actionbook/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?

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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 |

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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

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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? |

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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

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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
        }
    }
}

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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 |

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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-* |