MU SE 565 - Software System Requirements (29 pages)

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Software System Requirements



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Software System Requirements

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Pages:
29
School:
Monmouth University
Course:
Se 565 - Software Requirements Engineering
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SE 565 Software System Requirements IX Data Flow Diagrams Dr Jiacun Wang Department of Software Engineering Monmouth University 01 14 19 Jiacun Wang 1 Topics Requirements engineering methods Structured techniques for documenting and analyzing requirements Data Flow Diagrams Data Dictionaries 01 14 19 Jiacun Wang RE 9 2 Role of methods in RE Process of requirements engineering RE is usually guided by a requirements method Requirement methods are systematic ways of producing system models System models are important bridges between the analysis and the design process 01 14 19 Jiacun Wang RE 9 3 Necessary properties for a RE method Suitability for agreement with the end user using intuitive notation The precision of definition of its notation Assistance with formulating requirements Definition of the world outside Scope for malleability Scope for integrating other approaches Scope for communication Tool support 01 14 19 Jiacun Wang RE 9 4 No ideal RE method There is no ideal requirement method A number of methods use a variety of modelling techniques to formulate system requirements System models can be enriched by modelling different aspects of using modelling techniques 01 14 19 Jiacun Wang RE 9 5 SA and OOA Structured analysis SA focus on functionality focus on data Object oriented analysis OOA integrate data and functions 01 14 19 Jiacun Wang RE 9 6 Data Flow Modeling One of the most popular structured methods The Data Flow Diagram DFD method DeMarco 1979 provides a description of a system based on modeling the transformational processes of a system the collections stores of data that the system manipulates and the flows of data between the processes stores and the outside world The DFD describes the functional viewpoint of the system e g it describes the system in terms of its operation tasks 01 14 19 Jiacun Wang RE 9 7 Data Flow Modeling cont This method works well for transaction processing systems and other function intensive applications The DFD provides a way to represent the steps involved in a business process or the operations of a proposed software system A DFD illustrates how the functional requirements in the SRS combine to let the user perform specific tasks 01 14 19 Jiacun Wang RE 9 8 The four basic elements of the DFD notation The circle the bubble It denotes an operation and is labeled with a brief description of the operation Transformation of data into other data Notation 3 Reserve seat 01 14 19 Jiacun Wang RE 9 9 The box or rectangle terminator It denotes external entities source or sink of information outside the boundaries of the system The source sink can be a human user or other system Notation Customer Source of information 01 14 19 Jiacun Wang RE 9 10 The paralleled bars or parallel lines It denotes a data store or file A place where data are held for later transformation or reference Notation User database Update user details 01 14 19 Jiacun Wang RE 9 11 The arc arrow It denotes the flow of information between the other three elements data flow Notation Customer s card details PIN Customer Validate customer access Access map Network database 01 14 19 Jiacun Wang RE 9 12 The Data Flow Diagram Notation Summary Data Transformation Terminator Input Output Terminator Data dictionary 01 14 19 Jiacun Wang RE 9 13 Hierarchical functional decomposition The DFD can present the system at different levels of details Hierarchical functional decomposition details Context level Level 1 Level 2 Level n 01 14 19 The decomposition continues until the lowest level diagram contains only primitive process operations that can be clearly represented in narrative text pseudocode activity diagram etc Jiacun Wang RE 9 14 Guidelines for building DFDs 1 All names should be unique 2 Suppress logical decisions no diamond shaped boxes 3 Don t become bogged down in details Defer error conditions and error handling until the end of the analysis not in the case of a diagnostic system or patient monitoring system for ex 4 Preserve the number of inputs and outputs between the levels 5 As bubbles are decomposed into less abstract bubbles the corresponding data flows may also need to be decomposed 01 14 19 Jiacun Wang RE 9 15 Rules of decomposition A top level process I1 I2 A I O3 O A O I1 O3 O1 O2 I1 I I2 is decomposed into some number of lower level processes I2 01 14 19 Jiacun Wang RE 9 16 Guidelines for building DFDs cont 6 Place data stores only on the Level 1 and lower levels not on the context diagram 7 Name each process as a concise action verb object 8 Number the processes uniquely and hierarchically On level 1 diagram number each process with an integer If you create a child DFD for process 2 number the processes in that child diagram 2 1 2 2 etc 9 Don t show more than 10 processes on a single diagram because it becomes harder to understand and change Instead introduce another layer of abstraction by grouping related processes into a higher level process 01 14 19 Jiacun Wang RE 9 17 Example Simple library system The context level DFD for Issue library item Library card Library user Requested item 01 14 19 Issued item Issue library item Jiacun Wang Return date Library assistant RE 9 18 Example Simple library system Level 1 of the DFD for the Issue library item Library card User database User details Update user details Check user User status Library user Requested item Issued item User ID Item ID Check item Item details Item status Issue Item database 01 14 19 Updated details Jiacun Wang Return date item Library assistant Update details RE 9 19 Example A Distribution System Context level data flow diagram Reply to be mailed Greenbelt Depts Dept Queries Reports to Depts Mgmt Queries Greenbelt Distribution System Customer Correspondence Customer 01 14 19 Greenbelt Mgmt Mgmt Reports Response to Customer Correspondence with Supplier Supplier Response Supplier Discarded correspondence Jiacun Wang RE 9 20 Level 1 data flow diagram Discarded Correspondence Customer Correspondence Reply to be mailed Receiving Correspondence Supplier Response Order items Preparing Shipments Response to Customer 4 Invoice 1 Recorded Customer Order Inventory File Mgmt Queries supplies Dept Queries Order Copy Keeping Accounts Stocking Supplies 3 2 Reports to Depts Correspondence with Supplier 01 14 19 Mgmt Reports Credit File Jiacun Wang RE 9 21 Level 2 DFD Receiving correspondence Customer Correspondence Routed Correspondence Record Route Correspondence Supplier Response 1 2 1 1 Read Sign Draft Read


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