SAD - DFD Context Diagrams And Franchise Colleges 03 Data Flow Diagrams - Context Diagrams By MANSHA NAWAZ Section 03 SAD - DFD Context Diagrams

SAD - DFD Context Diagrams 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 Section 03 SAD - DFD Context Diagrams

SAD - DFD Context Diagrams Lecture Outline the nature of models basic notation building a context diagram building an event list Section 03 SAD - DFD Context Diagrams

SAD - DFD Context Diagrams Data Flow Diagrams What Is a Data Flow Diagram? Data Flow Diagrams (DFDs) are a System Modelling CASE 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. Section 03 SAD - DFD Context Diagrams

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). Section 03 SAD - DFD Context Diagrams

Data Flow Diagrams - Yourdon’s Notation Symbol Symbol used if duplicated in diagrams Description Entity – has an input into the system or receives output from it. Does not exist within the system itself Process – represents the processes which must take place inside the system Data store – where information is held over time Data flow – the direction of the arrow shows the direction in which the data flows within the system Section 03 SAD - DFD Context Diagrams

SAD - DFD Context Diagrams 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. a Customer b Sales Dept c Accounts Package 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). a Customer Section 03 SAD - DFD Context Diagrams

SAD - DFD Context Diagrams Process A process is an action on or transformation of data. It is shown as a circle and is identified by a number and name. The name is a description of the data processing taking place. 1 Check Credit Rating 2 Split Order 3 Adjust Stock 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. Section 03 SAD - DFD Context Diagrams

SAD - DFD Context Diagrams Data Flow Date A data flow is shown as a named arrow. Invoice 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. Stock Adjustment Section 03 SAD - DFD Context Diagrams

SAD - DFD Context Diagrams 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. 1 Customers 2 Invoices 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. 1 Customers 1 Stock Stock Level Section 03 SAD - DFD Context Diagrams

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 04 lecture 05 TL-DFD, DFD-0 or Level-0 A systems Sub-Systems View Low-Level Data Flow Diagrams lecture 06 lecture 07 LL-DFD, DFD-1 or Level-1 A sub-systems a Function view Section 03 SAD - DFD Context Diagrams

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 Section 03 SAD - DFD Context Diagrams

SAD - DFD Context Diagrams 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 Section 03 SAD - DFD Context Diagrams

SAD - DFD Context Diagrams Basic Notation III Representing inputs & outputs data that flows between the system and the outside shown as arrows have direction data in – information out named xdetail a XXX The XY System b YYY yreport Should Context Diagrams have datastores? Section 03 SAD - DFD Context Diagrams

SAD - DFD Context Diagrams 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. Section 03 SAD - DFD Context Diagrams

SAD - DFD Context Diagrams 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 Section 03 SAD - DFD Context Diagrams

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 Section 03 SAD - DFD Context Diagrams

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 Section 03 SAD - DFD Context Diagrams

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 04 lecture 05 Low-Level Data Flow Diagrams lecture 06 lecture 07 Section 03 SAD - DFD Context Diagrams

SAD - DFD Context Diagrams Summary & Self Study 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? Section 03 SAD - DFD Context Diagrams