Data Flow Diagrams

Introduction Data Flow Diagrams describe how information is used to produce the functions that are required by the current system what information is provided to the system and what information is provided from the system who is using the system what data will be stored the boundaries of the system to be built

Data Flow Diagrams (DFD) DFDs describe the flow of data or information into and out of a system what does the system do to the data? A DFD is a graphic representation of the flow of data or information through a system Use example of borrow library book here to illustrate the different components. Student is external passes in request (book and student details) into validate student process output valid student request into reserve book process. Two data stores, students and loans. Output back to student is book; Case study - Just A Line system BCS - GP1 3

Case study - Just A Line system Use example of borrow library book here to illustrate the different components. Student is external passes in request (book and student details) into validate student process output valid student request into reserve book process. Two data stores, students and loans. Output back to student is book; Case study - Just A Line system BCS - GP1 3

4 Main Elements external entity - people or organisations that send data into the system or receive data from the system process - models what happens to the data i.e. transforms incoming data into outgoing data data store - represents permanent data that is used by the system data flow - models the actual flow of the data between the other elements

Notation Data Flow Process External Entity Data Store Data Flow Process box External Entity D Data Store

Case study - Just A Line system Where/who Process identifier Process name Use example of borrow library book here to illustrate the different components. Student is external passes in request (book and student details) into validate student process output valid student request into reserve book process. Two data stores, students and loans. Output back to student is book; Case study - Just A Line system BCS - GP1 3

Data store description Data store identifier Data store description Use example of borrow library book here to illustrate the different components. Student is external passes in request (book and student details) into validate student process output valid student request into reserve book process. Two data stores, students and loans. Output back to student is book; Case study - Just A Line system Applicant’s name Employee record Payment Customer details 3 BCS - GP1 3

Levelled DFDs Even a small system could have many processes and data flows DFDs could be large and messy use levelled DFDs - view system at different levels of detail one overview and many progressively greater detailed views sides of context process box = system boundary Example context diagram for library, inputs= book request, book return and output = book; external entity = student. Go through the case study - context Fig 3.4 pg 32; level 1- Fig 3.5 pg 32 level 2 - Fig 3.6 pg 33 4 BCS - GP1 4

Context diagram (level 0 DFD) Box represents whole system boundary Context diagram (level 0 DFD) Level 1 DFD Level 2 DFD

Level 0 - Context Diagram models system as one process box which represents scope of the system identifies external entities and related inputs and outputs additional notation - system box

Level 0 Diagram Shows all the processes that comprise the overall system Shows how information moves from and to each process

Level 1 - overview diagram gives overview of full system identifies major processes and data flows between them identifies data stores that are used by the major processes boundary of level 1 is the context diagram

Level 1 Diagrams Shows all the processes that comprise a single process on the level 0 diagram Shows how information moves from and to each of these processes Shows data stores Shows in more detail the content of higher level process

Case study - Just A Line system Use example of borrow library book here to illustrate the different components. Student is external passes in request (book and student details) into validate student process output valid student request into reserve book process. Two data stores, students and loans. Output back to student is book; Case study - Just A Line system BCS - GP1 3

Level 2 - detailed diagram level 1 process is expanded into more detail each process in level 1 is decomposed to show its constituent processes boundary of level 2 is the level 1 process

Level 2 Diagrams Shows all processes that comprise a single process on the level 1 diagram Shows how information moves from and to each of these processes Level 2 diagrams may not be needed for all level 1 processes Correctly numbering each process helps the user understand where the process fits into the overall system

Types of DFD Data Flow Diagrams are either Logical or Physical Logical DFD - This type of DFD concentrates on the system process, and flow of data in the system. A logical diagram shows how the business operates but not how the system will be constructed. The logical diagram omits details of how the more physical aspects are implemented. Physical DFD - This type of DFD shows how the data flow is actually implemented in the system. Physical model deals with the way that information is stored, the manual procedures used, the necessity for temporary data collections etc.

DFD Samples

Level 0 – Food Ordering System https://www.visual-paradigm.com/tutorials/data-flow-diagram-example-food-ordering-system.jsp Level 0 – Food Ordering System

Level 1 – Food Ordering System