m09-domain_skill

Overview

This skill guides domain modeling decisions for Rust systems using domain-driven design (DDD) principles. It helps distinguish entities, value objects, aggregates, repositories, and services, and maps those domain concepts to Rust patterns and ownership models. Use it to design robust, invariant-preserving domain types before coding.

How this skill works

The skill inspects a domain concept and asks key design questions: identity, invariants, and ownership. It traces decisions upward to business rules and downward to implementation patterns (newtypes, private constructors, trait-based repositories, ownership choices like Arc/Rc). It provides pattern templates, quick references, and common pitfalls to avoid.

When to use it

• Starting a new domain model or feature design
• Deciding whether a concept is an Entity or Value Object
• Defining aggregate boundaries and consistency scopes
• Choosing ownership and reference strategies in Rust
• Designing validated domain types and repositories

Best practices

• Decide identity up front: use an Id type for entities and newtypes for semantic values
• Enforce invariants via private fields and validated constructors or type states
• Keep aggregate internals encapsulated; expose behavior on the root only
• Prefer newtype wrappers over primitives for domain semantics and validation
• Map persistence to repository traits to separate domain logic from I/O

Example use cases

• Modeling a Payment or Transaction where audit and precision rules dictate invariants
• Designing a User aggregate with UserId, Email value object, and add/remove behaviors on the root
• Creating a validated Email or Money value object with private construction and domain validation
• Defining a Repository trait for User or Order to abstract persistence in tests and services