Patterns for resilient service boundaries, message contracts, and operationally useful NestJS microservices.

Microservices become useful when boundaries map to ownership and failure modes. Message contracts, idempotent consumers, retry behavior, and observability matter more than the transport itself.

@MessagePattern("invoice.created")
async handleInvoiceCreated(event: InvoiceCreatedEvent) {
  const exists = await this.inbox.seen(event.id);
  if (exists) return;

  await this.billing.applyInvoice(event);
  await this.inbox.markSeen(event.id);
}