Introduction - Entity Life Histories ELHs

1. Introduction - Entity Life Histories (ELHs) ELHs model the system from the viewpoint of how information is changed Basic concepts Notation Examples

2. Entity Life Histories (ELHs) DFDs model the system from the viewpoint of processes and information flows. ERDs model the system from the viewpoint of the information structure. Entity Life Histories (ELHs) model the system from the viewpoint of how the information is changed.

3. Entity Life Histories (Cont�d) ELHs show the full set of changes that can occur to the information within the system. (ELHs): provide a visual picture of each entity (in isolation). put a time structure on every event that may effect an entity throughout its existence in the system. put a sequence on the processes in the DFDs. One ELH is drawn for each entity on the ER diagram. Third point (italics) - events may be inferred from data flows on DFDs, on ELHs they are made specific.Third point (italics) - events may be inferred from data flows on DFDs, on ELHs they are made specific.

4. Entity Life Histories (ELHs) Events when they occur are not totally at random. The sequence of events is important, things happen over a period of time. Charting this valid sequence gives a life history. ELHs show events in sequence: the things that happen which cause the system to do something which changes the state of the data. Point 1 examples: A life insurance policy would not be paid out until a proposal had been accepted, a premium paid, and the insured person had died. If you want to draw money from your bank account, you can�t do it unless you have first opened an account and deposited sufficient money.Point 1 examples: A life insurance policy would not be paid out until a proposal had been accepted, a premium paid, and the insured person had died. If you want to draw money from your bank account, you can�t do it unless you have first opened an account and deposited sufficient money.

5. Entity Life Histories (Cont�d) Each event that can effect an entity in any way during its life in the system is entered on the diagram (in the form of a hierarchy). Events are shown that: initially create an entity (its birth) occur during its life in the system, set out in the sequence in which they will logically occur (middle life) finally remove an entity from the system (its death). Point 2 - as far as the data is concerned, these events equate with additions to the system, amendments and deletions.Point 2 - as far as the data is concerned, these events equate with additions to the system, amendments and deletions.

6. Basic Structure of an Entity Life Most events in a DP system are initiated in the outside world. However, events can be triggered by things other than data coming into a system. A state of the data causing something to happen: amount owed to a supplier is now greater than the value of credit notes held, a cheque must be issued. four weeks since a book was due for return, a reminder must now be issued. A time: last day of the month, send out statements, 4.00pm print cheques. Most events in a DP system are initiated in the outside world. However, events can be triggered by things other than data coming into a system. A state of the data causing something to happen: amount owed to a supplier is now greater than the value of credit notes held, a cheque must be issued. four weeks since a book was due for return, a reminder must now be issued. A time: last day of the month, send out statements, 4.00pm print cheques.

7. An Entity Life Passage of time is a uniform flow from left to right of the diagram. At time zero the system gets to know about the entity by an event which results in the creation of the entity. The middle life is a set of recurring or iterative events causing changes to the entity. This period represents the majority of the life of the entity in the system. Eventually the life is terminated by the arrival of a �death� event causing the entity to be removed from the system.

8. ELH Constructs There are three different constructs which can be shown on an ELH: sequence selection iteration The development of ELHs and their corresponding constructs is a complex matter. Anything beyond a cursory overview is beyond the scope of this course today. As such h we will not consider parallel structures, quits and resumes or null eventsThe development of ELHs and their corresponding constructs is a complex matter. Anything beyond a cursory overview is beyond the scope of this course today. As such h we will not consider parallel structures, quits and resumes or null events

9. ELH Constructs - Sequence All events shown on the ELH must occur one after the other. The box labeled A will always be the first to occur, followed by B, which is followed by C, which is followed by D. This is the only possible sequence. Can think of it as a progression through time, but no indication of time is given (minutes, hours, days or even years).The box labeled A will always be the first to occur, followed by B, which is followed by C, which is followed by D. This is the only possible sequence. Can think of it as a progression through time, but no indication of time is given (minutes, hours, days or even years).

10. ELH Constructs - Selection Sometimes there is a choice of events that may effect an entity - selection. Defines nodes that are alternatives to one another at a particular point in the ELH. One and only one possible event will occur at a particular time in a sequence. As node A is at the beginning, an occurrence of Entity X must be created by one of three events: E, F or G.Defines nodes that are alternatives to one another at a particular point in the ELH. One and only one possible event will occur at a particular time in a sequence. As node A is at the beginning, an occurrence of Entity X must be created by one of three events: E, F or G.

11. ELH Constructs - Iteration (Repetition) Sometimes the same event can legitimately occur many times (zero or more) in the life of an entity - iteration. An iteration is where an effect or node may be repeated any number of times at the same point within an ELH. Each iteration must be complete before the next begins. After entity X has been created by E, F or G, the event H may effect the entity any number of times. �Any number of times� includes none.An iteration is where an effect or node may be repeated any number of times at the same point within an ELH. Each iteration must be complete before the next begins. After entity X has been created by E, F or G, the event H may effect the entity any number of times. �Any number of times� includes none.

12. ELH Constructs - Structure A higher level box is the parent and its lower level box is the child: all boxes with the same parent must be of the same type e.g. sequence, selection or iteration. if this is not possible new levels must be introduced. Events are represented at the lowest level.Events are represented at the lowest level.

13. Entity Life Histories An entity/event matrix is a useful grid that is often constructed before the ELHs are drawn. it is possible for each entity on the ERD to be described indicating events associated with it. More explicitly, for each event it is possible to identify Create, Modify and Death events. It is a matrix showing the relationship between events on an ELH and entities from the ERD

14. Entity/Event Matrix

15. Entity/Event Matrix The production of such a matrix has a number of associated benefits: It acts as a check that every entity has a Create and Delete event. It forces consideration to be given to all things that can happen to a particular entity. It adds an additional perspective to systems analysis, hence ensuring the process is complete. It ensures that all the entities that are affected by one event are considered together.

16. Entity/Event Matrix Guidelines Identify all Events for a given Entity Identify Creation Events (C) Identify Deletion Events (D) Identify Middle life Events (M) Add selection or iterations where appropriate.

17. Addition of State Indicators The value of the state indicator shows where in its ELH an entity is at any one time. The values given to the state indicator have no significance provided that each effect assigns a unique value to it. The convention is to start with a value of 1 and increment it by one each time an event effects the entity.

18. State Indicators (Cont�d) The first step is to assign a value to each of the effect boxes on the ELH: represents the value that the state indicator has been set to after the event has finished effecting the entity occurrence. The state indicator values can be used to help in determining whether or not we can allow an event to effect an entity. For each effect box we can define a set of values of the state indicator that are valid before the event can be allowed to effect the entity.

19. State Indicators State indicator = valid previous values/set to value Definition of state indicator: set to value :- 4 valid previous values :- 2 or 3 state indicator = 2,3/4

20. State Indicators (Cont�d) Creation effects have a null valid previous value. Where there is a selection, all effects that are alternatives to one another have the same valid previous state indicator values. Where there is an iteration, the value set by the repeating effect will also be included as one of the valid previous values for that effect. Death events have no set to value.

21. Example 2

22. Example 1

26. State Indicators

29. Example 3 Imagine that Maurice Moneybags has decided he wants to open a bank account at Cash & Grabbs Bank. When Maurice has persuaded Mr Cash, the manager, that he would be suitable as a customer, Mr Cash turns to his computer terminal and records Maurice's new bank account code in the system. The ER diagram of the bank computer system contains an entity called Bank Account. The event occurrence that creates the Maurice Moneybags occurrence of the entity Bank Account is the opening of the account by Mr Cash. At some point the account is closed and then deleted.

30. Entity/Event Matrix Entity/event Matrix for Bank Account

31. Example - ELH (Bank Account)

32. Example 4 Small College A small college wishes to maintain an information system relating to students. To enter the system, a student must enrol. Just after enrolment each student is assigned a personal tutor. Over the duration of their time at college, a student may change his/her address, change his/her name, or even change course. When a student changes course s/he must enrol on the new course and be re-assigned a personal tutor. A student may graduate, fail, or withdraw. Construct a life history for the STUDENT entity (include state indicators).