Week 4 - Beginning Analysis

System analysts describe information systems using with the help of a collection of a collection of models. Complex systems may require more than one type of models and simple systems may require only one model to be represented. The process of creating models helps the systems analysts to clarify and refine their design. Models assists communication with system users.

Modelling in systems analysis and design is done for various different reasons. First and foremost a lot of learning is made from the modelling process and it also reduces complexity by obstraction. Systems analysts will also be helped by modelling to remember all the details of the system and communication between the development team members is also enhanced. Communication between between a number of stakeholders and users is also facilitated by the use of modelling. Finally modelling allows documenting what was done for future maintenance and enhancement.

DIfferent types of models are used in Information Systems Development. These include: Mathematical Model, Descriptive model and the Graphical model. The mathematical model makes use of formulas that describe the technical aspect of the system whereas the descriptive models consist of narrative memos, reports or lists that describe different aspects of the system. 

As we saw earlier on the Analysis phase of Systems Analysis and design define system requirements, logical models and provide details without regards to a specific technology. The design phase on the other hand will define the physical models, provide technical details and extend logical models.

Events are occurences that take place at a specific time and place and they trigger all system processing.  Requirement definition will determine relevant events, starting with the external events first and then the temporal events second. The system gets decomposed into managable units. There are three types of events: External, Temporal and State events. External events take place outside the system and are initiated by external agent/actor. Temporal events occur as a result of reaching a particular point in time and are based on system deadlines. State events take place when something inside the system triggers a processing need.

Things have relationships and attributes: Relationships are naturally occuring associations between things and they occurs in 2 different directions. Thenumber of associations is cardinality or multiplicity, eg, binary, unary, ternary and n-ary. Attributes refer to a specific piece of information about a thing.

Data entities are things that the system needs to store about the data in the Traditional Information Systems Approach.  It is modelled with the entity relationship diagram - ERD. The requirements model is used to create the database design model for the relational database. Objects have behaviours and attributes and they are also encapsulated, that is they are a self containing unit. Class diagrams model classes of objects in stead of data entities and they define system components. It consists of the class name, its attributes and behaviours also known as methods or function in Object Oriented programming. The object oriented modelling notation is the Unified Modelling Language, UML.