Week 9 Part IV · Domain Logic and Security

Domain-Driven Design and Business Logic

Instructor lesson plan: lecture (2 h) and practice (2 h).

Learning objectives

Model entities, value objects, aggregates, domain services, and domain events.
Place business rules in the domain or application layer deliberately.
Review use cases around workflows and invariants.

Tools this week

domain modeluse-case servicesevent sketchesworkflow examples

🎓Lecture · 2 hours

0:00-0:20	20 min	Ubiquitous languageNaming concepts from the business domain, not from UI screens or database tables.
0:20-0:45	25 min	Aggregates and invariantsConsistency boundaries and transaction decisions.
0:45-1:05	20 min	Domain services and application servicesDistinguishing business calculations from orchestration.
1:05-1:15	10 min	Break
1:15-1:40	25 min	Domain eventsExpressing something meaningful that happened, without inventing distributed complexity.
1:40-2:00	20 min	Workflow examplesReviewing business behavior through use cases.

Common misconception to confront.

Students often think: all rules belong in services.
Set it straight: invariants that protect an entity or aggregate should live with that model when possible.

Check for understanding

What is an aggregate boundary?

A consistency boundary that groups entities and value objects changed together under invariants.

When use a domain service?

When a business operation does not naturally belong to one entity or value object.

Key takeaways.

The domain model should speak the business language.
Aggregates protect invariants.
Workflow examples reveal missing business rules.

📚Reading & resources

Domain-Driven Design Entities, value objects, aggregates, services, and events.
Domain model validations Microsoft guidance on validation and invariants.

💻Practice · 2 hours

0:00-0:25	25 min	Language reviewTeams rename vague concepts and write a glossary.
0:25-1:00	35 min	Aggregate designIdentify one aggregate and encode its invariants.
1:00-1:10	10 min	Break
1:10-1:45	35 min	Workflow examplesAdd examples for successful and failed business workflows.
1:45-2:00	15 min	Project-integration briefDocument aggregate boundaries and business rules.

Project integration (this week)

Strengthen the domain model and workflows.
Add at least one aggregate or domain service.
Add examples for core business rules.