Domain Modeling

Layer 2: Design Choices

Core Question

What is this concept's role in the domain?

Before modeling in code, understand:

• Is it an Entity (identity matters) or Value Object (interchangeable)?
• What invariants must be maintained?
• Where are the aggregate boundaries?

---

Domain Concept → Rust Pattern

Domain Concept	Rust Pattern	Ownership Implication
Entity	struct + Id	Owned, unique identity
Value Object	struct + Clone/Copy	Shareable, immutable
Aggregate Root	struct owns children	Clear ownership tree
Repository	trait	Abstracts persistence
Domain Event	enum	Captures state changes
Service	impl block / free fn	Stateless operations

---

Thinking Prompt

Before creating a domain type:

1. What's the concept's identity?

   • Needs unique identity → Entity (Id field)
   • Interchangeable by value → Value Object (Clone/Copy)

2. What invariants must hold?

   • Always valid → private fields + validated constructor
   • Transition rules → type state pattern

3. Who owns this data?

   • Single owner (parent) → owned field
   • Shared reference → Arc/Rc
   • Weak reference → Weak

---

Trace Up ↑

To domain constraints (Layer 3):

"How should I model a Transaction?"
    ↑ Ask: What domain rules govern transactions?
    ↑ Check: domain-fintech (audit, precision requirements)
    ↑ Check: Business stakeholders (what invariants?)

Design Question	Trace To	Ask
Entity vs Value Object	domain-*	What makes two instances "the same"?
Aggregate boundaries	domain-*	What must be consistent together?
Validation rules	domain-*	What business rules apply?

---

Trace Down ↓

To implementation (Layer 1):

"Model as Entity"
    ↓ m01-ownership: Owned, unique
    ↓ m05-type-driven: Newtype for Id

"Model as Value Object"
    ↓ m01-ownership: Clone/Copy OK
    ↓ m05-type-driven: Validate at construction

"Model as Aggregate"
    ↓ m01-ownership: Parent owns children
    ↓ m02-resource: Consider Rc for shared within aggregate

---

Quick Reference

DDD Concept	Rust Pattern	Example
Value Object	Newtype	struct Email(String);
Entity	Struct + ID	struct User { id: UserId, ... }
Aggregate	Module boundary	mod order { ... }
Repository	Trait	trait UserRepo { fn find(...) }
Domain Event	Enum	enum OrderEvent { Created, ... }

Pattern Templates

Value Object

struct Email(String);

impl Email {
    pub fn new(s: &str) -> Result<Self, ValidationError> {
        validate_email(s)?;
        Ok(Self(s.to_string()))
    }
}

Entity

struct UserId(Uuid);

struct User {
    id: UserId,
    email: Email,
    // ... other fields
}

impl PartialEq for User {
    fn eq(&self, other: &Self) -> bool {
        self.id == other.id  // Identity equality
    }
}

Aggregate

mod order {
    pub struct Order {
        id: OrderId,
        items: Vec<OrderItem>,  // Owned children
        // ...
    }

    impl Order {
        pub fn add_item(&mut self, item: OrderItem) {
            // Enforce aggregate invariants
        }
    }
}

---

Common Mistakes

Mistake	Why Wrong	Better
Primitive obsession	No type safety	Newtype wrappers
Public fields with invariants	Invariants violated	Private + accessor
Leaked aggregate internals	Broken encapsulation	Methods on root
String for semantic types	No validation	Validated newtype

---

Related Skills

When	See
Type-driven implementation	m05-type-driven
Ownership for aggregates	m01-ownership
Domain error handling	m13-domain-error
Specific domain rules	domain-*