03 Data Flow Diagrams - Context Diagrams

1. And Franchise Colleges 03 Data Flow Diagrams - Context Diagrams • By MANSHA NAWAZ SAD - DFD Context Diagrams

2. Learning Aims • Aim • to construct a context diagram for a proposed computerised information system • Objectives • to identify and represent the whole system • to identify and label external entities • to identify and label system in/outputs • to compile a consistent event list SAD - DFD Context Diagrams

3. Lecture Outline • the nature of models • basic notation • building a context diagram • building an event list SAD - DFD Context Diagrams

4. Data Flow Diagrams • What Is a Data Flow Diagram? • Data Flow Diagrams (DFDs) are a System ModellingCASE Tool • DFDs are graphical views used to represent the system, sub-systems and functional view (systems decomposition). • System Model is depicted by a series of DFD’s • Used to model a proposed system • DFD’s show how data travels within a system. • flows of data to and from external entities such as people, departments and organisations outside the system. • flows of data to and from a systems processes • show the flows to and from data stores within the system area. SAD - DFD Context Diagrams

5. Types of Data Flow Diagram Stage 1 – at the beginning of the systems analysis phase – current physical DFDs to represent graphically the current system. Stage 2 – as part of the systems analysis phase – current logical DFDs to depict the current system, without reference to where things happen or who does what. Stage 3 – also during the systems analysis phase – required logical DFDs including the new requirements. Stage 4 – as part of the systems design phase – required physical DFDs showing how the new system will be implemented. • Many methods make use of DFDs for some, or all, of these purposes. • We’re going to look at how to use DFDs to model systems at a logical level using Yourdon notation for new systems (focus on Stage 3 & 4). SAD - DFD Context Diagrams

6. Data Flow Diagrams - Yourdon’s Notation SAD - DFD Context Diagrams

7. a Customer Entity • An external entity(terminator or source) is something outside the system. It is shown as a square and is identified by a lower case letter and name. b Sales Dept c Accounts Package a Customer • Eg people, departments, organisations that provide information to the system (source) or receive information from the system (sink). • An external entity can be a source and a sink. • If external entities have to be duplicated, an • angled line is put in the corner of the box (all copies). SAD - DFD Context Diagrams

8. 1 Check Credit Rating 2 Split Order 3 Adjust Stock Process • A process is an action on or transformation of data. It isshown as a circle and is identified by a number and name. The name is a description of the data processing taking place. • The description should be an imperative statement, consisting of an active verb and object. • The description does not have a subject, ie we don’t say who carries out the process or where. • This helps our logical view of the system. SAD - DFD Context Diagrams

9. Invoice Date Stock Adjustment Data Flow • A data flow is shown as a named arrow. • The name of the data flow is the name of the piece of data passing between the 2 connected symbols. • The direction of the arrow shows the direction in which the data is flowing. • Two way flows of data are best shown by 2 arrows, but may be shown by one double-headed arrow. SAD - DFD Context Diagrams

10. 2 Invoices 1 Customers 1 Customers Stock Level 1 Stock Data Store • The final symbol is the data store, represented by parallel lines, identified by a number and name. The name is usually in the plural. • The data in a data store is data at rest (persistent data), ie a file or database. • If data stores have to be duplicated, a vertical line is added to the left (all copies). • When data is written to a data store, the data flow is shown going in. • When data is read from a data store, the data flow is shown coming out. • If data is being read and written, a double-headed arrow can be used. SAD - DFD Context Diagrams

11. System modelling using DFD’s • There are many ways to go about drawing DFDs models for system representation. • We will focus on 3 level system views • Context Diagram (CD) lecture 03 • System View • Top-Level Data Flow Diagram lecture 04lecture 05 • TL-DFD, DFD-0 or Level-0 • A systems Sub-Systems View • Low-Level Data Flow Diagrams lecture 06lecture 07 • LL-DFD, DFD-1 or Level-1 • A sub-systems a Function view SAD - DFD Context Diagrams

12. Context Diagrams (CD)Basic Notation I Representing the proposed system • whole system view • a process is shown as a circle • the system is represented as a single process • it is named The XY System SAD - DFD Context Diagrams

13. Basic Notation II Representing external entities • outside will be agencies that react with the system • who or what is going to communicate with our system • shown as squares • numbered a..z • named a XXX The XY System b YYY SAD - DFD Context Diagrams

14. Basic Notation III xdetail Representing inputs & outputs • data that flows between the system and the outside • shown as arrows • have direction • data in – information out • named a XXX The XY System b YYY yreport Should Context Diagrams have datastores? SAD - DFD Context Diagrams

15. Context Diagram (CD) • Yourdon also proposes the notion of a CD • This is a very high level diagram consisting of one process that represents the whole system, and all the external entities that interact with it. • The purpose of such a diagram is to show clearly all a system’s interfaces with its environment. • No stores appear on this diagram. SAD - DFD Context Diagrams

16. The Event List • things happen outside of the system • those that the system must respond to are termed events, these are listed • each event on the list corresponds to an input on the context diagram • An XXX sends in an xdetail • the exception are temporal events SAD - DFD Context Diagrams

17. Building Context Diagrams • Draw one process to represent the system • Name it appropriately • “Who or what does the system interact with” • Draw each of these entities onto the diagram • Identify the dataflow required by the system – Data in - Information out. • Do not expect to obtain all entities or dataflow on your first draft. • Expect a number of revisions – the more time you spend the more awareness you have on the system SAD - DFD Context Diagrams

18. Development : Context Diagram • Context Diagram (CD) • Provide a supporting text description of the system view as a introduction to the CD • Provide the CD • Event List • List all events and make sure it is consistent with your context diagram SAD - DFD Context Diagrams

19. Document : Analysis Specification • Terms Of Reference lecture 02 • Context Diagram lecture 03 • Context Diagram Support Text • Context Diagram • Event List • Top-Level Data Flow Diagram lecture 04lecture 05 • Low-Level Data Flow Diagrams lecture 06lecture 07 SAD - DFD Context Diagrams

20. Summary Structured methods arose to meet the problems of growth Context Diagrams show system and its interaction with the outside world Self Study review the lecture complete 3a & b from the course workbook can you achieve the objectives set out in slide 2? Summary & Self Study SAD - DFD Context Diagrams