System Modeling Concepts and Environmental Model Definition

System Modeling Fundamentals

A model is an abstract representation of a real object. The first model developed is crucial as it defines the interfaces between the system and its environment. This initial model is known as the environmental model, describing the immediate context in which the system operates according to its objectives.

Components of the Environmental Model

1. Statement of Purpose (Objectives)

   This is a short and concise textual statement defining the system's purpose.

2. Context Diagram

   The Context Diagram graphically shows how the system interacts with the environment, making it easy for a user to interpret.

   It is a special case of a Data Flow Diagram (DFD), where a single bubble represents the entire system.

   Important Features Represented:

   • Individuals, organizations, and systems with which the system communicates. These are known as terminators.
   • The data the system receives.
   • The data the system produces.
   • Data stores that the system shares with the terminators.
   • The border between the system and the rest of the world.

3. List of Events

   This component detects all events in the outside world that can activate the system.

Terminology

External Entities

External Entities are entities with which the system communicates. They are the sources where events are generated and the recipients of outputs.

They are represented graphically by a rectangle labeled with the Entity Name.

Note: The name of the entity must state the position or function, not the person.

Data

• Elemental Data

  The minimum unit of information within the system. E.g., Article Code.

• Data Structure

  A set of interrelated elemental data.

  Example: Address = Street + Number + Floor + Department

Data Flows

Data flows are sets of data produced by or moving due to an event related to the system.

Flow names should clearly identify the message content. They are represented graphically by an arrow that enters or leaves a system.

Example: Material Request Notification

Naming Conventions for Data Flows

Remember:

• Names like List, Details, or Report are not representative names for flows.
• Do not name the flow after the data store (e.g., avoid "Stores Flow").

Stimulus and Departures

• Stimulus

  Data streams that enter the system and activate it.

• Departures (External Responses)

  Data streams that the system sends to the environment.

Event Types

There are two primary types of events:

• External Events

  Events affecting an external entity, which reacts by generating a data stream that enters the system.

  • Generated by an External Entity.

  Syntax for External Events:

  External Entity + Verb + Object

  Example: Customer places an order

• Temporal Events

  Events that mobilize an action that must be performed periodically, generating a data stream that enters the system as a notice.

  • Generated in predetermined periods of time.
  • Always require an external response.

  Temporal Event Subtypes:

  1. Absolute
  2. Constraint

  Syntax for Temporal Events:

  1. Absolute Events:

  Period of Time, Response

  Example: Every day, Show Hours Worked

  2. Constraint Events:

  Term, Condition, Response