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Information Technology
ITEC 3010

Systems Analysis and Design I ITEC3010 – Fall 2010 – Luiz Cysneiros Lecture 5 – Modeling System Requirements: Events and Things – Oct 19 Models and Modeling - A model: a representation of some aspect of the system being built - A variety of models o Many are graphical (e.g. Data flow diagram, ER diagram) o Can show different levels of detail o Some focus on data o Some focus on processing - Purpose: creating a model can help clarify and refine the design o Developing the model raises questions that need to be considered o Models help to simplify complex aspects of systems Reasons for Modeling - Learning from the modeling process o New pieces are found to be needed o New questions arise that need to be answered to complete the model - Reducing complexity by abstraction o Systems can be complex and intangible o Models of parts of the system help to clarify and focus on important aspects - Remembering all of the details o Models are a way of storing information for later use in a form that can be digested (e.g. A picture can say a lot) - Communicating with other development team members o Can show others aspects of the system in a succinct way o Support communication – e.g. someone working on inputs and outputs can use the model to communicate with someone working on processing details - Communicating with a variety of users and stakeholders o Users need to see clear and complete models to understand what the analyst is proposing o Users often work with analyst to create models (this process can help users better understand what the system can do) - Documenting what was done for future maintenance/enhancement o Critical development team leaves behind a record of what was created o Need to package everything in a form future developers can understand and use when they modify and update the system o Much of the documentation consists of models (e.g. diagrammatic) as text mentions – also much of documentation also consists of textual reports Types of Models - Mathematical model: a series of formulas that describe technical aspects of a system o Comfortable for scientific or engineering applications o Sometimes used in business systems – e.g. simple formula for payroll where you model gross pay as regular pay plus overtime pay - Descriptive model: narrative memos, reports, or lists that describe some aspect of the system o E.g. might jot down notes from interviewing a user o Users may describe what they do in reports or memos o Analyst can then convert these descriptions to a modeling notation - Sometimes narratives are the best way for recording information - Simple lists of features, inputs, outputs, events or users are examples - A very important form of narrative model: writing a procedure in a very precise way – structured English or pseudocode - Pseudocode used a lot by programmers, can also be used to describe procedures during earlier phases o Example of Pseudocode description of a payroll function:  Input the employees payroll data  If hours worked is greater than 40 then calculate overtime pay  Calculate gross pay for the employee - Gross Pay = hourly pay times 40 plus overtime  Calculate tax for the employee ETC. - Graphical Model: diagrams and schematic representations of some aspect of a system o Easy to understand complex relationships (old saying: a picture is worth a thousand words) o Graphical models may look similar to a real-world part of the system (e.g. a screen design or report layout) o But often represent more abstract things, e.g. processes, data, objects, messages, connections o Key graphical models tend to represent abstract aspects of a system during Analysis Phase o The more concrete models (e.g. screen design) tend to appear later in the Design Phase Notes on Graphical Models - Many different types and formats - Variations in notation - However, still based on basic symbols o Circle o Square o Rectangle o Line Models Used in the Analysis Phase - The analysis phase named “Define System Requirements” involves creating models o Logical models (define what is required without committing to one specific technology) - Many different types of formalisms for defining logical models Models used in the Design Phase - These are physical models – since will be implemented with a specific technology - Some are extensions of requirements models created during systems analysis - Some may be used during both analysis and design (e.g. object-oriented class diagram) Events and System Requirements - Two key concepts to model: o Events o Things - Event – an occurrence at a specific time and place, that can be described and is worth remembering o E.g. customer placing an order, shipping identifies a backorder, nuclear reactor goes to meltdown - When defining system requirements it is useful to begin by asking what events occur that will affect the system being studied o What events will occur that the system will have to respond to? o Allows you to focus on external environment to keep you at high level view of system (not the workings of it) - Also allows you to focus on the interfaces of the system to outside people and systems - End users can easily describe system needs in terms of events that affect their work, very useful when working with users - Gives a way to divide (or decompose) the system requirements so you can study each separately (complex systems need to be decomposed based on events) Background to Event Concept - Structured analysis adapted to real-time systems in the early 1980’s o E.g. process control system, reactors, aerospace etc. o Events like “chemical vat is full”, “boiler overflowing” o System must respond to these types of events immediately o Extended to business applications since they have become more interactive Types of Events - External event o An event that occurs outside the system o Usually initiated by an external agent or actor (a person or organization that supplies or receives data from the system) – can also be another software !! o Classic example of an external agent: a customer o Possible event: the customer wants to place an order o Naming events:  Include the external agent in the name  E.g. Events: “customer places order”, “management checks order status”,”customer reports change of address” - External Events Checklist o External agent wants something resulting in a transaction o External agent wants some information o Data changed needs to be updated o Management wants some information - Temporal Events o An event that occurs as a result of reaching a point in time o Could be reports to management generated regularly o System automatically produces reports etc. at right time (so no external agent needed) o E.g. Payroll systems produce a paycheck every two week (or once a month) - Temporal Events Checklist o Internal outputs needed  Management reports (summary or exception)  Operational reports (detailed transactions)  Statements, status reports (including payroll) o External outputs needed  Statements, status reports, bills, reminders - State Events o An event that occurs when something happens inside the system that triggers the need for processing o E.g. the sale of a product results in an adjustment in the inventory (event “Reorder point reached) o This state change might occur as a result of external events or to temporal events (so could be similar to temporal event, but point in time can’t be defined) Identifyin
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