Jackson System Development
From Models to Networks
See web site for some JSD notes.
Entities are the components of our model
e.g., CUSTOMER, VEHICLE, INVOICE
Describe using Entity Structure Diagrams
Entity Structure Diagrams
The ESD shows the life cycle of each entity class from birth to death.
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
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
The normal ESD shows a life history which doesnít allow for abnormal termination.
Redrawing to allow for exceptional terminal conditions makes for a messy and hard to comprehend diagram
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 donít SELL it.
We might draw this as:
Messy CAR ESD
Sometimes concurrency clouds the issue
We need to define new roles for the entity = emerging marsupial
Problem: PLACE, PLACE
There is an entity hidden inside this one.
Itís 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
Remains to redraw CUSTOMER...
We are now modelling the concurrency inherent in the real world.
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
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.
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.