1. © Andrew IrelandSoftware Design F28SD2 Function-oriented Design Andrew Ireland School of Mathematical & Computer Sciences Heriot-Watt University Edinburgh

2. © Andrew IrelandSoftware Design F28SD2 Outline Motivations Data flow Structuring Detailed design

3. © Andrew IrelandSoftware Design F28SD2 Motivations Many significant software systems fall into this classification, e.g. transaction processing Many legacy applications were developed using function-oriented design

4. © Andrew IrelandSoftware Design F28SD2 Function-oriented Design Applicability: –Minimal system state –Information is typically communicated via parameters or shared memory –No temporal aspect to functions, i.e. the result of a function invocation is not dependent upon the function’s earlier invocations Approach: –Promotes a top-down functional decomposition style of design

5. © Andrew IrelandSoftware Design F28SD2 Function-oriented Activities Data flow design: –Model data flow through the system as a set of data flow diagrams Structural decomposition: –Model the functionality of the system as a hierarchy of sub-functions via the structure chart notation Detailed design: –Provide detailed data and functional descriptions, typically supported by the notion of a data dictionary

6. © Andrew IrelandSoftware Design F28SD2 Data Flow Diagrams Data Flow Diagrams (DFDs) are used to represent the flow of information through a system, i.e. how the system outputs are derived by the functional transformation of input data sources DFDs are used within many design methods and are supported by numerous computer assisted software engineering (CASE) tools

7. © Andrew IrelandSoftware Design F28SD2 DFD Notation User interaction with the system, i.e. input/output Data store, e.g. database, files, etc System function or sub-function Labelled data flow Note: data flows can be combined logically, i.e. and/or

8. © Andrew IrelandSoftware Design F28SD2 Example: Regression Testing Testing that code modifications have not regressed the correctness of the system as a whole: “Also as a consequence of the introduction of new bugs, program maintenance requires far more system testing per statement written than any other programming. Theoretically, after each fix one must run the entire batch of test cases previously run against the system, to ensure that it has not been damaged in an obscure way. In practice, such regression testing must indeed approximate this theoretical idea, and it is very costly." Fred Brooks, The Mythical Man Month (p 122)

9. © Andrew IrelandSoftware Design F28SD2 Example: Regression Testing Consider the following system requirements: 1.A user provides a name for a source code base 2.If valid, then the source code base is retrieved from a database along with its associated regression test suit, which is held in a separate database 3.The source code is executed against the regression tests 4.The test results are compared against the expected results, which are held in another database 5.If actual and expected results differ then a report is generated for the user, else a certificate is generated for the user 6.A log file summarizing the regression testing stored in a database

10. © Andrew IrelandSoftware Design F28SD2 DFD Example Source code db Test case db Expected results db Run testsCompare Gen Log Gen reports Gen certifs Validate Regression test db and or s_file t_file e_file a_res r_res id l_file r_file c_file report certif Data flow

11. © Andrew IrelandSoftware Design F28SD2 Exercise: Wanted Persons Consider the following system requirements: 1.A user provides facial features based upon crime reports 2.If valid, then the feature list is used to retrieve a list of relevant feature templates from a database. If the feature list is not valid then an error message is sent to the user 3.The feature template list is used to search for matching offender imagines which are stored within an offender records database 4.Any records for offenders already in prison are then pruned 5.From pruned list, a suspect list is dispatched to the user, while the corresponding records are sent to a “wanted persons” database Exercise: construct a DFD for the requirements above

12. © Andrew IrelandSoftware Design F28SD2 Structural Decomposition For a given DFD there will exist choices as to how it is implemented The structure chart notation provides a means of adding more structure to the design Each function (or procedure) is represented as a rectangle A high-level function is represented as a hierarchy of sub-functions Links between sub-functions are labelled with input/output data, i.e. parameters or shared data

13. © Andrew IrelandSoftware Design F28SD2 Structural Decomposition A y A y xx A xy A xx inputoutputtransformCo-ordinate

14. © Andrew IrelandSoftware Design F28SD2 Structural Decomposition Note that a structure chart shows the relationship between sub-functions, but not the order in which they are invoked Typically a series of structure charts are developed for a given DFD – incrementally expanding level by level, e.g. A BC A BC DEF x x x z v u r s t z

15. © Andrew IrelandSoftware Design F28SD2 Structural Decomposition A B CD EFGH K IJ input outputprocess

16. © Andrew IrelandSoftware Design F28SD2 Structural Decomposition A B CD EFGH K IJ decision making data processing

17. © Andrew IrelandSoftware Design F28SD2 Structure Chart: First Regression tester InitializeReportTest Source code Test cases Expected results Source code id Regression results: regression log defect report certificate id res

18. © Andrew IrelandSoftware Design F28SD2 Structure Chart: Second Regression tester InitializeReportTest Source code Test cases Expected results Regression log id ValidateRun testsCompare id res LoggerGenerator id a_res r_res Defect report Certificate r_res

19. © Andrew IrelandSoftware Design F28SD2 Structure Chart: Third Regression tester InitializeReportTest Source code db Test case db Regression test db id ValidateRun testsCompare id res LoggerGenerator Gen cert Gen report id a_res r_res r_filec_file Expected results db reportcert l_file s_filet_filee_file r_res

20. © Andrew IrelandSoftware Design F28SD2 Detailed Design Detailed design involves getting inside the functional blocks represented with the structure chart notation At this level, Process Description Languages (PDL), i.e. pseudo code A useful intermediate representation is a Data Dictionary (DD) – a structured, but informal, record of data items and functional DDs are useful to designers, programmers as well as during maintenance

21. © Andrew IrelandSoftware Design F28SD2 Example: Data Dictionary EntityTypeDescription idstringName of the source code base under test resbooleanThe result of validating the name of the source code base provided by the user ……… r_resvectorThe regression test results, i.e. the actual results vs expected results a_resvectorThe actual results obtained by executing the code base against the regression tests testprocedureInput: name of source code base Function: co-ordinates the execution of the regression tests and the comparison with the expected results Output: results of regression testing ………

22. © Andrew IrelandSoftware Design F28SD2 Sequential vs Concurrent Note that no explicit reference has been made as to how a design is implemented, i.e. sequential vs concurrent DFDs in particular do not impose a control paradigm A DFD representation of a function could be implemented as a: –sequence of sub-function invocations or –set of communicating processes

23. © Andrew IrelandSoftware Design F28SD2 Summary Learning outcomes: –Applicability of function-oriented design –Data flow and DFDs –Structure charts and functional hierarchy –Detailed design and data dictionaries Recommended reading: –D. Budgen, “Software Design”, Addison-Wesley 2003 –I. Sommerville, “Software Engineering”, Addison- Wesley 2007