Introduction
When 20 services communicate with each other, how to ensure that changing the API in service A does not break service B? Contract Testing solves this problem without running all services at the same time.
1. Problem: API Breaking Changes
Service A (Consumer) gọi Service B (Provider):
Service B thay đổi response:
Before: { "price": 999 } ← number
After: { "price": "999.00" } ← string!
→ Service A parse Price as number → crash!
→ Integration test phát hiện? Chỉ nếu chạy cả A + B cùng lúc
→ E2E test? Chậm, flaky, expensive
→ Contract test? ✅ Phát hiện sớm, CI tốc độ cao
2. Consumer-Driven Contract Testing
2.1 Concepts
Consumer (Order Service):
"Tôi expect response từ Product Service có format:
{ id: string, name: string, price: number }"
→ Ghi thành Pact (contract file)
Provider (Product Service):
"Để tôi verify rằng API của tôi satisfy contract này"
→ Run Pact verification against real API
Nếu fail → Provider biết thay đổi sẽ break Consumer
→ Fix API hoặc coordinate migration
2.2 Flow
┌──────────────┐ 1. Generate ┌──────────────┐
│ Consumer │────Pact File───►│ Pact Broker │
│ (Order Svc) │ (contract) │ (central │
└──────────────┘ │ registry) │
└──────┬───────┘
│
2. Verify│
▼
┌──────────────┐
│ Provider │
│ (Product Svc)│
└──────────────┘
3. Pact Implementation
3.1 Consumer Side (Order Service)
// order-service/tests/pact/product.consumer.test.js
const { PactV3 } = require('@pact-foundation/pact');
describe('Product Service Contract', () => {
const provider = new PactV3({
consumer: 'OrderService',
provider: 'ProductService',
});
it('get product by id', async () => {
// Define expected interaction
provider
.given('product 123 exists')
.uponReceiving('a request for product 123')
.withRequest({
method: 'GET',
path: '/api/products/123',
})
.willRespondWith({
status: 200,
body: {
id: '123',
name: like('Laptop'),
price: like(999),
inStock: like(true),
},
});
// Execute test against Pact mock
await provider.executeTest(async (mockService) => {
const product = await fetchProduct(mockService.url, '123');
expect(product.id).toBe('123');
expect(typeof product.price).toBe('number');
});
});
});
3.2 Provider Verification (Product Service)
// product-service/tests/pact/provider.verify.test.js
const { Verifier } = require('@pact-foundation/pact');
describe('Pact Verification', () => {
it('validates OrderService contract', async () => {
const verifier = new Verifier({
providerBaseUrl: 'http://localhost:8080',
pactBrokerUrl: 'https://pact-broker.company.com',
provider: 'ProductService',
providerVersion: process.env.GIT_SHA,
publishVerificationResult: true,
stateHandlers: {
'product 123 exists': async () => {
await seedDB({ id: '123', name: 'Laptop', price: 999 });
},
},
});
await verifier.verifyProvider();
});
});
4. CI/CD Integration
# Consumer CI (Order Service)
consumer-contract-test:
steps:
- run: npm test -- --testPathPattern=pact
- run: npx pact-broker publish pacts/
--consumer-app-version=$GIT_SHA
--branch=$BRANCH
# Provider CI (Product Service)
provider-contract-verify:
steps:
- run: npm run pact:verify
# can-i-deploy check before deploymentt
- run: npx pact-broker can-i-deploy
--pacticipant=ProductService
--version=$GIT_SHA
--to-environment=production
4.1 Can-I-Deploy
Before deploying Product Service v2.1:
$ pact-broker can-i-deploy \
--pacticipant ProductService \
--version 2.1 \
--to production
✅ OrderService (1.5) → ProductService (2.1): VERIFIED
✅ CartService (2.0) → ProductService (2.1): VERIFIED
Result: Safe to deploy!
5. Schema Evolution Best Practices
| Change Type | Breaking? | Strategy |
|---|---|---|
| Add field | No | Just add (consumers ignore unknown) |
| Remove fields | YES | Deprecate → verify no consumers use → remove |
| Rename field | YES | Add new → migrate consumers → remove old |
| Change type | YES | New field → migrate → remove old |
| Add endpoint | No | Just add |
| Remove endpoint | YES | Contract test catches usage |
Summary
- Contract Testing = verify API contracts between services
- Consumer-Driven: consumer defines expectations, provider verifies
- Pact: industry standard tool, supports many languages
- Pact Broker: central registry + can-i-deploy safety check
- CI/CD: run on every PR, block deploy if contracts broken
- Catches breaking changes before integration testing
Next article: Lesson 23: Mono-Repo vs Multi-Repo — Source Code Organization