Domain Modeling

Domain Modeling

Purpose

Make the code speak the business's language precisely. Most long-lived complexity comes from a mismatch between the words the business uses and the structures the code has — where "user" means four different things and none of them are named.

When to Use

• The same concept has different names in code, in the database, and in conversation.
• A single model class is used by three subsystems that each need something different from it.
• New requirements consistently require changes in unrelated modules.
• Designing a new subsystem in a domain with real business rules.

Capabilities

• Ubiquitous language: a shared, enforced vocabulary between code and domain experts.
• Modeling entities, value objects, and aggregates with clear invariants.
• Bounded-context identification and mapping.
• Replacing primitive obsession with domain types.
• Making illegal states unrepresentable.

Inputs

• The domain vocabulary, ideally from the people who use it daily.
• The current model and where it hurts.
• The business rules, especially the ones that are currently enforced by convention.

Outputs

• A glossary of terms, each with exactly one meaning.
• Domain types that carry their invariants.
• Named bounded contexts and the translation between them.

Workflow

1. Collect the vocabulary — Listen to how the domain experts speak. When they say "a subscription lapses", the code should have a lapse() and not a status = 3.
2. Find the collisions — The same word meaning two things is the highest-value finding. "Order" in fulfilment and "Order" in billing are usually different concepts.
3. Draw the context boundaries — Where the meaning of a word changes, a context boundary exists. Draw it explicitly and translate at the edge.
4. Replace primitives with types — Money, EmailAddress, SubscriptionId. A type that validates on construction cannot be invalid downstream.
5. Move invariants into the model — If a rule is currently enforced in a controller, the model permits an invalid state. Move it.
6. Rename ruthlessly — Code that uses a word the business does not use is code the business cannot verify.

Best Practices

• One word, one meaning, per context. If you need to say "billing order" to be clear, the type should be BillingOrder.
• A value object with no identity should be immutable and compared by value. Money(1000, "USD") equals Money(1000, "USD").
• An aggregate is a consistency boundary. Everything inside it is transactionally consistent; everything outside it is eventually consistent. Choose the boundary deliberately.
• Never share a database table across bounded contexts. That is a hidden coupling that no interface can undo.
• Do not build a domain model for a CRUD application. If there are no invariants, a form and a table is the correct architecture.
• The glossary belongs in the repository, and disagreements about it are the most productive meetings you will have.

Examples

Primitive obsession versus a domain type:

// Before: every caller must remember the currency and the unit.
function refund(orderId: string, amount: number, currency: string) { ... }
refund("ord_1", 19.99, "USD");   // floating point money, in an unvalidated string id

// After: the types make the mistakes impossible.
class Money {
  private constructor(readonly cents: bigint, readonly currency: Currency) {}

  static of(cents: bigint, currency: Currency): Money {
    if (cents < 0n) throw new DomainError("Money cannot be negative");
    return new Money(cents, currency);
  }

  plus(other: Money): Money {
    if (other.currency !== this.currency) {
      throw new DomainError(`Cannot add ${other.currency} to ${this.currency}`);
    }
    return new Money(this.cents + other.cents, this.currency);
  }
}

function refund(orderId: OrderId, amount: Money): Refund { ... }

Currency mismatches and negative amounts are now compile-time or construction-time errors rather than production incidents.

Notes

• Aggregate boundaries that are too large create lock contention; boundaries that are too small push consistency into the application layer. Start with the smallest set of objects that must change together atomically.
• Anti-corruption layers exist so that a bad upstream model does not leak into a good one. When integrating a vendor API, translate at the edge — do not let their nouns into your domain.