14 functional design

Function-oriented design Design with functional units which transform inputs to outputs

Function-oriented design Practiced informally since programming began Thousands of systems have been developed using this approach Supported directly by most programming languages Most design methods are functional in their approach CASE tools are available for design support

A function-oriented view of design

Natural functional systems Some systems are naturally function-oriented Systems which maintain minimal state information i.e. where the system is concerned with processing independent actions whose outcomes are not affected by previous actions Information sharing through parameter lists Transaction processing systems fall into this category. Each transaction is independent

ATM software design loop loop Print_input_message (” Welcome - Please enter your card”) ; exit when Card_input ; end loop ; Account_number := Read_card ; Get_account_details (PIN, Account_balance, Cash_available) ; if Validate_card (PIN) then loop Print_operation_select_message ; case Get_button is when Cash_only => Dispense_cash (Cash_available, Amount_dispensed) ; when Print_balance => Print_customer_balance (Account_balance) ; when Statement => Order_statement (Account_number) ; when Check_book => Order_checkbook (Account_number) ; end case ; ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 7 Eject_card ; Print (“Please take your card or press CONTINUE”) ; exit when Card_removed ; end loop ; Update_account_information (Account_number, Amount_dispensed) ; else Retain_card ; end if ; end loop ;

Functional and object-oriented design For many types of application, object-oriented design is likely to lead to a more reliable and maintainable system Some applications maintain little state - function-oriented design is appropriate Standards, methods and CASE tools for functional design are well-established Existing systems must be maintained - function-oriented design will be practiced well into the 21st century

Functional design process Data-flow design Model the data processing in the system using data-flow diagrams Structural decomposition Model how functions are decomposed to sub-functions using graphical structure charts Detailed design The entities in the design and their interfaces are described in detail. These may be recorded in a data dictionary and the design expressed using a PDL

Data flow diagrams Show how an input data item is functionally transformed by a system into an output data item Are an integral part of many design methods and are supported by many CASE systems May be translated into either a sequential or parallel design. In a sequential design, processing elements are functions or procedures; In a parallel design, processing elements are tasks or processes

DFD notation Rounded rectangle - function or transform Rectangle - data store Circles - user interactions with the system Arrows - show direction of data flow keywords and/ or. Used to link data flows

Structural decomposition Structural decomposition is concerned with developing a model of the design which shows the dynamic structure i.e. function calls This is not the same as the static composition structure The aim of the designer should be to derive design units which are highly cohesive and loosely coupled In essence, a data flow diagram is converted to a structure chart

Decomposition guidelines For business applications, the top-level structure chart may have four functions namely input, process, master-file-update and output Data validation functions should be subordinate to an input function Coordination and control should be the responsibility of functions near the top of the hierarchy

Decomposition guidelines The aim of the design process is to identify loosely coupled, highly cohesive functions. Each function should therefore do one thing and one thing only Each node in the structure chart should have between two and seven subordinates

Process steps Identify system processing transformations Transformations in the DFD which are concerned with processing rather than input/output activities. Group under a single function in the structure chart Identify input transformations Transformations concerned with reading, validating and formatting inputs. Group under the input function Identify output transformations Transformations concerned with formatting and writing output. Group under the output function

Detailed design Concerned with producing a short design specification (mini-spec) of each function. This should describe the processing, inputs and outputs These descriptions should be managed in a data dictionary From these descriptions, detailed design descriptions, expressed in a PDL or programming language, can be produced

A comparison of design strategies An example of an office information retrieval system (OIRS) is used to compare different design strategies Functional design, concurrent systems design and object-oriented design are compared The OIRS is an office system for document management. Users can file, maintain and retrieve documents using it

Interface description Operation field. Pull-down menu allowing an operation to be selected. Known and current indexes fields Pull-down menus of indexes Document name. Name under which the document is to be filed. Qualifier field Pattern used in retrieval. Current workspace Contains the documents currently being used. May be edited with word processor

Fetch-execute model procedure Interactive_system is begin loop Command := Get_command; if Command = “quit” then -- Make sure files etc. are closed properly Close_down_system ; exit ; else Input_data := Get_input_data ; Execute_command (Command, Input_data, Output_data) ; end if ; end loop ; end Interactive_system ;

Design decisions What strategy should be adopted in decomposing Execute command? Are the input and output data flows processed independently or are they inter-dependent. If independent, there should be a central transform for each processing unit Is the central transform a series of transforms? If so, each logical element in the series should be a single transformation

OIRS design description procedure OIRS is begin User := Login_user ; Workspace := Create_user_workspace (User) ; -- Get the users own document database using the user id DB_id := Open_document_database (User) ; -- get the user’s personal index list; Known_indexes := Get_document_indexes (User) ; Current_indexes := NULL ; -- command fetch and execute loop loop Command := Get_command ; exit when Command = Quit ; Execute_command ( DB_id, Workspace, Command, Status) ; if Status = Successful then Write_success_message ; else Write_error_message (Command, Status) ; end if ; end loop ; Close_database (DB_id) ; Logout (User) ; end OIRS ;

Concurrent systems design Data flow diagrams explicitly exclude control information. They can be implemented directly as concurrent processes. Logical groups of transformations can also be implemented as concurrent processes e.g. input data collection and checking The OIRS system can be implemented as a concurrent system with command input, execution and status reporting implemented as separate tasks

Detailed process design procedure Office_system is task Get_command ; task Process_command is entry Command_menu ; entry Display_indexes ; entry Edit_qualifier ; -- Additional entries here. One for each command end Process_commands ; task Output_message is entry Message_available ; end Output_message ; task Workspace_editor is entry Enter ; entry Leave ; end Workspace_editor ;

Detailed process design task body Get_command is begin -- Fetch/execute loop loop loop Cursor_position := Get_cursor_position ; exit when cursor positioned in workspace or (cursor positioned over menu and button pressed) Display_cursor_position ; end loop ; if In_workspace (Cursor_position) then Workspace_editor.Enter ; elsif In_command_menu (Cursor_position) then Process_command.Command_menu ; elsif In_Known_indexes (Cursor_position) then Process_command.Display_indexes ; elsif In_Current_indexes (Cursor_position) then ... Other commands here ... end loop ; -- Fetch/execute end Get_command ; -- other task implementations here end Office_system ;

Key points Function-oriented design relies on identifying functions which transform inputs to outputs Many business systems are transaction processing systems which are naturally functional The functional design process involves identifying data transformations, decomposing functions into sub-functions and describing these in detail 35

Key points Data-flow diagrams are a means of documenting end-to-end data flow. Structure charts represent the dynamic hierarchy of function calls Data flow diagrams can be implemented directly as cooperating sequential processes Functional and object-oriented design result in different system decompositions. However, a heterogeneous approach to design is often necessary

14 functional design

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14 functional design