Entity Event Modelling Entity Event Matrix, Entity Life History, Effect Correspondence Diagram

Entity-event modelling An entity may be effected by several events. An event may effect several entities. This can be represented by a matrix (Entity Event Matrix) separately for each entity (Entity Life History) separately for each event (Effect Correspondence Diagram)

First, let’s recap Joe’s Yard has a current logical DFD A set of Requirements A Logical Data structure

Current Logical DFD

Set of New Requirements “We want Joe to have a stock level on his stock, so that we know what needs to be ordered.” No new process “We want Joe to know who his customers are, so that he can tout for repeat business.” New process – mail shot “We want Joe to be able to lodge requested items, so that he can make a decision on whether to stock them in the future or so that he can order them on demand.” New processes – lodge requests, contact customers re requests.

New Logical Data Structure

Entity-event matrix List all entities across the top of the page. List all events (system functions) down the side of the page. Fill in the matrix as follows: If an event creates an entity, mark with C. If an event deletes an entity, mark with D. If an event modifies an entity, mark with M. All entity columns should have at least one C, D and M/R.

Entity / Event Matrix

Entity / Event Matrix

ENTITY LIFE HISTORY An Entity life history consists of a tree, the top node of which is the entity. The next level contains nodes indicating the organisation of events. The almost lowest level contains nodes representing the individual events which change the entity. The lowest level contains the processing operations which achieve the effects of the higher nodes.

SSADM ideal “For each entity, make out an entity life history” In a real situation Identify entities that Effect a lot of other entities Change states a lot

Entity Life Histories SEQUENCE - left to right

SELECTION - either or

ITERATION *

Stock Entity Life History

Standard Payment ELH not Joe’s Yard! Cheque created Issued Removed Payment entered Authorised Printed cashed lost reconciled expired cancelled

Steps in development Make out a 'normal' life: Creation, amendment, deletion Include complications - irregularities Include all known events Check importance of timing of attribute creation / modification / deletion Check alterations to entity's relationships Anything else missing from ELH?

Effect Correspondence Diagrams These are used to ensure that the Entity Life Histories are completed satisfactorily.

Construction List all entities effected (updated) by an event. Draw as for LDM, including only entities and relationships effected by the event. If one event can effect an entity in different ways, use selection boxes, listing the effect roles in the box.

If one event can effect several occurrences of an entity, use an iteration box, describing the set of occurrences in the box. Entities required by the event for enquiry purposes can also be listed, along with the reasons for enquiry and attributes needed. Entities needed more than once for different reasons, will be displayed more than once, but boxed together.