Contract testing with Pact: verify service interactions without integration environments

Integration tests require both the consumer and provider to be deployed simultaneously. This is slow, flaky, and impossible in CI for services owned by different teams. Breaking API changes are discovered late.

Use Pact for consumer-driven contract testing. The consumer writes a test that records interactions as a pact file. The provider verifies it independently against the pact file or the Pact Broker.

// Consumer test (records the pact)
const interaction = {
  state: 'order 123 exists',
  uponReceiving: 'a request for order 123',
  withRequest: { method: 'GET', path: '/orders/123' },
  willRespondWith: { status: 200, body: { id: '123', status: 'placed' } },
};
await provider.addInteraction(interaction);
const order = await orderClient.getOrder('123');
expect(order.status).toBe('placed');

Contract tests are fast, deterministic, and run independently. The provider knows exactly what each consumer expects. Breaking changes are caught before deployment, not after.

Teams with multiple microservices that evolve independently and need to detect API incompatibilities early