Events and System Requirements
·
Events
o
Occurrences at a specific time and place
o
Trigger all system processing
·
Requirement definition
o
Determine relevant events
1.
External events first
2.
Temporal events second
o
Decompose system into manageable units
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Events Affecting a Charge Account Processing System
Types of Events
1.
External
· Outside system
· Initiated by external agent or actor
External events are events that occur outside the system, usually initiated by an external agent or actor. An external agent or actor is a person or organizational unit that supplies or receives data from the system. To identify the key external events, the analyst first tries to identify all of the external agents that might want something from the system.
2.
Temporal
· Occurs as result of reaching a point in time
· Based on system deadlines
An event that occurs as a result of reaching a point of time. No external agent or actor is making a demand in temporal event, but the system is supposed to generate needed information or other outputs when they are needed. The analyst begins identifying the temporal events by asking about the specific deadlines that the system must accommodate.
3.
State
· Something inside system triggers processing need
An event
that occurs when something happens inside the system that triggers the need for
processing. For example, if the sale of
a product results in an adjustment to an inventory record and the inventory in
stock drops below a reorder point, it is necessary to reorder. Often state events occur as a consequence of
external events. Sometimes they are
similar to temporal events, except the point in time cannot be defined.
Identifying Events Events versus Conditions and Responses
It
is difficult to distinguish an event and part of a sequence of conditions that
leads up to the event. The analyst has
to think through a sequence of activities before arriving to the point where an
event directly affects the system.
Sequence of
events: Tracing a Transaction’s Life
Cycle
It
is often useful in identifying events to trace the sequence of events that
might occur for a specific external agent or actor.
Technology-Dependent
Events and System Controls
Sometimes
the analyst is concerned about events that are important to the system but do
not directly concerns the users or transactions. Such events typically involve design choices
or system controls. Most of these events
involve system controls – which checks or safety procedures put in
place to protect the integrity of the system.
One technique used to help decide which events apply to controls is to
assume that technology is perfect. The
perfect technology assumptions state that events should be included during
analysis only if the system would be required to respond under perfect
conditions – that is, with equipment never breaking down, capacity for
processing and storage being unlimited, and people operating the system being
completely honest and never making mistakes.
Sequence of
“Transactions” for One Specific Customer Resulting in Many Events
Events Deferred Until
the Design Phase
 Looking at each Event
While
developing the list of events, the analyst should note additional information
about each event. This information is
entered in an event table. An event
table is a table that lists events in rows and key pieces of
information about each event in the columns.
The event table is a convenient way to record key information about the
requirements for the information system.
Information in an events table includes:
Trigger. An occurrence that tells the system than an
event has occurred, either the arrival of data needing processing or of a point
in time.
Source. An external agent or actor that supplies data
to the system.
Activity. Behavior that the system performs when an
event occurs.
Response. An output, produced by the system that goes
to a destination.
Destination. An external agent or actor that receives data
from the system.
Information about each event in an event table
Things and System Requirements
Another
key concept used to define system requirements involves understanding and
modeling things that the system needs to store information about. To the users, these are the things they deal
when they do their work – products, orders, invoices and customers – that need
to be part of the system. Often these
things are similar to the external agents or actors that interact with the
systems. In other cases, these things
are distinct from external agents.
In
the traditional approach to development, these things make up the data about
which the system stores information. In
object-oriented approach, these things are the objects that interact in the
system.
Types of
Things
1.
Tangible
things. Airplane, book, vehicle, document, worksheet
2. Roles
played. Employee, customer, doctor, patient, end
user, system administrator
3. Organizational
units. Division, department, section, task force,
workgroup
4.
Devices. Sensor, timer, controller, printer, disk
drive, keyboard, display window, mouse, menu buttons
5.
Incidents,
Events, or Interactions. Flight, service call, log on, log off,
contract, purchase, order, payment
6. Sites/locations. Warehouse, branch office, factory, retail
store, desktop
Procedure for Developing an Initial List of Things
Step
1: Using the event table and
information about each event, identify all nouns about system
Step
2: Using other information from
existing systems, current procedures, and current reports or forms, add items
or categories of information needed
Step
3: Refine list and record
assumptions or issues to explore
Characteristics
of Things
There
are many important relationships among things of important in the system. A relationship is a naturally occurring
association among specific things, such as an order is placed by a customer. Relationships between things apply in two directions. It is also important to understand the nature
of each relationship in terms of the number of associations for each thing. Cardinality or multiplicity – is the
number of associations that occur between specific things. Cardinality of relationships: zero to many, one
to one, one to many.
Types of
relationships
1.
unary
(recursive) relationship – a relationship between two things of the same type.
2.
binary
relationship – relationships between two different types of things.
3.
ternary
relationship – a relationship between three different types of things.
4.
n-ary
relationship – a relationship between n (any number) different types of things.
Attributes
of Things
Most
information system store and use specific pieces of information about thing. The specific piece of information about a
thing is called attribute. The attribute
that uniquely identifies the thing is called an identifier or key (i.e. Order
number, name, address). A compound
attribute is an attribute that contains a collection of related attributes.
Relationships Naturally Occur Between Things
Cardinality / Multiciplicity of Things
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