Contract Testing: The Safety Net Your Microservices Architecture Needs

// Consumer test
it('should authorize a payment', async () => {
  // 1. Set up expectations
  await provider.addInteraction({
    state: 'a valid payment card',
    uponReceiving: 'a request to authorize payment',
    withRequest: {
      method: 'POST',
      path: '/api/payments/authorize',
      headers: { 'Content-Type': 'application/json' },
      body: {
        cardNumber: '4111111111111111',
        amount: '100.00',
        currency: 'USD'
      }
    },
    willRespondWith: {
      status: 200,
      headers: { 'Content-Type': 'application/json; charset=utf-8' },
      body: {
        transactionId: like('txn_abc123'),
        status: 'authorized',
        amount: '100.00'
      }
    }
  });

  // 2. Test your actual code
  const result = await paymentClient.authorizePayment({
    cardNumber: '4111111111111111',
    amount: '100.00',
    currency: 'USD'
  });

  // 3. Verify it worked
  expect(result.status).toBe('authorized');
});

// Provider verification test
describe('Pact Verification', () => {
  it('validates the expectations of payment-client', async () => {
    await new Verifier({
      provider: 'payment-gateway-api',
      providerBaseUrl: 'http://localhost:3000',
      
      // Pull contracts from broker
      pactBrokerUrl: 'http://localhost:9292',
      
      // Handle test data setup
      stateHandlers: {
        'a valid payment card': async () => {
          // Set up test data for this scenario
          await database.seed('valid-card');
        }
      },
      
      // Publish results back
      publishVerificationResult: true,
      providerVersion: '2.1.0'
      
    }).verifyProvider();
  });
});