COM 117 Introduction to Application Building

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COM 117 Introduction to Application Building

• Overview of module
• Lecture on DFDs
• Practical activity
• Tutorial activity

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Timetable
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Data Flow Diagrams
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Introduction

• SSADM
• What is a Data Flow Diagram?
• Why do we use DFDs?
• Levelling
• Conventions
• Decomposition and Abstraction
• The Elements
• Process and Data Stores
• Outside Entity
• Data Flow
• The Levels
• Rules
• The Procedure for Constructing DFDs
• The Document Flow Diagram
• The Context Diagram
• Draw the external entities and data stores
• Level 1 Physical DFD
• Example Hairdressing Salon Level 1 Physical DFD
• Summary

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S.S.A.D.M.

• S.S.A.D.M. - Structured Systems Analysis and
  Design Method
• Uses different techniques to model a system
• Data Flow Diagrams
• Entity Relational Model (Logical Data Stores)
• Normalisation

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What is a Data Flow Diagram?

• Known as DFDs
• A way to model a real world situation
• They model the real world situation which aids
  discussion and clarity amongst users, analysts
  and designers.

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Why do we use DFDs?

• It is a way of taking the physical view and
  converting it into a logical view.
• The physical view - all documents involved
• The logical view - the data they contain
• Their main purpose is to communicate with the
  user, the analysts understanding of the scope of
  the required system

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Levelling

• Levels determine the amount of information shown
• Context diagrams show environment
• Each level shows more info. than the last
• DFDs are expanded or decomposed into levels.
• Separating each process into sub processes
• Uncovers more and more detail

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Conventions

• Balancing
• Process at lower level should have identical data
  flows if they flow out of a process
• Modelling Data Stores
• Only use DATA STORES used within this process on
  the diagram
• Numbering
• 1 - 1.1 - 1.1.1
• 1.2 - 1.2.1
• Labels
• Should carry as much meaning as possible

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Decomposition and Abstraction

• Decomposition - Divide and subdivide into
  manageable size problems
• Abstraction - Concentrate on the important issues
  and ignore the irrelevant

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The Elements

• The four main elements of DFDs notation
• Data Flows, with a label to indicate what data is
  flowing
• Processes, that handle the data
• Data stores, within the system (diary, filing
  cabinet or computer file)
• Outside entities, outside sources of data

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Process and Data Stores

• A process is made up of
• Data Stores

Destination (Place or Name)
Process Number
Process description Should be descriptive,
starting with a verb.
Can be M for manual or D for computer base data
stores.
Name of Store
M1
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Outside Entity

• Is anything outside the system that is of
  interest to the system. Can be a person, a
  company or another system.

Customer a
Outside entity shows the Name and a lowercase
alpha character is used to uniquely identify it.
Customer a
If an outside entity is repeated for the purpose
of neat layout a line is added across the top.
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Data Flow

• Is shown by a line with an arrowhead, indicating
  the direction of the flow of data. Each data
  flow should be named to indicate what data is
  being passed. Nouns or adjectives only no verbs
  are permitted.

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The Levels

• Context - Overview - contains only one process
• Level 1 - Utilises all four elements
• Level 2 - A breakdown of a level 1 process
• Level 3 - A breakdown of a level 2 process
• There is no rule as to how many levels of DFD
  that can be used.

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Rules

• Sequence not important - getting the Process
  correct is
• Context or Level 0 - Identifies the system/
  boundary/External Links
• Level 1 - Overview of function
• Level 2 - Breakdown to Understand
• Hard to know where to stop
• Rule of Thumb
• If there are more than 8 data flows break it
• Process of Identifying major Processes

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Process 3 Level 2
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Hair/Reception
Client a
Clie a
3.1
Hairdresser
Appointment Details
Conduct Appointment
Hair Details
M2
Diary
3.2
Hairdresser
Inform Reception
Change of Hair Details
3.3
Receptionist
Complete Appointment
M3
Client Card
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Naming of DFD processes
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Summary
SSADM What a DFD is Why we use them The
conventions What the elements are Example