Ch5: Software Specification

1 Overview  Use of specifications  Specification qualities  Classification of specification styles  Verification of specifications  Operational specifications  Data flow diagrams – Information/usage  Finite state machines – Event based  Petri nets – Resource base  Queuing and simulation models  ER and Object-oriented specifications  Logic and algebraic specifications  Specification notations and writing specifications

2 Software specification  Software systems must be specified. Why?  Requirements specification:  Design specification:  Module specification:  In general:  Specification vs. implementation: Specification – What  Implementation - How

3 Uses of specification  Statement of user’s needs:  Statement of the interface between the machine and the controlled environment:

4 Uses of specification (contd..)  Statement of requirements for implementation:  A reference point during maintenance:

5 Specification qualities  Clear, unambiguous, understandable:

6 Clear, unambiguous and understandable (example) 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.

7 Precise, unambiguous, clear (example) 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.

8 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.

9 Specification qualities (contd..)  Completeness:  Incremental:

10 Formal vs. informal specifications  Informal:  Formal:  Advantages of formal specifications:  Formal specifications may not be widely used:  Semi-formal: Notation without completely precise semantics:

11 Operational vs. Descriptive specifications  Operational specification: Describes desired behavior of the system:  Descriptional specification: Describes desired properties of system in a declarative manner.  Actual specifications may be a mix of operational and declarative styles

12 Verification of specifications

13 Verification of specifications  “Observe” dynamic behavior:  Analyze system properties: