Contract Testing with Pact

Overview

Contract testing validates that service consumers and providers agree on request/response expectations. Pact implements consumer-driven contracts (CDC) with shareable pact files and provider verification.

Table of Contents

1. •What is Contract Testing
2. •Consumer-Driven Contracts
3. •Pact Fundamentals
4. •Pact vs Integration Testing
5. •Writing Consumer Tests
6. •Provider Verification
7. •Pact Broker
8. •CI/CD Integration
9. •Bi-Directional Contracts
10. •Async Messaging
11. •GraphQL
12. •Language Support
13. •Patterns and Anti-Patterns
14. •Breaking Change Strategy
15. •Monitoring Compliance

What is Contract Testing

Contract tests validate API interactions without full end-to-end setups. They prevent breaking changes by verifying expectations explicitly.

Consumer-Driven Contracts

Consumers define expectations; providers must satisfy them:

• •Faster feedback for consumers
• •Clear API expectations
• •Reduced integration surprises

Pact Fundamentals

• •Consumer tests: Generate pact files.
• •Provider verification: Validate pact files.
• •Pact files: JSON contracts.
• •Pact Broker: Store and manage contracts.

Pact vs Integration Testing

• •Pact: Validates interface compatibility.
• •Integration: Validates full system behavior.

Use both for comprehensive coverage.

Writing Consumer Tests

Define interactions with matchers:

• •Exact value
• •Type-based matcher
• •Regex matcher

Example (TypeScript):

await provider.addInteraction({
  state: 'user exists',
  uponReceiving: 'get user',
  withRequest: { method: 'GET', path: '/users/123' },
  willRespondWith: {
    status: 200,
    body: { id: like(123), name: like('Alice') }
  }
});

Provider Verification

Provider verifies against published contracts:

• •Implement state handlers
• •Validate response headers/body/status
• •Support pending and WIP pacts for safe changes

Pact Broker

Key features:

• •Publish and version contracts
• •Webhooks on new contracts
• •Can-I-Deploy checks for promotion
• •Visibility into compatibility

CI/CD Integration

Recommended flow:

1. •Consumer tests generate pact
2. •Publish pact to broker
3. •Provider verifies in CI
4. •Use Can-I-Deploy before release

Bi-Directional Contracts

Support for provider-defined expectations plus consumer constraints:

• •Useful for GraphQL or schema-first APIs
• •Prevents provider drift

Async Messaging

Pact supports message-based contracts:

• •Define message payload expectations
• •Verify consumer and provider handlers

GraphQL

Contract test schema and query responses:

• •Use schema as a contract baseline
• •Validate query response shapes

Language Support

SDKs available for:

• •JavaScript/TypeScript
• •Python
• •Java
• •Go

Patterns and Anti-Patterns

Good:

• •Use flexible matchers
• •Keep interactions focused

Avoid:

• •Over-specifying exact values
• •One contract covering multiple unrelated cases

Breaking Change Strategy

• •Use pending pacts for backward-compatible changes.
• •Version APIs when breaking changes are required.
• •Maintain old contract until consumers migrate.

Monitoring Compliance

Track contract verification in CI and alert on failed verification runs.

Related Skills

• •16-testing/integration-testing
• •09-microservices/service-design
• •03-backend-api/express-rest