contract-testing-expert

CONTRACT-TESTING-EXPERT -- Consumer-Driven Contract Testing Specialist

Domain: Contract Testing / Pact / Schema Validation / API Compatibility / Provider Verification

Why Contract Testing

Integration tests:  Slow, flaky, require full environment
E2E tests:          Even slower, even flakier
Contract tests:     Fast, isolated, catch breaking changes BEFORE deploy

The contract is the agreement between consumer and provider:
"I will send X, and you will respond with Y"

Contract Testing vs Other Approaches

Approach	Speed	Isolation	Catches Breaking Changes	Maintenance
Contract (Pact)	Fast	Full	Yes (before deploy)	Medium
Integration	Slow	None	Yes (at deploy time)	High
Schema validation	Fast	Full	Structural only	Low
E2E	Very slow	None	Yes (late)	Very high
OpenAPI diff	Fast	Full	Structural only	Low

Best practice: Contract tests + OpenAPI schema validation together.

Pact Workflow (Consumer-Driven)

Step 1: CONSUMER writes a test
  - Define expected interaction (request + response)
  - Run test against Pact mock server
  - Generate pact file (contract JSON)

Step 2: Publish contract to Pact Broker
  - pact-broker publish ./pacts --consumer-app-version=$(git rev-parse HEAD)

Step 3: PROVIDER verifies the contract
  - Fetch contracts from broker
  - Replay each interaction against real provider
  - Report results back to broker

Step 4: Can-I-Deploy check (CI gate)
  - pact-broker can-i-deploy --pacticipant MyConsumer --version $(git rev-parse HEAD) --to production
  - Blocks deploy if any contract is broken

Consumer Test Example (Pact JS)

// Consumer side: order-service tests against payment-api

const { PactV4 } = require('@pact-foundation/pact')

const provider = new PactV4({
  consumer: 'order-service',
  provider: 'payment-api',
})

describe('Payment API Contract', () => {
  it('should process a valid payment', async () => {
    await provider
      .addInteraction()
      .given('a valid payment method exists')        // Provider state
      .uponReceiving('a request to process payment')
      .withRequest('POST', '/payments', (builder) => {
        builder
          .headers({ 'Content-Type': 'application/json' })
          .jsonBody({
            amount: 2999,          // cents
            currency: 'USD',
            payment_method_id: Matchers.string('pm_123'),
          })
      })
      .willRespondWith(201, (builder) => {
        builder.jsonBody({
          id: Matchers.uuid(),
          status: Matchers.oneOf('succeeded', 'pending'),
          amount: 2999,
          created_at: Matchers.iso8601DateTime(),
        })
      })
      .executeTest(async (mockServer) => {
        const client = new PaymentClient(mockServer.url)
        const result = await client.processPayment({
          amount: 2999,
          currency: 'USD',
          payment_method_id: 'pm_123',
        })
        expect(result.status).toBe('succeeded')
      })
  })
})