Why should you care?

Auditors and new hires need clear seams between HTTP, business rules, and Mongo persistence.

See it live — copy this example

Paste into your MeanVerse project (Angular + Express + MongoDB), then run with ng serve / node / mongosh as noted.

// modules/billing/billing.controller.ts -> service -> repository
export class BillingService {
  constructor(private repo: InvoiceRepository, private tax: TaxCalculator) {}

  async issueInvoice(tenantId: string, dto: CreateInvoiceDto) {
    const lineItems = dto.items.map(i => ({ ...i, tax: this.tax.rate(i) }));
    const total = lineItems.reduce((s, i) => s + i.amount + i.tax, 0);
    return this.repo.insert({ tenantId, lineItems, total, status: 'open' });
  }
}

What happened?

• Controller parses HTTP.
• Service computes tax and totals.
• Repository only talks Mongoose.
• TaxCalculator is swappable for country rules.

Practice next

1. Refactor one fat route into controller + service + repo.
2. Define interfaces for external payment gateway.
3. Add integration tests against in-memory Mongo.
4. Extract TaxCalculator interface with US and EU impl.
5. Add domain events emitted after invoice issued.

Remember

Thin HTTP, rich domain services. Repositories isolate Mongoose. Adapters wrap Stripe, email, Kafka.

SaaS billing compliance

MeanVerse must recalculate tax when EU VAT rules change.

Outcome: Swap TaxCalculator implementation without touching routes.