Specification

1. Specification Ch. 5

2. Outline • Discussion of the term "specification" • Types of specifications • operational • Data Flow Diagrams • (Some) UML diagrams • Finite State Machines • Petri Nets • descriptive • Entity Relationship Diagrams • Logic-based notations • Algebraic notations • Languages for modular specifications • Statecharts • Z Ch. 5

3. Specification • A broad term that means definition • Used at different stages of software development for different purposes • Generally, a statement of agreement (contract) between • producer and consumer of a service • implementer and user • All desirable qualities must be specified Ch. 5

4. Uses of specification • Statement of user requirements • major failures occur because of misunderstandings between the producer and the user • "The hardest single part of building a softwarem system is deciding precisely what to build" (F. Brooks) Ch. 5

5. Uses of specification (cont.) • Statement of the interface between the machine and the controlled environment • serious undesirable effects can result due to misunderstandings between software engineers and domain experts about the phenomena affecting the control function to be implemented by software Ch. 5

6. Uses of specification (cont.) • Statement of requirements for implementation • design process is a chain of specification (i.e., definition)–implementation–verification steps • requirements specificationrefers to definition of external behavior • design specification must be verified against it • design specification refers to definition of the software architecture • code must be verified against it Ch. 5

7. Uses of specification (cont.) • A reference point during maintenance • corrective maintenance only changes implementation • adaptive and perfective maintenance occur because of requirements changes • requirements specification must change accordingly Ch. 5

8. Specification qualities • Precise, clear, unambiguous • Consistent • Complete • internal completeness • external completeness • Incremental Ch. 5

9. Clear, unambiguous, understandable • Example: specification fragment for a word-processor Selecting is the process of designating areas of the document that you want to work on. Most editing and formatting actions require two steps: first you select what you want to work on, such as text or graphics; then you initiate the appropriate action. can an area be scattered? Ch. 5

10. Precise, unambiguous, clear • Another example (from a real safety-critical system) The message must be triplicated. The three copies must be forwarded through three different physical channels. The receiver accepts the message on the basis of a two-out-of-three voting policy. can a message be accepted as soon as we receive 2 out of 3 identical copies of message or do we need to wait for receipt of the 3rd? Ch. 5

11. Consistent • Example: specification fragment for a word-processor The whole text should be kept in lines of equal length. The length is specified by the user. Unless the user gives an explicit hyphenation command, a carriage return should occur only at the end of a word. What if the length of a word exceeds the length of the line? Ch. 5

12. Complete • Internal completeness • the specification must define any new concept or terminology that it uses • glossary helpful for this purpose • the specification must document all the needed requirements • difficulty: when should one stop? Ch. 5

13. Incremental • Referring to the specification process • start from a sketchy document and progressively add details • Referring to the specification document • document is structured and can be understood in increments Ch. 5

14. Classification of specification styles • Informal, semi-formal, formal • Operational • Behavior specification in terms of some abstract machine • Descriptive • Behavior described in terms of properties Ch. 5

15. Example 1 • Specification of a geometric figure E: E can be drawn as follows: 1.Select two points P1 and P2 on a plane 2. Get a string of a certain length and fix its ends to P1 and P2 3.Position a pencil as shown in next figure 4.Move the pen clockwise, keeping the string tightly stretched, until you reach the point where you started drawing this is an operational specification Ch. 5

16. Ch. 5

17. A descriptive specification • Geometric figure E is describe by the following equation ax2 + by2 + c = 0 where a, b, and c are suitable constants Ch. 5

18. Another example “Let a be an array of n elements. Theresult of its sorting is an array b of n elements such that the first element of b is the minimum of a (if several elements of a have the same value, any one of them is acceptable); the second element of b is the minimum of the array of n-1 elements obtained from a by removing its minimum element; and so on until all n elements of a have been removed.” OP “The result of sorting array a is an array b which is a permutation of a and is sorted.” DES Ch. 5

19. How to verify a specification? • “Observe” dynamic behavior of specified system (simulation, prototyping, “testing” specs) • Analyze properties of the specified system • Analogy with traditional engineering • physical model of a bridge • mathematical model of a bridge Ch. 5

20. Data Flow Diagrams (DFDs) • A semi-formal operational specification • System viewed as collection of data manipulated by “functions” • Data can be persistent • they are stored in data repositories • Data can flow • they are represented by data flows • DFDs have a graphical notation Ch. 5

21. Graphical notation • bubbles represent functions • arcs represent data flows • open boxes represent persistent store • closed boxes represent I/O interaction Ch. 5

22. Example specifies evaluation of (a + b) * (c + a * d) Ch. 5

23. A construction “method” (1) 1. Start from the “context” diagram Ch. 5

24. A construction “method” (2) 2. Proceed by refinements until you reach “elementary” functions (preserve balancing) Ch. 5

25. A library example Ch. 5

26. Refinement of“Get a book” Ch. 5

27. Patient monitoring systems The purpose is to monitor the patients’ vital factors--blood, pressure, temperature, …--reading them at specified frequencies from analog devices and storing readings in a DB. If readings fall outside the range specified for patient or device fails an alarm must be sent to a nurse. The system also provides reports. Ch. 5

28. A refinement Ch. 5

29. More refinement Ch. 5

30. An evaluation of DFDs (1) • Easy to read, but … • Informal semantics • How to define leaf functions? • Inherent ambiguities • Outputs from A, B, C are • all needed? • Outputs for E and F are • produced at the same time? Ch. 5

31. An evaluation of DFDs (2) • Control information is absent Possible interpretations: (a) A produces datum, waits until B consumes it (b) B can read the datum many times without consuming it (c) a pipe is inserted between A and B Ch. 5

32. Formalization/extensions • There have been attempts to formalize DFDs • There have been attempts to extend DFDs (e.g., for real-time systems) Ch. 5

33. borrow book return book librarian customer library update UML use-case diagrams • Define functions on basis of actors and actions Ch. 5

34. UML sequence diagrams • Describe how objects interact by exchanging messages • Provide a dynamic view Ch. 5

35. UML collaboration diagrams • Give object interactions and their order • Equivalent to sequence diagrams Ch. 5

36. Finite state machines (FSMs) • Can specify control flow aspects • Defined as a finite set of states, Q; a finite set of inputs, I; a transition function d : Q x I  Q (d can be a partial function) Ch. 5

37. Example: a lamp Ch. 5

38. Another example:a plant control system Ch. 5

39. A refinement Ch. 5

40. Classes of FSMs • Deterministic/nondeterministic • FSMs as recognizers • introduce final states • FSMs as transducers • introduce set of outputs • . . . Ch. 5

41. FSMs as recognizers qf is a final state Ch. 5

42. FSMs as recognizers Ch. 5

43. Limitations • Finite memory • State explosion • Given a number of FSMs with k1, k2, … kn states, their composition is a FSM with k1 * k2 *… * kn. This growth is exponential with the number of FSMs, not linear (we would like it to be k1 + k2 +… + kn ) Ch. 5

44. State explosion: an example Ch. 5

45. The resulting FSM Ch. 5

46. Petri nets A quadruple (P,T,F,W) P: places T: transitions (P, T are finite) F: flow relation (F  {PT}  {TP}) W: weight function (W: F  N – {0})Properties: (1) P  T = Ø (2) P  T  Ø (3)F  (P  T)  (T  P) (4) W: F  N-{0} Default value of W is 1 State defined by marking: M: P  N Ch. 5

47. P P 1 2 t 1 t P 2 3 P P 4 5 t 4 t 3 P P 6 7 t 5 t 6 Graphical representation Ch. 5

48. Semantics: dynamic evolution • Transition t is enabled iff • pt's input places, M(p)  W(<p,t>) • t fires: produces a new marking M’ in places that are either t's input or output places or both • if p is an input place: M'(p) = M(p) - W(<p,t>) • if p is an output place: M'(p) = M(p) + W(<t,p>) • if p is both an input and an output place: M'(p) = M(p) - W(<p,t>) + W(<t,p>) Ch. 5

49. Nondeterminism • Any of the enabled transitions may fire • Model does not specify which fires, nor when it fires Ch. 5

50. Modeling with Petri nets • Places represent distributed states • Transitions represent actions or events that may occur when system is in a certain state • They can occur as certain conditions hold on the states Ch. 5