Ch 5-8: System Modeling & Design

Chapter 5–8 — System Modeling & Design

(Sommerville Ch. 5–8)

Cohesion and Its Influence on Maintenance

Cohesion measures how strongly related the responsibilities of a module are.

• High cohesion (good): a module does one thing well → easy to understand, test, and maintain
• Low cohesion (bad): a module has many unrelated responsibilities → changes in one part risk breaking another

High cohesion reduces maintenance effort because:

• Changes are localized to a single module
• Modules are independently understandable
• Testing and reuse are simpler

ER Diagram Generation

Level 1 ER Diagram:

• Identify entities (nouns from problem description)
• Identify relationships (verbs between entities)
• Draw entity-relationship diagram with cardinalities

Level 2 ER Diagram:

• Add attributes to entities
• Identify primary keys
• Resolve many-to-many relationships with junction entities

Level 3 ER Diagram:

• Normalize to 3NF
• Identify foreign keys
• Define all constraints