Aims and Objectives In this session you will

Aims and Objectives In this session you will be able to: • • • Define ELHs and describe why they are used in systems analysis. Recognise the structure of an ELH. Describe ELH notation. Construct an ELH from a given example. Explain the use of state indicators.

Entity Life Histories • We are still in the Systems Analysis stage, still trying to model a system. • DFDs model the system from the viewpoint of information flows and the processes. ERDs model the system from the viewpoint of the information. Normalisation models the attributes of the system’s entities. • Entity Life Histories (ELHs) model the system from the viewpoint of how the information is changed. This is necessary because we want to provide up-to-date, accurate information.

Entity Life Histories • Entity Life Histories (ELHs) show the full set of changes that can occur to information within the system. • One ELH is drawn for each entity on an ERD and: Provides a visual picture of the life of each, individual entity. The life of an entity is the sequence of events that cause the entity to change right through from creation to deletion. The sequence of events is important because it tracks what happens over a period of time. If we chart this sequence we get a life history. • Models the changes rather than the entity

Entity Life Histories • Before developing an ELH model, we need to know about three components: • System entities. • The events which affect the system entities during their lifetime in the system. When trying to identify events we need to ask: • • • How does the system get to know about this entity? Why does it leave the system? What causes changes to its attributes? • A basic notation for describing graphically the sequence in which events happen.

Entity Life Histories - Notation • • We use ELH notation, after we have identified the entities and events, to record the sequence in which events can occur to an entity. We do not show we identify entities or events - this task is done before developing the ELH diagram. Entity names, group headings, and events. * Event repetition or iteration. Selection between two or more events.

Entity Life Histories - Notation • A typical ELH has the following appearance:

Entity Life Histories - Notation • • • 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. As we see, there are three types of events that must be shown on an ELH: • Birth events. • Death events. • Middle life events - these will be set out in the sequence in which they will logically occur. These events can cause an entity to be: • Created. • Deleted or removed. • Modified (in terms of changes to its attribute values).

Entity Life Histories - Notation • The boxes in the diagram are called nodes. Birth, Iteration of Events, and Death events are all leaf nodes. Middle Life Events is an intermediate node. The root of the hierarchy gives the name of the entity whose life we are analysing. • The passage of time is assumed to be a uniform flow from left to right: • Time zero in the life is where the system gets to know about the entity due to some event/s which create the entity. • The middle life is usually a set of recurring/iterating events causing changes to the entity, and represents most of the life. • Life is ended by some event/s that causes the entity to be deleted from the system.

Entity Life Histories - Control Structures • As well as the basic notation, there is the idea of structure and control of structure which can be shown on an ELH: – – Event Structure Event Sequence Event Selection Event Iteration

Entity Life Histories - Control Structures • Event Structure within the ELH: • Events are presented in the form of a hierarchy of levels, where a higher level box is the parent and its lower level box is the child. Bank Account Opened Account Life Account Closure * Transaction Pay Deposit Direct Deposit Cheque Cashed Account Deletion

Entity Life Histories - Control Structures Event Sequence control within the ELH: • All events shown on the ELH must occur one after the other. • ENTITY X A • B C D In the entity X, A must occur first, followed by B, and so on. This is the only possible sequence (through time - the length of time isn’t indicated however).

Entity Life Histories - Control Structures • Event Selection control within the ELH: • Sometimes there can be a choice of events that may effect an entity. ENTITY X A E • B F C D G As A is at the beginning, this shows that an occurrence of entity X must be created by only one of the three events: E, F, G

Entity Life Histories - Control Structures • Event Iteration control within the ELH: • Sometimes the same event can legitimately occur many times (zero or more) in the life of an entity ENTITY X A E • B F C G D H * After entity X is created by E, F, or G, the event H may affect the entity any number of times - which could also mean none.

Source/Sink Data Store Process DFD ER Entity roughly equates to ELH Entity Event

Entity Life Histories - Example This ELH show that the first event to affect the entity Bank Account is Account Opened. Then, for the life of the account, there is a series of transactions, which can be one of three types of transactions. After an undefined number of transactions, the account will be closed and then deleted.

Addition of State Indicators A state indicator can be thought of as a data item within an entity which is updated every time an event effects it. • The value of the state indicator shows where in its life history an entity is at any one time. • The convention is to start with a value of 1 and increment it by one each time an event effects the entity.

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.

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.

Summary You should now be able to: • • • Define ELHs and describe why they are used in systems analysis. Recognise the structure of an ELH. Describe ELH notation. Construct an ELH from a given example. Explain the use of state indicators.