M09 Domain

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

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

Related Skills