1. 1SSPD Jackson System Development From Models to Networks

2. 2SSPD 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

3. 3SSPD Entity Structure Diagrams The ESD shows the life cycle of each entity class from birth to death. VEHICLEPURCHASEDISPOSE WORKING LIFE HIRING HIRE RETURN *

4. 4SSPD 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

5. 5SSPD 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

6. 6SSPD 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

7. 7SSPD 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:

8. 8SSPD Messy CAR ESD CAR ORDER check DELIVERcheck o.k. damage? SELL O RETURN O o.k. O O RETURN O O

9. 9SSPD With backtracking CAR POSIT normal ADMIT abnormal ORDERDELIVERSELL RETURN Wrong car Damaged Quit ! ! ! ? ?

10. 10SSPD Marsupials Sometimes concurrency clouds the issue We need to define new roles for the entity = emerging marsupial

11. 11SSPD SOP Example Problem: PLACE, PLACE Customer Place Possible Amends Deliver/ Cancel AmendDeliverCancel * oo

12. 12SSPD 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

13. 13SSPD Marsupial emerges... Remains to redraw CUSTOMER... Customer Order Place Possible Amends Deliver/ Cancel AmendDeliverCancel * oo

14. 14SSPD Redrawn Customer We are now modelling the concurrency inherent in the real world. Customer Place Action AmendDeliver Cancel * o ooo

15. 15SSPD 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

16. 16SSPD 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

17. 17SSPD 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.