Domain

The domain is the model of the business: its objects, its rules and what happens to them. It lives in domain/ and imports nothing but the domain and @alveolus/core.

Why

When the rule "an order cannot be placed empty" sits in a controller, a SQL query and a cron job, each copy drifts and nobody knows which one is right. When it imports an ORM or a framework, it cannot be read or tested without them.

The fix

The rules live in one place, in plain TypeScript classes named after the business. Everything else calls them.

How the blocks fit together

1ModelAn aggregate groups entities and value objects behind a root that keeps the rules.

2OutcomesA change records a domain event; a refusal returns a domain error.

3Outside worldRepositories and ports say what the domain needs, in its words. Adapters do the rest.

The building blocks

Building block	What it is	Use it when
Aggregates	A group of objects changed together through one root, which keeps their rules.	Some rules must hold after every change.
Entities	An object defined by its identity, inside an aggregate.	A part of an aggregate changes over time and must be told apart.
Value objects	An immutable value compared by its attributes, and the typed identifiers.	A number or a string has rules or a unit: an amount, an email, an id.
Domain events	Something that happened in the domain, in the past tense.	Something else must react to a change.
Domain errors	An expected business failure, returned as a value.	A business rule refuses a request.
Domain services	A stateless operation that belongs to no single object.	A rule needs several aggregates and belongs to none.
Ports	What the domain needs from the outside world, in its own words.	The domain needs the time, an id, a payment, another context.
Repositories	How aggregates are loaded and saved, and how views are read.	An aggregate must be stored, or a query must read data.
Views	What a query returns.	A screen or an API needs data shaped for reading.

See also

• Application, the use cases that call the domain
• Strategic, how contexts meet
• Rules: layers/no-impure-domain, tactical/no-misplaced-class