Data Flow Diagram

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Data Flow Diagram Levelling

• COMM1B
• Information Systems Analysis

2
Level 1 Data Flow Diagrams

• It is not always necessary to start with a
  Document Flow diagram and/or a context diagram
• These are not really DFDs
• But can help identify system boundary - external
  entities, agencies and data.
• IT IS IMPOSSIBLE to produce a complete accurate
  level 1 DFD at the first attempt - further
  investigations are always necessary to complete
  the picture

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Level 1 Data Flow Diagrams

• you decide how much detail to show
• useful to group together processes done in
  functional areas for clarity (personally
  speaking max. 5 or 6 processes approx.)
• must be an accurate and complete picture
• processes descriptions should summarise the
  overall processing
• all DFDs must adhere to strict notation and
  convention rules (forms part of the system
  documentation). We use SSADM

4
Lower Level Data Flow Diagrams

• Processes are expanded (or decomposed) into lower
  layers of detail
• each process separated into sub-processes, then
  sub-sub-processes and so on
• level 2 followed by level 3 and so on (level 3
  usually the lowest)

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Lower Level Data Flow Diagrams

• Lowest level reached when further decomposition
  becomes impossible
• next stage is to write a detailed (elementary)
  process description
• Commonly Structured English, decision trees and
  tables

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Stages ofa Data Flow Model
Level 1
Context (Level 0)
Agency
Overall process
Agency
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Stages ofa Data Flow Model
Level 1
Level 2
1
Responsibility
Major Process description
Responsibility
2
Major Process description
3
Responsibility
Major Process description
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Stages ofa Data Flow Model
Level 3
Level 2
9
Stages ofa Data Flow Model
Level 3
Level 2 process description
2.3
2.3.1 Elementary Process Description Specified
using Decision Tree Decision Table Structured
English
2.3.1
Responsibility
Sub-sub Process Description

2.3.2
Responsibility
Sub-sub Process Description
2.3.3
Responsibility
Sub-sub Process Description
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More Data Flow Diagram Symbols
Data flow in both directions eg.simultaneous read
and update (used occasionally)
A process - which is at its lowest level and will
not be expanded any further

A repeated data store - on the same diagram
M2
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More Data Flow Diagram Symbols
Multiple occurrence of same external entity on
same diagram
a
Customer
a
Use of identifier for entity
Customer
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Context Diagram(level 0)
Accounts
Customer
Goods received
note
Copy delivery
note
Delivery note
Re-order card
Weekly credit
limit list
Order
Order
acknowledgement
Order Processing System
Goods received note
Order Processing System
Supplier
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Level 1Data Flow Diagram
Order Processing System
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Level 2 DFDControl Stock
15
Level 3 DFDMonitor Stock Levels
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Rules and Guidelines

• Data Stores used by more than one process are
  shown on that level diagram

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Rules and Guidelines

• Data Stores used by only one process at that
  level are not shown
• They are internal to that process
• Appear on the next level diagram

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Rules and Guidelines

• Process numbering uses a decimal extension
• Sub-processes from level 1 DFD process 2
• become 2.1, 2.2, 2.3, 2.4 and so onat second
  level
• Sub-processes from level 2 process 2.3
• become 2.3.1, 2.3.2, 2.3.3, 2.3.4 and so onat
  third level

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Rules and Guidelines

• New data stores at lower levels
• inherit process number followed by data store
  number

3
3.1
3.2
M3/1
3.3
M3/1
M3/2
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Rules and Guidelines
Level 1 DFD
Data flows across process boundaries
Level 2 DFD
Must be consistency between levels
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Rules and Guidelines

• All data flows which cross the the process
  boundary at the level above must be drawn
• The only new data flows and data stores are
  internal

Level 2 DFD
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Rules and Guidelines

• Avoid system flowchart
• DFDs do not show sequence or timing
• Data may pass directly from one process to
  another
• However, consider possible data stores

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Checking your Data Flow Model

• If a process has less than four data flows then
  it is probably at its lowest level.
• If a process has approx. eight or more data flows
  then it is probably too busy to be a bottom
  level process, and should be further decomposed.
• Processes that act as a dead end for data flows
  are incorrectly defined - they should transform
  data, not store it. There must be a flow of data
  out
• Processes need data to process. If there are no
  data flows in, it cannot be correct.

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Checking your Data Flow Model

• Data Stores that have flows in (updates) but none
  out (reads) should be checked - is this data not
  being used?
• Data Stores that have no flows going in, only
  flows out, should be checked - is this a
  reference file maintained by another system?
• All data flows on the lowest level diagram should
  be single direction flows.
• ALL detail must be covered. Do not omit or skim
  over details which appear to be trivial
• Users will not fully understand the model
• Important constraints could be missed out of the
  requirements

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Data Flow Diagrams Summary

• A complete set of levelled DFDs form the Process
  Model of a System, in conjunction with elementary
  process descriptions (and sometimes Use Cases)

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Data Flow Diagrams Summary

• You will see many examples of DFDs which do NOT
  conform to the rules and guidelines
• They are not representative of good practice
• Indicative of weak system documentation
• Often demonstrate a lack of understanding of the
  existing/proposed system and can lead to poor
  system design
• Remember - your work will be assessed on its
  quality and accuracy

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Further Reading

• Lejk and Deeks
• An Introduction to Systems Analysis Techniques
  Chapters 4 5 (essential reading)
• M Goodland and C Slater
• SSADM version 4 a Practical Approach McGraw Hill
  1995
• Philip L Weaver
• Practical SSADM version 4. Chapters 2 and 3.
  Pitman Publishing.