API Contract Testing

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

Key Concepts

Term	Definition
Consumer	Service that calls an API
Provider	Service that exposes an API
Contract	Agreed request/response format
Pact	Consumer-driven contract testing tool
Schema	Structure definition (OpenAPI, JSON Schema)
Broker	Central repository for contracts

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('john@example.com')
        })
      })
      .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();

OpenAPI Validation (Express)

const OpenApiValidator = require('express-openapi-validator');

app.use(OpenApiValidator.middleware({
  apiSpec: './openapi.yaml',
  validateRequests: true,
  validateResponses: true,
  validateSecurity: true
}));

Additional Implementations

• Python JSON Schema: See references/python-json-schema.md
• Java REST Assured: See references/java-rest-assured.md
• Pact Broker CI/CD: See references/pact-broker-cicd.md

Best Practices

Do:

• Test from consumer perspective
• Use matchers for flexible matching
• Validate structure, not specific values
• Version contracts explicitly
• Test error responses
• Run tests in CI pipeline
• Test backward compatibility

Don't:

• Test business logic in contracts
• Hard-code specific values
• Skip error scenarios
• Ignore versioning
• Deploy without verification

Tools

• Pact - Multi-language consumer-driven contracts
• Spring Cloud Contract - JVM ecosystem
• OpenAPI/Swagger - Schema-first validation
• Dredd - API blueprint testing
• Spectral - OpenAPI linting