slide1
Download
Skip this Video
Download Presentation
Lecture 3: Introducing Data Flow Diagrams (DFDs)

Loading in 2 Seconds...

play fullscreen
1 / 24

Lecture 3: Introducing Data Flow Diagrams (DFDs) - PowerPoint PPT Presentation


  • 146 Views
  • Uploaded on

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.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Lecture 3: Introducing Data Flow Diagrams (DFDs)' - clio


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1
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
slide6
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.
slide20
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
ad