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Lecture 3: Introducing Data Flow Diagrams (DFDs). Section 1 - The Concept of Diagrams Why use Diagrams? Diagrams as Working Documents Systems Analysis Modelling Techniques. Why Use Diagrams?. To help overcome the communication problem between users and developers.

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Lecture 3:Introducing Data Flow Diagrams (DFDs)

Section 1 - The Concept of Diagrams

  • Why use Diagrams?

  • Diagrams as Working Documents

  • Systems Analysis Modelling Techniques


Why use diagrams
Why Use Diagrams?

  • To help overcome the communication problem between users and developers.

  • Use of natural language, written or spoken open to

    • misinterpretation

    • ambiguities

    • omissions

  • Diagrams

    • help communication

    • help understanding

    • are unambiguous


Why use diagrams cont d
Why Use Diagrams (Cont’d)

DEVELOPERS (have discussions with users etc)

CREATE DIAGRAMS (to record understanding)

AMEND DIAGRAMS (with further information)

USE DIAGRAMS as the

 BASE OF SYSTEM STRUCTURE


Diagrams as working documents
Diagrams as Working Documents

  • Diagrams are working documents:

    • changes will occur (for correctness)

    • changes must be documented and tracked

  • Change is integral to the development process.


Systems analysis modelling techniques examples
Systems Analysis - Modelling Techniques (Examples)

  • Data Flow Diagrams

  • Entity Relationship Diagrams

  • Normalisation

  • Entity-Life Histories


Lecture 3:Introducing Data Flow Diagrams (DFDs)

Section 2 - Data Flow Diagrams (DFDs)

  • Data Flow Diagramming

  • DFD Notation

  • DFD Levelling

  • Guidelines for Drawing DFDs

  • Connectivity & Validation


Data flow diagramming
Data Flow Diagramming

  • Business users normally define their business operation in terms of the processes of that operation.

A process may be defined as an action or series of

actions which produce a change or development.

  • The process view of a system may be modelled by a Data Flow Diagram (DFD).


Data flow diagramming cont d
Data Flow Diagramming (Cont’d)

  • DFDs concentrate upon the data needed to support the information requirements of a system:

    • what data is needed

    • the processes that convert it.

  • Data flow diagrams provide:

    • the notion of structure

    • static pieces of documentation

    • communication tool

  • DFDs are one of the most powerful and useful techniques available to the systems analyst.


Dfd notation
DFD Notation

A DFD has four key components.

External Entity

Process

Data Store

Data Flows


Processes
Processes

  • Processes are the individual tasks which when completed in a certain sequence fulfil the overall goal of the system of which they are a part.

2

Sales Clerk

Validate

Customer Order

Processes transform input into output.


Data flows
Data Flows

  • Data Flows depict the fact that some data in the form of documents, phone calls etc is moving.

  • From an external entity to a process (or vice versa).

  • From one process to another.

  • From a process to a store (or vice versa).

invoice

address


Data flows cont d
Data Flows (Cont’d)

Write data to a file

Take data from a file


Data flows cont d1

Customer

order

1

Sales Clerk

Validate

Order

new_order

part_number

D1

Orders

D2

Stock

Data Flows (cont’d)

  • Data flow names should be different if the form of their data elements changes on entry to and exit from a process.


Data stores
Data Stores

D1

Stock

  • Data Stores contain data which is needed by a process in order for it to be completed.

  • These show data ‘at rest’ within the system.

  • They carry a description and are numbered D1, D2, D3 etc (computer data) or M1, M2, M3 etc (manual files).


External entity
External Entity

Customer

  • External entities exist in the system’s environment and either provide data to the system (sources) or receive data from the system (sinks).

  • They are usually people, places, departments, organisations etc.


Dfd levelling
DFD Levelling

  • DFDs allow the analyst to look at the system at different levels of detail.

  • A business operation may contain many processes.

  • The inclusion of all processes on a single diagram can:

    • make it look cluttered

    • make it difficult to see exactly what a process does

  • To overcome this it is usual to ‘break down’ the diagram, a process known as levelling.


Dfd levelling see lejk deeks p67
DFD Levelling (See Lejk & Deeks p67)

Context

Level 1

Level 2

1

2.1

External

Entity

Sub-process

Process

2

2.2

Overall

Process

Sub-process

Process

3

2.3

External

Entity

Sub-process

Process


Context diagrams overview or level 0
Context Diagrams (Overview or Level 0)

  • Represent the system at a high level of detail.

    Comprised of:

  • One single process box for the entire system.

  • External entities.

  • The data flows that pass between the external entities and the system.


Context diagrams cont d
Context Diagrams (cont’d)

Purpose:

  • to identify and examine the interfaces between the external entities and the system.


Example Context Diagram

Customer

order

Simple Order

System

invoice

delivery_details

Customer


Level 1 diagrams
Level 1 Diagrams

Show:

  • the system in more detail

  • how data enters the system

  • how these data items are transformed by the processes

  • how they leave the system

  • A Level 1 diagram must have the same number of inputs and outputs as its context diagram.

  • The flows are connected to and from the actual processes which create, receive or change them.

  • Processes are numbered 1, 2, 3 etc on a Level 1 diagram.


  • Guidelines for drawing a context diagram
    Guidelines for Drawing a Context Diagram

    • Read the case study a number of times.

    • Try to list potential external entities.

    • Establish what flows are sent to the system from the external entities.

    • Establish what flows are sent from the system to the external entities.

    • Draw the Context diagram.


    Guidelines for drawing a level 1 diagram
    Guidelines for Drawing a Level 1 Diagram

    • Taking one sentence at a time try to identify potential processes (look for verbs).

    • Identify and list the data flows.

    • Identify and list the data stores.

    • Draw the Level one diagram (using the correct notation).


    Connectivity validation
    ‘Connectivity & Validation’

    • Make sure the Level 1 diagram is fully connected.

    • That is, a process must receive inputs from other processes or from data stores to be triggered.

    • Check the Level 1 diagram against the context diagram for consistency:

      • flows across the boundaries must be the same

      • names must be the same


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