Systems Analysis & Design Data Flow Diagrams

End Home Data Flow Diagrams – Definition  A data flow diagram is a pictorial model that shows the flow of data between the various subsystems or processes of a system  The purpose of the data flow diagram is to highlight… the functions of a system and to show how the system stores information and the how information is transferred between data stores

End Home Data Flow Diagrams – Definition(2) A data flow diagram shows how data flows around a system An external source of data Some Process A System Abstracted to DFD Data Storage

End Home Data Flow Diagrams – Logical/Physical  Logical DFDs o Show how the business operates o Show processes independently of the implemented system  Physical DFDs o Show how the system operates or how the system will be implemented o Processes are not independent of the implemented system

End Home Data Flow Diagrams – General A data flow diagram is an important analysis tools for analysing data-oriented systems In a DFD, data is organized in several ways. We have… data stores data flow data flow processes processes data transfer to or from the external environment data transfer to or from the external environment

End Home Data Flow Diagrams – External Entity External Entity The source or destination of the data outside the system  This may be a person or organisation, outside the system boundaries that supplies data to the system or receives data from the system

End Home Data Flow Diagrams – Data Flow Data Flow Data flow is data or information that is transferred to or from a process (input or output)  The data flow name must be a noun  a data flow must be input to a process or output from a process

End Home Data Flow Diagrams – Data Process Data Process A process is an activity of the system  A process name should be either a subsystem name or a kind of verb-adjective-noun for describing the process System e.g. Southwark Library System Subsysteme.g. Student Loan Subsystem Verb-adjective-noune.g. Update Loan Info  every process must have at least one input and one output data flow

End Home Data Flow Diagrams – Data Stores Data Store A data store is an inventory of data (i.e. data that is stored for later use)  The data store name must be a noun  in physical DFD’s, the format of storage may also be included

End Home Data Flow Symbols  External source or destination data   Process that transforms data   File or other data storage   Flow of data A data flow diagram can consist of four symbols

End Home Data Flow Symbols – External Entity A square rectangle indicates an external entity that sends data into the system, or receives data from the system. The entity may be a person or organisation that is not part of the system. E1 Customer Identifier Name

End Home Data Flow Symbols – Data Flow A line represents a route by which data flows from one element to another Customer Details

End Home Data Flow Symbols – Data Process A rounded rectangle represents a process that the system performs on the data. Processes are the changing of data by performing calculations, updating or sorting, either manually or by computerisation. Location Identifier Process Description P1Accounts Verify customer details

End Home Data Flow Symbols – Data Store  The open rectangle indicates stored data. It is referred to as a data repository or data store. Examples of data stores are filing cabinets, floppy disks, magnetic tape, paper records. S1Inventory Name Identifier

End Home Valid Data Flow  External entity to process E.G. Data flowing into the system E.G. Data flowing into the system and vice versa  Process to external entity E.G. Data flowing out of the system

End Home Valid Data Flow  Process to data store E.G. Writing data to a disk E.G. Writing data to a disk and vice versa  Data store to process E.G. Reading data from a disk

End Home Valid Data Flow  Process to Process E.G. Passing data from one process to another E.G. Passing data from one process to another

End Home Data Flow Diagrams – Rules All data flows to stores must come from a process External Entity Data Store

End Home Data Flow Diagrams – Rules  No Miracles Process A miracle is a process with no input

End Home Data Flow Diagrams – Rules  No Black Holes Process Data Storage A black hole is a process with no output

End Home Data Flow Diagrams – Common Errors  Miracle: Process with no input  Black Hole: Process with no output  Data flows not named  Processes that leave the data unchanged  Data flows to processes that are neither inputs or outputs.

End Home Data Flow Diagrams – Guidelines  Place external entities on the outside of the diagram  Place data stores and processes in the middle of the diagram  Try not to cross flow lines  Do not duplicate processes, external entities or data stores  For each individual data flow fragment, try to fit the diagram on one page.

End Home Data Flow Diagrams – Example Library System Data Flow Diagram Fragment P1 Joining Library CustomerE1 Confirmation Details Library Joining Details Update Database Borrowers DatabaseS1