Domain-Driven Design

// an aggregate: the wallet enforces its own invariant; nothing else edits balances directly
class Wallet {                                             // aggregate root
  private constructor(readonly id: WalletId, private available: Money, private holds: Map<HoldId, Money>) {}

  placeHold(id: HoldId, amount: Money): DomainEvent[] {
    if (amount.greaterThan(this.available)) throw new InsufficientFunds(this.id, amount);   // invariant
    this.available = this.available.minus(amount);
    this.holds.set(id, amount);
    return [{ type: 'HoldPlaced', walletId: this.id, holdId: id, amount }];
  }
  captureHold(id: HoldId): DomainEvent[] { /* … */ }
  releaseHold(id: HoldId): DomainEvent[] { /* … */ }
}
// Money is a value object: integer minor units + currency, immutable, compared by value
// another aggregate (Transfer) references the wallet by WalletId only and reacts to its events