M09 Domain Agent Skill

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:

• What's the concept's identity?
• Needs unique identity → Entity (Id field)
• Interchangeable by value → Value Object (Clone/Copy)

• What invariants must hold?
• Always valid → private fields + validated constructor
• Transition rules → type state pattern

• 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-* |