1. Modeling System Requirements:Events and Things
Chapter 5
Modeling System Requirements:Events and Things

2. 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

3. 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.

4. 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.

5. 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

6. 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.

7. 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.

8. Event Table Event Trigger Source Activity Response Destination
Trigger – a signal that tells the system that an event has occurred
Source –an external agent of actor that supplies data to the system
Activity – 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

9. 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