onion-lasagna-domain

Onion Lasagna Domain

Overview

Model domain code as pure business behavior: value objects validate facts, entities carry identity, aggregate roots enforce consistency, and domain events record meaningful past occurrences.

Always First

Inspect actual implementations before writing code:

rg -n "class .* extends Base(AggregateRoot|Entity|ValueObject|DomainEvent)" packages/onion-lasagna/src starters -g '*.ts'
rg -n "InvariantViolationError|pullDomainEvents|peekDomainEvents|reconstitute|static create" packages/onion-lasagna/src starters -g '*.ts'

Read the matching base class and tests before copying a constructor shape. Use starter files for folder and naming conventions unless they contain complete implementations.

Class Choices

• BaseValueObject<T>: immutable, compared by value, self-validating in create().
• BaseEntity<TId, TProps>: identity-based object with mutable state and versioning.
• BaseAggregateRoot<TId, TProps>: consistency boundary that raises domain events.
• BaseDomainEvent<TPayload>: immutable past-tense event raised by an aggregate.

Domain Rules

• Entity, aggregate, and VO constructors stay private or protected; expose static create() for new objects.
• Add static reconstitute() for loading existing state without emitting creation events.
• Validate VOs before construction; validate cross-property invariants in factories or domain methods.
• Throw domain errors such as InvariantViolationError for business rule violations.
• Use value objects for ids and meaningful primitives instead of raw strings in entity props.
• Raise events only for facts that happened in the domain, and include primitive payload data handlers need.
• Event constructors or factories should follow the local BaseDomainEvent examples/tests.

Forbidden In Domain

• No HTTP, GraphQL, Hono, Elysia, Express, Fastify, NestJS, SDK, or external API imports.
• No Drizzle, Prisma, SQL, database schema, repository adapter, cache, queue, or transport imports.
• No request DTOs, response DTOs, persistence rows, framework exceptions, or console.log.
• No app/use-case authorization or infra retry behavior.

Test Checklist

• VO accepts valid input and rejects invalid input.
• Aggregate create() sets initial state and emits the relevant events.
• reconstitute() restores state and does not emit creation events.
• Domain methods enforce invalid transitions when transitions are introduced.
• Events are inspectable with peekDomainEvents() and cleared with pullDomainEvents() when relevant.