Bài giảng Systems Analysis and Design - Chapter 7: Using Dataflow Diagrams

Using Dataflow DiagramsSystems Analysis and Design, 7eKendall & Kendall7© 2008 Pearson Prentice HallLearning ObjectivesComprehend the importance of using logical and physical data flow diagrams (DFDs) to graphically depict movement for humans and systems in an organizationCreate, use, and explode logical DFDs to capture and analyze the current system through parent and child levelsDevelop and explode logical DFDs that illustrate the proposed systemProduce physical DFDs based on logical DFDs you have developedUnderstand and apply the concept of partitioning of physical DFDs2Data Flow DiagramsGraphically characterize data processes and flows in a business systemDepict:System inputsProcessesoutputs3Major TopicsData flow diagram symbolsData flow diagram levelsCreating data flow diagramsPhysical and logical data flow diagramsPartitioningCommunicating Using Data Flow Diagrams 4Advantages of the Data Flow ApproachFreedom from committing to the technical implementation too earlyUnderstanding of the interrelatedness of systems and subsystemsCommunicating current system knowledge to usersAnalysis of the proposed system5Basic SymbolsA double square for an external entityAn arrow for movement of data from one point to anotherA rectangle with rounded corners for the occurrence of a transforming processAn open-ended rectangle for a data store6Figure 7.1 The four basic symbols used in data flow diagrams, their meanings, and examples7External EntitiesRepresent another department, a business, a person, or a machineA source or destination of data, outside the boundaries of the systemShould be named with a noun8Data FlowShows movement of data from one point to anotherDescribed with a nounArrowhead indicates the flow directionRepresents data about a person, place, or thing9ProcessDenotes a change in or transformation of dataRepresents work being performed in the systemNaming conventionAssign the name of the whole system when naming a high-level process To name a major subsystem attach the word subsystem to the nameUse the form verb-adjective-noun for detailed processes10Data StoreA depository for data that allows examination, addition, and retrieval of dataNamed with a noun, describing the dataData stores are usually given a unique reference number, such as D1, D2, D3 Represents a:Filing cabinetDatabaseComputerized file11Figure 7.2 Steps in developing data flow diagrams12Creating the Context DiagramThe highest level in a data flow diagramContains only one process, representing the entire systemThe process is given the number 0All external entities, as well as Major data flows are shown13Figure 7.3 Context diagram14Drawing Diagram 0The explosion of the context diagramMay include up to nine processesEach process is numberedMajor data stores and all external entities are included15Drawing Diagram 0 (Continued)Start with the data flow from an entity on the input sideWork backwards from an output data flowExamine the data flow to or from a data storeAnalyze a well-defined processTake note of any fuzzy areas16Figure 7.3 Note the greater detail in diagram 017Data Flow Diagram LevelsData flow diagrams are built in layersThe top level is the Context levelEach process may explode to a lower levelThe lower level diagram number is the same as the parent process numberProcesses that do not create a child diagram are called primitive18Creating Child DiagramsEach process on diagram 0 may be exploded to create a child diagramA child diagram cannot produce output or receive input that the parent process does not also produce or receiveThe child process is given the same number as the parent processProcess 3 would explode to Diagram 319Creating Child Diagrams (Continued)Entities are usually not shown on the child diagrams below Diagram 0If the parent process has data flow connecting to a data store, the child diagram may include the data store as wellWhen a process is not exploded, it is called a primitive process20Figure 7.4 Differences between the parent diagram (above) and the child diagram (below)21Checking the Diagrams for ErrorsForgetting to include a data flow or pointing an arrow in the wrong direction22Checking the Diagrams for Errors (Continued)Connecting data stores and external entities directly to each other23Checking the Diagrams for Errors (Continued)Incorrectly labeling processes or data flowIncluding more than nine processes on a data flow diagram24Checking the Diagrams for Errors (Continued)Omitting data flowCreating unbalanced decomposition (or explosion) in child diagrams25Figure 7.5 Typical errors that can occur in a data flow diagram (payroll example)26Logical and Physical Data Flow DiagramsLogicalFocuses on the business and how the business operatesNot concerned with how the system will be constructedDescribes the business events that take place and the data required and produced by each eventPhysicalShows how the system will be implementedDepicts the system27Figure 7.7 Features common of logical and physical data flow diagrams28Figure 7.8 The progression of models from logical to physical29Developing Logical Data Flow DiagramsBetter communication with usersMore stable systemsBetter understanding of the business by analystsFlexibility and maintenanceElimination of redundancy and easier creation of the physical model30Developing Physical Data Flow DiagramsClarifying which processes are performed by humans and which are automatedDescribing processes in more detailSequencing processes that have to be done in a particular orderIdentifying temporary data stores Specifying actual names of files and printoutsAdding controls to ensure the processes are done properly31Figure 7.10 Physical data flow diagrams contain many items not found in logical data flow diagrams32Event Modeling and Data Flow DiagramsAn input flow from an external entity is sometimes called a trigger because it starts the activities of a processEvents cause the system to do something and act as a trigger to the systemAn approach to creating physical data flow diagrams is to create a data flow diagram fragment for each unique system event33Event Response TablesAn event table is used to create a data flow diagram by analyzing each event and the data used and produced by the eventEvery row in an event table represents a data flow diagram fragment and is used to create a single process on a data flow diagram34Figure 7.12 An event response table for an Internet storefront35Figure 7.13 Data flow diagrams for the first three rows of the Internet storefront event response table36Use Cases and Data Flow DiagramsEach use case defines one activity and its trigger, input, and output Allows the analyst to work with users to understand the nature of the processes and activities and then create a single data flow diagram fragment37Partitioning Data Flow DiagramsPartitioning is the process of examining a data flow diagram and determining how it should be divided into collections of manual procedures and computer programsA dashed line is drawn around a process or group of processes that should be placed in a single computer program 38Reasons for PartitioningDifferent user groupsTimingProcesses may be separated into different programs for securitySimilar tasksEfficiencyConsistencySecurity39Partitioning Web SitesImproves the way humans use the siteImproves speed of processingEase of maintaining the siteKeep the transaction secure40Communicating Using Data Flow Diagrams Use unexploded data flow diagrams early when ascertaining information requirementsMeaningful labels for all data components41SummaryData flow diagramsStructured analysis and design tools that allow the analyst to comprehend the system and subsystems visually as a set of interrelated data flowsDFD symbolsRounded rectangleDouble squareAn arrowOpen-ended rectangle42Summary (Continued)Creating the logical DFDContext-level data flow diagramLevel 0 logical data flow diagramChild diagramsCreating the physical DFDCreate from the logical data flow diagramPartitioned to facilitate programming43Summary (Continued)Partitioning data flow diagramsWhether processes are performed by different user groupsProcesses execute at the same timeProcesses perform similar tasksBatch processes can be combined for efficiency of dataProcesses may be partitioned into different programs for security reasons44