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 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
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