Testing Strategy

Preview — 3 of 10 questions

When testing a service that orchestrates several microservices via ClientProxy.send(), what is the key to a reliable distributed test without real brokers?

javascript

import { of, throwError } from 'rxjs';
import { delay } from 'rxjs/operators';

inventoryClient.send.mockReturnValue(of({ stock: 5 }).pipe(delay(10)));
paymentClient.send.mockReturnValue(throwError(() => new Error('declined')));

await expect(orderService.place(order)).rejects.toThrow('declined');

ASpin up the full broker topology for every test

BReplace all RPC calls with synchronous local function calls in production code

CUse random data and accept flaky results

DMock each ClientProxy so send() returns a controlled of(response) (or a deferred observable to simulate latency/ordering), letting you assert orchestration logic deterministically

What does property-based testing with fast-check add over example-based unit tests for a NestJS pricing service?

javascript

import fc from 'fast-check';

it('total is never negative', () => {
  fc.assert(
    fc.property(fc.nat(), fc.float({ min: 0, max: 1 }), (price, discount) => {
      const total = pricingService.apply(price, discount);
      return total >= 0;
    }),
  );
});

AIt generates many randomized inputs to verify invariants/properties hold across the input space, surfacing edge cases hand-picked examples miss

BIt replaces the need for the service entirely

CIt only tests UI components

DIt guarantees 100% branch coverage automatically

Why adopt consumer-driven contract testing (Pact) between NestJS microservices?

javascript

// Consumer side records expected interaction; provider verifies it:
await provider.addInteraction({
  state: 'user 1 exists',
  uponReceiving: 'a request for user 1',
  withRequest: { method: 'GET', path: '/users/1' },
  willRespondWith: { status: 200, body: { id: '1' } },
});

ATo replace all unit tests with one contract

BTo encrypt traffic between services

CTo verify that a provider's API still satisfies the expectations (the contract) recorded by its consumers, catching breaking changes without full end-to-end environments

DTo measure code coverage across services

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