Process Modelling and Entity Modelling Cross Checks

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Process Modelling and Entity Modelling Cross
Checks
Information System AnalysisCOMM1B
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Entity Relationship Model
CUSTOMER
be issued with
be issued to
ESTIMATE
contain
become
be derived from
ORDER
made up of
appear on
belong to
refer to
ORDER LINE
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Representing Data in a System

• Entities in a system must have data stored about
  them
• otherwise they are not entities
• these are shown as Entities on Data Models

• But process models must use this data.
• DFDs have data stores where the data is kept.
• Shown on level 1 DFDs or lower level diagrams.

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Level 2 DFD
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Entity/Data Store Cross-reference

• For physical reasons ie. Because of the way in
  which the current system actually operates, a
  data store in your DFD may represent more than
  one entity
• For example the Customer file (a card wallet)
  will have detailed information about each
  customer. But it also contains estimates for each
  of the customers (which can become orders)

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Entity/Data store Cross Reference
M1
M3/1
M2
M3.3/1
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Entity/Data store Cross Reference
Entities
Data Store M1 Customer File M2 Orders
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Entity / Data Store Cross-reference

• Data is often duplicated in existing systems
• eg. details about stock items are kept in the
  stock file.
• For re-ordering purposes details about stock
  items are also kept in the re-order card file
• So an entity may be represented in more than one
  data store on your DFDs

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Level 3 DFD
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Entity/Data Store Cross-reference

• Data Store
• M3/1 Stock File

• Entities

Stock Category
M3.3/1 Re-order Cards
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Constructing an Entity/Data Store
Cross-reference

• List every data store shown on Data Flow Model
  (all levels)
• Reproduce extracts from entity relationship model
  (logical data structure) showing groups of
  entities which have details stored about them in
  each of the data stores

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Entity/Data Store Cross-reference

• Entities

• Data Store
• M1 Customer File
• M2 Orders

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Entity/Data Store Cross-reference

• Data Store
• M3/1 Stock File

• Entities

M3.3/1 Re-order Cards
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Entity/Data Store Cross-reference

• Ensure that each entity is represented by at
  least one data store
• Ensure that each data store represents at least
  one entity
• This cross-check helps identify any discrepancies
  between the process model and the data model of
  the system

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Further Reading

• Lejk and Deeks
• An Introduction to Systems Analysis Techniques
• Chapter 9 Logicalisation
• Philip L Weaver
• Practical SSADM version 4 Pitman Publishing
• Chapter 3 p 112-115