api-contract-testing

API Contract Testing

Verify that APIs honor their contracts between consumers and providers without requiring full integration tests.

Key Concepts

Pact Consumer Test (TypeScript)

import { PactV3, MatchersV3 } from '@pact-foundation/pact';

const provider = new PactV3({
  consumer: 'OrderService',
  provider: 'UserService'
});

describe('User API Contract', () => {
  it('returns user by ID', async () => {
    await provider
      .given('user 123 exists')
      .uponReceiving('a request for user 123')
      .withRequest({ method: 'GET', path: '/users/123' })
      .willRespondWith({
        status: 200,
        body: MatchersV3.like({
          id: '123',
          name: MatchersV3.string('John'),
          email: MatchersV3.email('[email protected]')
        })
      })
      .executeTest(async (mockServer) => {
        const response = await fetch(`${mockServer.url}/users/123`);
        expect(response.status).toBe(200);
      });
  });

  it('returns 404 for non-existent user', async () => {
    await provider
      .given('user does not exist')
      .uponReceiving('a request for non-existent user')
      .withRequest({ method: 'GET', path: '/users/999' })
      .willRespondWith({
        status: 404,
        body: MatchersV3.like({
          error: { code: 'NOT_FOUND', message: MatchersV3.string() }
        })
      })
      .executeTest(async (mockServer) => {
        const response = await fetch(`${mockServer.url}/users/999`);
        expect(response.status).toBe(404);
      });
  });
});

Provider Verification

import { Verifier } from '@pact-foundation/pact';

new Verifier({
  provider: 'UserService',
  providerBaseUrl: 'http://localhost:3000',
  pactBrokerUrl: process.env.PACT_BROKER_URL,
  publishVerificationResult: true,
  providerVersion: process.env.GIT_SHA,
  stateHandlers: {
    'user 123 exists': async () => {
      await db.users.create({ id: '123', name: 'John' });
    },
    'user does not exist': async () => {
      await db.users.deleteAll();
    }
  }
}).verifyProvider();