Presentation on theme: "Software Engineering-II"— Presentation transcript:
1 Software Engineering-II Data Flow DiagramsSoftware Engineering-IISir Zubair Sajid
2 Introduction SSADM What is a Data Flow Diagram? Why do we use DFDs? LevellingConventionsDecomposition and AbstractionThe ElementsProcess and Data StoresOutside EntityData FlowThe LevelsRulesThe Procedure for Constructing DFD’sThe Document Flow DiagramThe Context DiagramDraw the external entities and data storesLevel 1 Physical DFD - Complete
3 S.S.A.D.M. Data Flow Diagrams S.S.A.D.M. - Structured Systems Analysis and Design MethodUses different techniques to model a systemData Flow DiagramsEntity Relational Model (Logical Data Stores)Normalisation
4 What is a Data Flow Diagram? Known as DFDsA way to model a real world situationThey are the interface between the real world activities and an understanding of how this can be converted into a computer system.
5 Why do we use DFDs?It is a way of taking the physical view and converting it into a logical view.The physical view - all documents involvedThe logical view - the data they containTheir main purpose is to communicate with the user, the analyst’s understanding of the scope of the required system
6 Levelling DFDs are expanded or decomposed into levels. Separating each process into sub processesUncovers more and more detail
7 Conventions Balancing Process at lower level should have identical data flows if they flow out of a processModelling Data StoresOnly use DATA STORES used within this process on the diagramNumberingLabelsShould carry as much meaning as possible
8 Decomposition and Abstraction Decomposition - Divide and subdivide into manageable size problemsAbstraction - Concentrate on the important issues and ignore the irrelevant
9 The Elements Data Flows, with a label to indicate what data is flowing The four main elements of DFDs notationData Flows, with a label to indicate what data is flowingProcesses, that handle the dataData stores, within the system (diary, filing cabinet or computer file)Outside entities, outside sources of data
10 Process and Data Stores A process is made up ofData StoresDestination (Place or Name)Process NumberProcess description Should be descriptive, starting with a verb.Can be M for manual or D for computer base data stores.Name of StoreM1
11 Outside EntityIs anything outside the system that is of interest to the system. Can be a person, a company or another system.CustomeraOutside entity shows the Name and a lowercase alpha character is used to uniquely identify it.CustomeraIf an outside entity is repeated for the purpose of neat layout a line is added across the top.
12 Data FlowIs shown by a line with an arrowhead, indicating the direction of the flow of data. Each data flow should be named to indicate what data is being passed. Nouns or adjectives only no verbs are permitted.
13 The Levels Context - Overview - contains only one process Level 1 - Utilises all four elementsLevel 2 - A breakdown of a level 1 processLevel 3 - A breakdown of a level 2 processThere is no rule as to how many levels of DFD that can be used.
14 RulesSequence not important - getting the Process correct isContext or Level 0 - Identifies the system/ boundary/External LinksLevel 1 - Overview of functionLevel 2 - Breakdown to UnderstandHard to know where to stopRule of ThumbIf there are more than 8 data flows break itProcess of Identifying major Processes