Contract Testing with Pact

The End-to-End Trap

In mid-to-large distributed systems, end-to-end (E2E) tests often become the bottleneck. They are slow, flaky, and require a stable environment with all dependencies deployed. When a test fails in a 50-service ecosystem, finding the root cause is a nightmare.

What is Pact?

Pact is a code-first tool for implementing consumer-driven contract testing. It allows the consumer (e.g., a frontend app) to define the expectations they have for a provider (e.g., a REST API). These expectations are saved as a "Pact file".

// Defining a Pact interaction
pactWith({ consumer: 'MyOrderApp', provider: 'OrderService' }, provider => {
  describe('getting an order', () => {
    it('returns the order details', async () => {
        // Test logic...
    });
  });
});

Why it Matters

Contract tests are decoupled. You can verify the consumer without the provider, and vice-versa. This enables independent deployments and high-frequency release cycles with absolute confidence.