Consumer-Driven Contract Testing Concepts

1# Contract testing flow:
2
31. Consumer writes Pact test
4   → Interaction: POST /auth/login with {email, password}
5   → Expectation: 200 {accessToken: string, user: object}
6
72. Pact runs consumer test against a mock server
8   → Records the interaction as a pact file (JSON)
9
103. Pact publishes to Pact Broker
11   → Other teams can see what you depend on
12
134. Provider runs Pact verifier
14   → Replays each interaction against the real API
15   → Verifies responses match the consumer's expectations
16
175. can-i-deploy check before deployment
18   → Ensures consumer and provider are compatible

1npm install --save-dev @pact-foundation/pact

1import { PactV3 } from '@pact-foundation/pact';
2import { like, eachLike } from '@pact-foundation/pact/src/dsl/matchers';
3import * as path from 'path';
4
5const provider = new PactV3({
6  consumer: 'web-frontend',
7  provider: 'api-service',
8  dir: path.resolve(process.cwd(), 'pacts'),
9});
10
11test('GET /courses contract', async () => {
12  await provider
13    .given('courses exist')
14    .uponReceiving('a request for courses')
15    .withRequest({ method: 'GET', path: '/courses' })
16    .willRespondWith({
17      status: 200,
18      body: {
19        data: eachLike({ id: like('id'), title: like('title') }),
20        meta: like({ total: 42 }),
21      },
22    })
23    .executeTest(async (mockServer) => {
24      const res = await fetch(`${mockServer.url}/courses`);
25      const body = await res.json();
26      expect(res.status).toBe(200);
27      expect(body.data).toBeInstanceOf(Array);
28    });
29});

1import { Verifier } from '@pact-foundation/pact';
2
3test('provider verification', async () => {
4  await new Verifier({
5    providerBaseUrl: `http://localhost:${port}`,
6    pactUrls: ['pacts/web-frontend-api-service.json'],
7    provider: 'api-service',
8    stateHandlers: {
9      'courses exist': async () => {
10        await prisma.course.createMany({ data: testCourseData });
11      },
12    },
13  }).verifyProvider();
14});

1# Publish pacts after consumer tests pass:
2npx pact-broker publish pacts/ \
3  --broker-base-url https://pact-broker.example.com \
4  --broker-token $TOKEN \
5  --consumer-app-version $(git rev-parse --short HEAD)
6
7# Check deployment safety:
8npx pact-broker can-i-deploy \
9  --pacticipant web-frontend \
10  --version $(git rev-parse --short HEAD) \
11  --to-environment staging \
12  --broker-base-url https://pact-broker.example.com
13
14# can-i-deploy exits 0 (safe) or 1 (not safe)
15# Add to deploy script before every deployment

1import { like, eachLike, term, integer, decimal } from '@pact-foundation/pact/src/dsl/matchers';
2
3// like(): matches the type, not the value
4body: { id: like('any-string-id'), count: like(42) }
5// Verifies: id is a string, count is a number
6
7// eachLike(): array where each item matches the pattern
8body: { data: eachLike({ id: like('id'), price: like(10.0) }) }
9// Verifies: data is an array, each item has id (string) and price (number)
10
11// term(): regex match
12body: { email: term({ generate: '[email protected]', matcher: '^[\\w.-]+@[\\w.-]+\\.[a-z]{2,}$' }) }
13
14// integer(): whole numbers only
15body: { count: integer(5) }
16
17// decimal(): decimal numbers
18body: { price: decimal(79.99) }
19
20// NEVER use exact values in pact matchers:
21// BAD: body: { id: 'cuid-specific-value' } — breaks when ID changes
22// GOOD: body: { id: like('any-id') } — stable across test runs