DATA FLOW DIAGRAMS DFDs

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DATA FLOW DIAGRAMS (DFDs)

• USED TO CHART THE FLOW OF DATA THROUGH AN ACTUAL
  OR POTENTIAL SYSTEM OR ORGANISATION
• THEY SHOW
• EXTERNAL ENTITIES
• DATA FLOWS
• PROCESSES
• DATA STORES
• THEY DO NOT SHOW ENOUGH DETAIL

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DATA DICTIONARIES

• A DATA DICTIONARY (DD) CONTAINS DATA ABOUT DATA.
• A DATA DICTIONARY IS AN ESSENTIAL BACKUP TOOL TO
  DFDs.
• IT MAY BE MANUALLY COMPILED OR
• IT MAY BE AUTOMATED.
• AUTOMATED PACKAGES HAVE SEVERAL ADVANTAGES

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DATA DICTIONARY CONTENTS

• A DATA DICTIONARY SHOULD CONTAIN ENTRIES FOR
• DATA ELEMENTS
• DATA STRUCTURES.
• DATA STORES
• DATA FLOWS
• PROCESSES
• EXTERNAL ENTITIES

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DATA ELEMENTS AND STRUCTURES

• A DATA ELEMENT IS AN ITEM OF DATA THAT IT IS NOT
  MEANINGFUL TO DECOMPOSE FURTHER.
• IT IS THE FUNDAMENTAL BUILDING BLOCK OF THE
  SYSTEM.
• ELEMENTS ARE SELDOM ABSOLUTE
• WHAT IS AN ELEMENT IN ONE SYSTEM MAY NOT BE IN
  ANOTHER.

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DATA ELEMENTS AND STRUCTURES

• DATA STRUCTURES ARE MADE UP OF DATA ELEMENTS
• AND/OR OTHER DATA STRUCTURES
• DATA FLOWS ARE DATA STRUCTURES IN MOTION
• DATA STORES ARE DATA STRUCTURES AT REST.

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DEFINING DATA ELEMENTS.

• NAME A MEANINGFUL, UNIQUE NAME.
• DESCRIPTION SHORT DESCRIPTION OF THE ELEMENT.
• ALIASES ALTERNATIVE NAMES OR TERMS FOR THE
  ELEMENT
• TYPE CHARACTER, NUMERIC OR ALPHABETIC ETC.
• FORMAT 99XX99 OR 9999XX.
• VALUES WHAT IS THE VALID RANGE OF VALUES FOR
  THE ELEMENT.

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DEFINING DATA ELEMENTS.

• SECURITY WHO IS ALLOWED TO MODIFY THE DATA
  ELEMENT
• EDITING HOW SHOULD DATA BE SHOWN E.G. -30, 30
  30CR
• AUTHOR WHO CREATED THIS ENTRY.
• DATE THE DATE THE ENTRY WAS CREATED.
• COMMENTS ANY OTHER USEFUL INFORMATION ABOUT
  THIS DATA ELEMENT

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DEFINING DATA STRUCTURES.

• A DATA STRUCTURE CAN BE DEFINED BY LISTING THE
  ELEMENTS AND/OR STRUCTURES THAT IT CONTAINS.
• THESE WILL BE DOCUMENTED ELSEWHERE IN THE DATA
  DICTIONARY.
• HOWEVER THE LIST MAY CONTAIN ELEMENTS OR
  STRUCTURES THAT ARE
• OPTIONAL
• REPEATED
• MUTUALLY EXCLUSIVE

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DATA DICTIONARY SYMBOLS.

• WHEN WE ARE SHOWING THE STRUCTURE OF A DATA
  STRUCTURE WE NEED SYMBOLS TO SHOW THOSE ITEMS
  WHICH ARE OPTIONAL, REPEATED OR ALTERNATIVES
• OPTIONAL
• ALTERNATIVES
• REPETITION
• NUMBER OF TIMES (2).

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DEFINING DATA FLOWS.

• DATA FLOW NAME
• ALIASES
• DESCRIPTION
• FROM
• TO
• DATA STRUCTURES
• VOLUMES
• GROWTH
• COMMENTS

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DESCRIBING DATA STORES.

• DATA STORE NAME
• ALIASES
• DESCRIPTION
• INBOUND DATA FLOWS
• OUTBOUND DATA FLOWS
• DATA STRUCTURES
• VOLUME
• GROWTH
• ORGANISATION/KEY
• COMMENTS

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DESCRIBING PROCESSES.

• PROCESS NAME
• ALIASES
• DESCRIPTION
• INPUTS
• OUTPUTS
• LOGIC SUMMARY
• COMMENTS

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DESCRIBING EXTERNAL ENTITIES.

• ENTITY NAME
• ALIASES
• DESCRIPTION
• FLOWS FROM
• FLOWS TO
• COMMENTS

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MAINTAINING A DATA DICTIONARY (1)

• TO BE USEFUL A DATA DICTIONARY MUST BE
• UP TO DATE
• COMPLETE
• EASILY ACCESSIBLE.

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MAINTAINING A DATA DICTIONARY (2)

• IT IS OFTEN NECESSARY FOR A DATA DICTIONARY
• BE LOOKED AFTER BY A DATA DICTIONARY
  ADMINISTRATOR OR BY A PROJECT LIBRARIAN.
• A DATA DICTIONARY IS DYNAMIC THE INFORMATION IN
  IT WILL BE BUILT UP OVER A PERIOD OF TIME.

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MAINTAINING A DATA DICTIONARY (3)

• HOWEVER THE INFORMATION FROM PREVIOUS PROJECTS
  MAY BE USEFUL IN FUTURE PROJECTS
• USED WITH DFDs A DATA DICTIONARY IS AN IMPORTANT
  TOOL IN SYSTEMS ANALYSIS AND DESIGN.
• A COMPLEX WEB OF INTER- RELATIONSHIPS EXISTS
  BETWEEN DATA ITEMS. A SINGLE DATA ELEMENT MAY
  APPEAR IN MANY DATA STRUCTURES, FLOWS, STORES AND
  PROCESSES.

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MAINTAINING A DATA DICTIONARY (4)

• THE EFFECT OF AMENDING A DATA ELEMENT HAS TO BE
  'TRACED' THROUGH THE ENTIRE SYSTEM TO IDENTIFY
  POSSIBLE PROBLEMS AND SIDE- EFFECTS. THEREFORE
  IN ALL BUT THE MOST TRIVIAL APPLICATIONS AN
  AUTOMATED DATA DICTIONARY IS REQUIRED.

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AUTOMATED DATA DICTIONARIES(1)

• AN AUTOMATED DATA DICTIONARY HAS SUBSTANTIAL
  ADVANTAGES OVER A MANUAL DATA DICTIONARY.
• IT CAN BE USED FOR THE FOLLOWING PURPOSES.
• TO PRODUCE ORDERED LISTINGS OF ALL ENTRIES OR
  CLASSES OF ENTRY, FULL OR SUMMARY
• TO ENSURE THAT ALL ELEMENTS OR STRUCTURES,
  MENTIONED IN A DATA DICTIONARY DEFINITION, HAVE
  AN ENTRY.

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AUTOMATED DATA DICTIONARIES(2)

• TO ENSURE THAT ALL NECESSARY SECTIONS IN A
  DEFINITION ARE FILLED IN.
• TO ENSURE THAT ALL DATA ELEMENTS APPEAR ON A DATA
  STRUCTURE.
• TO CHECK THAT ALL DATA ELEMENTS OR STRUCTURES
  MENTIONED IN MINI SPECIFICATIONS ARE DEFINED.
• TO FIND THE NAMES OF ITEMS FROM THEIR
  DESCRIPTION.

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AUTOMATED DATA DICTIONARIES(3)

• TO GENERATE MACHINE-READABLE DATA DEFINITIONS
• TO EXTRACT DATA DICTIONARY DEFINITIONS FROM
  EXISTING PROGRAMS.
• TO GENERATE TEST DATA. .
• TO HELP PRODUCE UP TO DATE DOCUMENTATION.