m09-domain

What the source does

• FilesystemNone observedNo reads or writes in the source
• NetworkNone observedNo outbound calls in the source
• ShellNone observedNo commands run in the source
• DependenciesPinnedNothing floats
• SecretsNone foundNo keys in source

Five audit layers

Gates applied: no behavioural pass, no license.

Audit 0.4.1 · source sha f4ff8c82542d · 31 Oct 2026Report an issue or request a re-scan

• Publisherzhanghandong GitHub organizationOwner of the repository this skill is read from
• Source folderzhanghandong/rust-skills/skills/m09-domainThe exact folder we audited
• LicenceNot declaredAs the repository states it
• Commit pinned for this verdictf4ff8c82542dA new audit runs when the source changes
• Repository statusNot archivedLast push 69 days ago
• Sandboxed runNot runSkills are read, not executed; layer L4 is skipped

Its description: CRITICAL: Use for domain modeling. Triggers: domain model, DDD, domain-driven design, entity, value object, aggregate, repository pattern, business rules, validation, invariant, 领域模型, 领域驱动设计, 业务规则

---
name: m09-domain
description: "CRITICAL: Use for domain modeling. Triggers: domain model, DDD, domain-driven design, entity, value object, aggregate, repository pattern, business rules, validation, invariant, 领域模型, 领域驱动设计, 业务规则"
user-invocable: false
---

# 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

```rust
struct Email(String);

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

### Entity

```rust
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

```rust
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-* |

Excerpt, first 4,189 characters of SKILL.md, verbatim from the audited commit. It is text from someone else’s repository, quoted so you can judge it before your agent reads it.

git clone https://github.com/zhanghandong/rust-skills
git -C rust-skills checkout f4ff8c82542d

Works in

1. 2026-10-31
   ✓SAFE B 89 / 100

   first audit · source f4ff8c82542d

Questions about m09-domain

• github.com/zhanghandong/rust-skills/tree/main/skills/m09-domain — the audited source at commit f4ff8c82542d and every finding aboveObserved · 31 Oct 2026

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