Contract testing verifies that two services agree on their API without running them together in a full integration test. In consumer-driven contracts (CDC), the consumer defines what it needs, and the provider verifies it can deliver it.
Full end-to-end tests across many services are slow, flaky, and hard to run on every commit. Contract tests catch breaking API changes early, in isolation.
1. Consumer writes expectations ─▶ generates a CONTRACT (Pact file)
2. Contract published to a broker
3. Provider runs the contract against itself in its own pipeline
4. Provider build FAILS if it would break the consumer
// consumer declares exactly what it expects from the provider
provider.given('order 42 exists')
.uponReceiving('a request for order 42')
.withRequest({ method: 'GET', path: '/orders/42' })
.willRespondWith({
status: 200,
body: { id: '42', total: 99.5 } // the shape the consumer relies on
});
The provider's CI replays this and fails if it no longer returns that shape.
| Contract test | Integration test | |
|---|---|---|
| Services running | One at a time | All together |
| Speed | Fast | Slow |
| Catches | Contract breaks | Real wiring issues |
Contract tests give you confidence that a provider change won't break consumers, without the cost and flakiness of spinning up the whole system.
They make independent deployment safe: each team's pipeline knows immediately if its change would violate a contract someone depends on.