System Analysis: Events, Processes, Data, and Context Diagrams

Understanding System Events

Example: At the end of the month, if there was staff turnover, this event might trigger the system to provide a "List of high and low" relevant data points.

Events may act on the system in three different ways:

• Parameter: This type of event occurs when the system is relatively inactive.
• Variable: This type of event occurs when the system reacts moderately.
• Operator: This type of event occurs when the system's reaction actively involves or spreads to other components.

Data Stores in System Architecture

Stores are sets of data at rest. They can be categorized as:

• Internal Storage: Storage warehouses used exclusively by the system.
• External Storage: Stores that are shared with other systems.

Personal Data

System Processes and Transformations

A process is a set of tasks performed by the system to transform one or more stimuli (inputs) into external responses (outputs or departures). It expresses the evolution of the system's states over time.

This can be represented as:

Stimuli (Inputs) + Process (Transitions) = External Responses (Outputs/Departures)

Example of a Process

Calculating the Final Capital (CF) obtained by investing an Initial Capital (IC) at an interest rate (i):

CF = IC + (i * IC)

Defining Visions of Context

Visions of context describe how a task is operated or performed, or the conditions under which the task can be executed. This includes conditions that may block, deny, or otherwise affect its implementation.

What is NOT a View of Context

• Data entering the system.
• Information coming out of the system.
• Company rules that do not affect the system under study.
• The frequency of temporary events.
• Determined values that a detail or attribute can take.

Essential System Activities and Memory

A system is composed of essential core activities and essential memory.

Essential Memory

Essential Memory comprises all data that the system would need to store, even if it were implemented using perfect technology. Essential memory consists of stores (or data warehouses).

Essential Core Activities

Essential Core Activities are all those tasks, internal activities, or processes that the system would perform, even if implemented using perfect technology. An event can be considered an essential activity.

Essential activities can be distinguished into two types:

1. Key Activities

• They help justify the existence of the system.
• They are part of the real purpose of the system.
• They are recognizable only in light of the system's objectives.

2. Custodial Activities

These activities capture, store, and maintain data from the environment that is needed for key activities.

Composite Activities

Key activities that perform fundamental responses while also maintaining the system's essential memory are called composite activities.

Constructing a Context Diagram: Steps

Follow these steps to build a context diagram:

1. For each source or subject of events, draw an external entity.
2. For each event, identify the data packet that acts as a stimulus and assign it a name.
3. For each named stimulus, draw a data flow from the external entity to the system.
4. Sketch the system's response to each stimulus (the notation '0.1 or higher' might refer to a diagramming convention or response multiplicity).
5. Review the narrative or system description to check for and include any missing stimuli and responses.
6. Detect if there are external stores and identify how the system uses them.
7. Add these external stores and their interactions to the Context Diagram.