Download week5a

Jackson System
Development
From Models to Networks
SSPD
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Entity Models
• See web site for some JSD notes.
• Events  entities
• Entities are the components of our
model
• e.g., CUSTOMER, VEHICLE,
INVOICE
• Describe using Entity Structure
Diagrams
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Entity Structure Diagrams
The ESD shows the life cycle of each
entity class from birth to death.
VEHICLE
WORKING
LIFE
PURCHASE
HIRING
HIRE
DISPOSE
*
RETURN
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Give events meaning
• The ESDs give a meaning and context
to the events
• We use them to build our real world
model
• They form the basis of our
specification and our system design
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Some Considerations
• Entities with ambiguous/deeply nested
selection trees can be simplified using
backtracking techniques.
• Real world concurrency is not shown
by a single ESD. We need marsupial
entities to solve this problem
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Backtracking
• The normal ESD shows a life history
which doesn’t allow for abnormal
termination.
• Redrawing to allow for exceptional
terminal conditions makes for a messy
and hard to comprehend diagram
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Backtracking Example
• Consider an entity CAR
– We ORDER a car. If the order
confirmation is for the right car then we
take DELIVERy of the car.
– If the right car is DELIVERed then we put
the car in the showroom to SELL it. If the
car becomes damaged then we don’t
SELL it.
– We might draw this as:
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Messy
CAR ESD
CAR
check
ORDER
o.k.
DELIVER
O
RETURN
O
check
o.k.
damage?
O
O
RETURN
RETURN
O
O
SELL
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With backtracking
CAR
ADMIT ?
abnormal
POSIT ?
normal
ORDER
Quit !
Wrong car
DELIVER
SELL
Quit !
Wrong car
RETURN
Quit !
Damaged
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Marsupials
• Sometimes concurrency clouds the
issue
• We need to define new roles for the
entity = emerging marsupial
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SOP Example
Customer
Place
Possible
Amends
Amend
*
Deliver/
Cancel
o
Deliver
Cancel
o
• Problem: PLACE, PLACE
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Hidden Marsupial
• There is an entity hidden inside this
one.
• It’s really just a different role for
customer
• It represents the actions of a customer
as they relate to a single order
• Effectively, reduces data to first
normal form
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Marsupial emerges...
Customer
Order
Place
Possible
Amends
Amend
*
Deliver/
Cancel
o
Deliver
Cancel
o
• Remains to redraw CUSTOMER...
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Redrawn Customer
Customer
Action
Place
o
Amend
o
*
Deliver
o
Cancel
o
• We are now modelling the
concurrency inherent in the real world.
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Structure Text
• The entities can also be represented
by Structure Text (JSP Schematic
Logic).
• Complete the picture with an entity
description for each entity in our
model
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Entity description
Entity BOOK
– Summary: This entity models the life of a copy of a book,
from acquisition by the library through to disposal by
either sale or inter-library swap.
– Action List:
• ACQUIRE
• CLASSIFY
• LEND
• RENEW
• RETURN
• SELL
• WITHDRAW
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Networking
• Models of real world objects and
events complete
• Next draw a network diagram
showing the real world interactions
amongst these objects
• This network will become our system
specification.
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