthrough and has five formal steps: overview, preparation, inspection, rework and follow-up • Inspection team should consist of 4 members • For example, design inspection team includes • Moderator (inspection team leader), designer, implementer, tester

  10. Inspection Steps • Overview • An overview of the document to be reviewed is given by one of the authors • At the end of the overview session, the document is distributed to the participants • Preparation • Participants try to understand the document in detail, aided by statistics of fault types • Participants prepare the lists of unclear/incorrect items as well • Inspection • An author walks through the document with the inspection team • Faults are detected and recorded – DO NOT correct here • Within one day, the moderator generates a written report containing faults detected during inspection

  11. Inspection Steps (contd) • Rework • The author resolves all faults and problems noted in the written report • Follow-up • The rework is thoroughly checked • All fixes must be checked to ensure that no new faults have been introduced

  12. Statistics on Inspections • 82% of all detected faults (IBM, 1976) • 70% of all detected faults (IBM, 1978) • 93% of all detected faults (IBM, 1986) • 90% decrease in cost of detecting fault (Switching system, 1986) • 4 major faults, 14 minor faults per 2 hours (JPL, 1990). Savings of $25,000 per inspection • Number of faults decreased exponentially by phase (JPL, 1992)

  13. Metrics for Inspections • Fault density (e.g., faults per KLOC) • Fault detection rate (e.g., faults detected per hour) • Fault detection efficiency (e.g., the number of major and minor faults detected per person-hour)

  14. Execution-Based Testing • Definitions • Fault is the IEEE Standard terminology for “bug” • Failure is the observed incorrect behavior of the product as a consequence of the fault • Error is the mistake made by programmer • “Programming testing can be a very effective way to show the presence of bugs, but it is hopelessly inadequate for showing their absence [Dijkstra, 1972]

  15. What is execution-based testing? • “Execution-based testing is a process of inferring certain behavioral properties of a product based, in part, on the results of executing the product in known environment with selected inputs.” [Goodenough, 1979] • Inference (i.e., deriving logical conclusion) • Known environment • Selected inputs • What must be tested? => Behavioral properties must be tested • Correctness, utility, reliability, robustness, and performance

  16. Utility • Utility is the extent to which a user’s needs are met when a correct product is used under conditions permitted by its specifications • That is, does it meet user’s needs? • Ease of use • Useful functions • Cost-effectiveness

  17. Reliability • Reliability is a measure of the frequency and criticality of product failure • How often the product fails? (Mean time between failures, MTBF) • How long it takes to restore? (Mean time to restore, MTTR) • But often more important is how long it takes to repair the results of the failure?

  18. Robustness • Robustness is a function of a number of factors such as • Range of operating conditions • Possibility of unacceptable results with valid input • Effect of invalid input

  19. Performance • Extent to which space and time constraints are met • Real-time systems have hard time constraints

  20. Correctness • A product is correct if it satisfies its output specifications • But what if the specifications themselves are incorrect?

  21. Correctness of specifications • Incorrect specification for a sort • Function trickSort which satisfies this specification:

  22. Correctness of specifications (contd) • Incorrect specification for a sort: • Corrected specification for the sort:

  23. Correctness • NOT necessary • NOT sufficient

  24. Correctness Proofs • Alternative to execution-based testing

  25. Example of Correctness Proof • Code segment to be proven correct

  26. Example of Correctness Proof (contd) • Flowchart of code segment

  27. Example of Correctness Proof

  28. Correctness Proof Case Study • Never prove a program correct without testing it as well

  29. Episode 1 • 1969 — Naur Paper • “Naur text-processing problem” Given a text consisting of words separated byblankor bynl (new line) characters, convert it to line-by-line form in accordance with following rules: (1) line breaks must be made only where given text has blankor nl ; (2) each line is filled as far as possible, as long as (3) no line will contain more than maxpos characters • Naur constructed a procedure (25 lines of Algol 60), and informally proved its correctness

  30. Episode 2 • 1970 — Reviewer in Computing Reviews • The first word of the first line is preceded by blank unless the first word is exactly maxpos characters long

  31. Episode 3 • 1971 — London finds 3 more faults • Including: • The procedure does not terminate unless a word longer than maxpos characters is encountered

  32. Episode 4 • 1975 — Goodenough and Gerhart find three further faults • Including: • The last word will not be output unless it is followed by blank or nl

  33. Proofs and Software Engineering • Lesson: • Even if product is proved correct, it must STILL be tested.

  34. Three Myths • Software engineers do not have enough math for proofs • Proving is too expensive to be practical • Proving is too hard

  35. Proofs and Software Engineering (contd) • Can we trust a theorem prover? • How to find input–output specifications, loop invariants • What if the specifications are wrong? • Can never be sure that specifications or a verification system are correct

  36. Proofs and Software Engineering (contd) • Correctness proofs are a vital software engineering tool, WHERE APPROPRIATE. • If • Human lives are at stake • Indicated by cost/benefit analysis • Risk of not proving is too great • Also, informal proofs can improve the quality of the product • Assert statement

  37. Who Performs Execution-Based Testing? • Testing is destructive • A successful test finds a fault • Solution • 1. The programmer does informal testing • 2. SQA does systematic testing • 3. The programmer debugs the module • All test cases must be • Planned beforehand, including expected output • Retained afterwards

  38. When Testing Can Stop? • Only when the product has been irrevocably retired

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