Chapter 5

Modeling System Requirements:Events and Things

Models and Modeling• Uses many different types of models to show the system at different

levels of detail -- Models for inputs, output and data stores -- show the same problem from different perspectives• Purpose – to help the analyst clarify and refine the design. (learn from the modeling process) -- helps the analyst focus on a few aspects of the systems at a time. (reduce complexity) -- some models serve to show how other models fit together. -- provide a way of storing information for later use. -- support communications among project team members and with system users

Events and System Requirements• An Event – an occurrence at a specific time and place that can be described and should be remembered by

the system. {Events trigger or drive all processing of the sys}Key ? What events occur that will require the system to respond? -- view the system as a black box -- focus on a high-level view of the sys. -- focus on systems interfaces to people and other systems -- aids in decomposing sys. Req.

Models and Modeling (cont)

-- Used as documentation for future development teams when the maintain or

enhance the system.• Types of models -- Mathematical model – a series of formulas that describe

technical aspects of a system. -- Descriptive model – Narrative memos, reports, or lists that describe some aspect of a system. (e.g., pseudocode for modeling algorithms.) -- Graphical Models – diagrams and schematic

representations of some aspect of a system.

Types of Events

• External events – an event that occurs outside the system, usually initiated by an

external agent or actor. an agent or actor– is a person or organizational unit that

supplies or receives data from the system. The analyst must identify all of the key external events--- to accomplish this the analyst must identify all of the external agents (e.g., a customer). External Events – transactions, agent info. request, changes, management requests -- name the external events

Type of Events (cont)

• Temporal Events – an event that occurs as a

result of reaching a point in time (time

interval initiated event or periodic events)

-- analyst ask what deadlines that the system must accommodate to identify temporal events

Temporal events do not have to occur on fixed calendar dates, they can occur after a

defined period of time has elapsed.

Type of Events (cont)

• State Event – an event that occurs when something happens inside the system that

triggers the need for processing.Identifying Events --Events versus Conditions and Responses

--Technology-Dependent Events and System Controls

-- do not directly concern users

system controls – checks or safety procedures

put in place to protect the integrity of the sys. perfect technology assumption– states that events should be included

during analysis only if the system would be required to respond under perfect conditions.

Event Table

Event Trigger Source Activity Response Destination

Trigger – a signal that tells the system that an event has occurredSource –an external agent of actor that supplies data to the systemActivity – behavior that the system performs when an event occurs (use case)Use case – a series of actions the a system performs that result in a defined outcome (similar to an activity)Response– an output, produced by the system, that goes to a destination

Destination – and agent or actor receives data from the system

Things and System Requirements

• Identify the “Things” or entities about which the systems needs to store information

• Relationships among “things”

--binary relationships

-- unary (recursive) relationship

-- n –ary relationship• Attributes of “things”

--identifier (key)

--compound attributes