Quick summary / thoughts as part of the Catalyst Book Club meeting.

Chapter 19: Policy and Level

• Things that change together are on the same level.
• Good architecture identifies levels and keeps them clearly defined/separated.
• The further away from I/O the higher the level.
• The closer to I/O the lower the level.

Chapter 20: Business Rules

• Entities are logical groups of critical business data and operations.
• These entities transcend the software system and apply outside the system.
• Use cases are business rules that only exist in the application.
• A use case deals with entities and can help transform input to output.
• Changes to I/O should not affect business rules.