functional Design DFD and data flow diagram.ppt
- 1. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 1 Function-oriented Design Design with functional units which transform inputs to outputs
- 2. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 2 Topics Covered Data flow design Structural decomposition Detailed design
- 3. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 3 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.
- 4. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 4 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 ; Eject_card ; Print (“Please take your card or press CONTINUE”) ; exit when Card_removed ; end loop ; Update_account_information (Account_number, Amount_dispensed) ;
- 5. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 5 Functional and Object-oriented Design For many types of applications, object-oriented design is likely to lead to a more reliable and maintainable system. For applications that 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.
- 6. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 6 Functional Design Process Data flow design • Model the data processing in the system using data flow diagrams Structural decomposition • Model how functions are decomposed into 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
- 7. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 7 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
- 8. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 8 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
- 9. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 9 Design Report Generator Sor tentity names Getentity names Design database Get design name Design name Lookup entitynames Data dictionar y Sor tb y type Entity nam es Sorted nam es Designentity descr iptions Produce noderepor t Produce linkrepor t Integ rate repor ts Print repor t Link descr iptions Node descr iptions Integ rated repor t Node repor t Link repor t and and
- 10. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 10 Structural decomposition is concerned with developing a model of the design which shows the dynamic structure; i.e., function calls. 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. Structural Decomposition
- 11. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 11 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.
- 12. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 12 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.
- 13. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 13 Process Steps Identify system processing transformations • Transformations in the DFD that 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
- 14. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 14 Initial Structure Chart Produce designrepor ts Collate entities Gener ate repor t Get design entitynam es Design nam e Design entity nam es Design repor t entity nam es entity data entity data entity nam es
- 15. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 15 Expanded Structure Chart Produce designrepor ts Collate entities Gener ate repor t Get design entitynam es entity nam es nam es sor ted entity data nam es Get design name Getentity names Sor tentities byname Getentity data Sor tentities bytype Produce integ ratedrepor t Print report design nam e entity nam es repor t entity data design nam e nam es sor ted nam es entity data sor ted entity data sor ted entity data Integ rated repor t sor ted entity data
- 16. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 16 Final Structure Chart Data dictionar y Produce designrepor ts Collate entities Gener ate repor t Get design entitynam es entity nam es nam es sor ted entity data nam es Get design name Getentity names Sor tentities byname Getentity data Sor tentities bytype Produce integ ratedrepor t Print report design nam e entity nam es repor t entity data design nam e nam es sor ted nam es entity data sor ted entity data sor ted entity data Integ rated repor t Design database design nam e entity nam e Produce linkrepor t Produce noderepor t Link data Link repor t Node data Node repor t sor ted entity data
- 17. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 17 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, can be produced.
- 18. ©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 15 Slide 18 Data Dictionary Entries Entity name Type Description Design name STRING The name of the design assigned by the design engineer. Get design name FUNCTION Input: Design name Function: This function communicates with the user to get the name of a design that has been entered in the design database. Output: Design name Get entity names FUNCTION Input: Design name Function: Given a design name, this function accesses the design database to find the names of the entities (nodes and links) in that design. Output: Entity names Sorted names ARRAY of STRING A list of the names of the entities in a design held in ascending alphabetical order.