Entity Event Modelling

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Entity Event Modelling

• Entity Event Matrix,Entity Life History,Effect
  Correspondence Diagram

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Entity-event modelling

• An entity may be effected by several events.
• An event may effect several entities.
• This can be represented by
• a matrix (Entity Event Matrix)
• separately for each entity (Entity Life History)
• separately for each event (Effect Correspondence
  Diagram)

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First, lets recap

• Joes Yard has a current logical DFD
• A set of Requirements
• A Logical Data structure

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Current Logical DFD
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Set of New Requirements

• We want Joe to have a stock level on his stock,
  so that we know what needs to be ordered.
• No new process
• We want Joe to know who his customers are, so
  that he can tout for repeat business.
• New process mail shot
• We want Joe to be able to lodge requested items,
  so that he can make a decision on whether to
  stock them in the future or so that he can order
  them on demand.
• New processes lodge requests, contact customers
  re requests.

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New Logical Data Structure
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Entity-event matrix

• List all entities across the top of the page.
• List all events (system functions) down the side
  of the page.
• Fill in the matrix as follows
• If an event creates an entity, mark with C.
• If an event deletes an entity, mark with D.
• If an event modifies an entity, mark with M.
• All entity columns should have at least one C, D
  and M/R.

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Entity / Event Matrix
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Entity / Event Matrix
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ENTITY LIFE HISTORY

• An Entity life history consists of a tree, the
  top node of which is the entity.
• The next level contains nodes indicating the
  organisation of events.
• The almost lowest level contains nodes
  representing the individual events which change
  the entity.
• The lowest level contains the processing
  operations which achieve the effects of the
  higher nodes.

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SSADM ideal

• For each entity, make out an entity life
  history
• In a real situation
• Identify entities that
• Effect a lot of other entities
• Change states a lot

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Entity Life Histories

• SEQUENCE - left to right

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SELECTION

• - either or

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ITERATION

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Stock Entity Life History
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Standard Payment ELH not Joes Yard!
payment
Removed
Issued
Cheque created
reconciled
lost
cashed
Printed
Authorised
Payment entered
expired
cancelled
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Steps in development

• Make out a 'normal' life Creation, amendment,
  deletion
• Include complications - irregularities
• Include all known events
• Check importance of timing of attribute creation
  / modification / deletion
• Check alterations to entity's relationships
• Anything else missing from ELH?

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Effect Correspondence Diagrams

• These are used to ensure that the Entity Life
  Histories are completed satisfactorily.

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Construction

• List all entities effected (updated) by an event.
• Draw as for LDM, including only entities and
  relationships effected by the event.
• If one event can effect an entity in different
  ways, use selection boxes, listing the effect
  roles in the box.

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• If one event can effect several occurrences of an
  entity, use an iteration box, describing the set
  of occurrences in the box.
• Entities required by the event for enquiry
  purposes can also be listed, along with the
  reasons for enquiry and attributes needed.
  Entities needed more than once for different
  reasons, will be displayed more than once, but
  boxed together.