Data-Flow Diagrams Week 10 Lecture 1

Data Flow Diagrams (DFDs) One of most important modelling tools used by system analysts In use since late 1970’s Model –System processes –Data used by these processes –External entities that interact with the system –Information flows in the system

Components of a DFD(1) Processes: –These show what systems do –Each process has: One or more data inputs One or more data outputs –Represented by circles in DFD –Unique name and number that appear inside circle that represents the process

Components of a DFD(2) Files or Data Stores –Repository of data –Data that is retained in the system –Processes can enter data into data store or retrieve data from a data store –Each data store has a unique name –Represented by a thin line or a rectangle (with a label and a name) in the DFD

Components of a DFD (3) External Entities –Outside the system –Either supply input data (a source) or use system output (a sink) –System designer has no control over external entities –May be organisation’s customers –If modelling one section of an organisation, other sections are modelled as external entities

Components of a DFD (4) Data Flows –Model the passage of data through the system –Represented by lines joining system components –Arrow at one end of line denotes the direction of the flow –Flows of data can take place: Between two processes From a data source to a process From a process to a data source From a source to a process From a process to a sink –No data flow between one data source and another

Data Flow Diagrams (DFDs) Process name – action oriented – what the process does Consider the ‘Check Attendance’ process –Data Stores Class list Attendance list Each flow represents a type of data being passed to or from a process DFD doesn’t specify the order of data arriving or being generated

Data Flow Diagrams 1 Check Attendance Class List Attendance List (How is the procedural detail of Check Attendance modelled?)

Data Flow Diagrams 1 Validate Phone number Valid Phone Number Invalid Phone Number Phone Number (Input Flow) (Output flows)

Data Flow Diagrams Student Academic Administration Check Attendance Request Class List Class List Attendance List D1Attendance List Store

Guidelines for constructing Choose meaningful names for processes Number the processes Redraw as often as necessary Avoid complex DFDs Make sure the DFD is logically consistent –Process with input and no output –Process with output and no input

Data Flow Diagrams 1 Enrol Student Applicant Application Forms Invalid Forms (A context Diagram – just one process. Illustrates how external entities interact with this process) Module Selection

DFDs (Enrolling in a University – Level 1 diagram) Applicant 1.1 Inspect Forms 1.2 Input Student Info 1.3 Enrol For Modules Application Forms Invalid forms D1 D2Timetable Student DB Student Info Course Choice Valid Forms Enrolments Module Selection