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

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?)

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

Pattern Templates

Value Object

rust
1struct Email(String);
2
3impl Email {
4    pub fn new(s: &str) -> Result<Self, ValidationError> {
5        validate_email(s)?;
6        Ok(Self(s.to_string()))
7    }
8}

Entity

rust
1struct UserId(Uuid);
2
3struct User {
4    id: UserId,
5    email: Email,
6    // ... other fields
7}
8
9impl PartialEq for User {
10    fn eq(&self, other: &Self) -> bool {
11        self.id == other.id  // Identity equality
12    }
13}

Aggregate

rust
1mod order {
2    pub struct Order {
3        id: OrderId,
4        items: Vec<OrderItem>,  // Owned children
5        // ...
6    }
7
8    impl Order {
9        pub fn add_item(&mut self, item: OrderItem) {
10            // Enforce aggregate invariants
11        }
12    }
13}

Common Mistakes

Related Skills