Function-oriented Design Andrew Ireland School of Mathematical & Computer Sciences Heriot-Watt University Edinburgh

Outline Motivations Data flow Structuring Detailed design

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

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

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

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 No standard notation …

DFD Notation User interaction with the system, i.e. input/output Data store, e.g. database, files, etc System function/operation/process Labelled data flow Note: data flows can be combined logically, i.e. and/or

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)

Example: Regression Testing Consider the following system requirements: A user provides a name for a source code base 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 The source code is executed against the regression tests The test results are compared against the expected results, which are held in another database If actual and expected results differ then a report is generated for the user, else a certificate is generated for the user A log file summarizing the regression testing stored in a database

DFD Example and or Data flow l_file Regression test db s_file r_res Source code db Gen Log s_file r_res report t_file a_res Test case db Gen reports and Run tests Compare r_res id r_file id Gen reports or Validate c_file e_file Expected results db certif Gen certifs Data flow

Exercise: Wanted Persons Consider the following system requirements: A user provides facial features based upon crime reports 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 The feature template list is used to search for matching offender imagines which are stored within an offender records database Any records for offenders already in prison are then pruned 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

Exercise: Wanted Persons Consider the following system requirements: A user provides facial features based upon crime reports 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 The feature template list is used to search for matching offender imagines which are stored within an offender records database Any records for offenders already in prison are then pruned 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

DFD Example or and feature list features error-msg get feature list validate features or feature list feature template db templates filter templates template list offender image db images match images suspect list images and prison records db records extract suspects records wanted persons db

Hierarchical DFDs As system complexity grows, so will the DFDs Hierarchical, or layered, DFDs provide a way of managing this complexity, i.e. Firstly, a Context Diagram (Level 0) is used to model the system as a single system process together with the environment in which it operates, e.g. users and external data stores Then, Level 1, 2, 3 … DFDs are developed, which model the system’s sub-processes

A Context Diagram (Level 0)

Level 1 DFD 1 2

Level 2 DFD 1.1 1.2 2.1 2.2

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

Structural Decomposition y x y y A A A A x x x x input output transform Co-ordinate

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 A z z x x B C B C x v r t u s D E F

Structural Decomposition B E F G H I J K input process output

Structural Decomposition decision making A C D B E F G H I J K data processing

Structure Chart: First Regression tester id res id res Initialize Test Report Source code id Source code Test cases Expected results Regression results: regression log defect report certificate

Structure Chart: Second Regression tester id r_res id r_res Initialize Test Report id r_res r_res r_res id id res a_res Validate Run tests Compare Logger Generator Source code Test cases Expected results Regression log Defect report Certificate

Structure Chart: Third Regression tester id r_res id r_res Initialize Test Report id r_res r_res r_res id id res a_res Validate Run tests Compare Logger Generator l_file s_file t_file e_file r_file c_file Gen report Gen cert Source code db Test case db Expected results db report cert Regression test db

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

Example: Data Dictionary Entity Type Description id string Name of the source code base under test res boolean The result of validating the name of the source code base provided by the user … r_res vector The regression test results, i.e. the actual results vs expected results a_res The actual results obtained by executing the code base against the regression tests test procedure Input: 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

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

Summary Learning outcomes: Recommended reading: 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