Practice : Contract Testing

Purpose and Strategic Importance

Contract Testing is a quality assurance practice that ensures services and their consumers agree on the format and behaviour of their interactions. By testing the "contract" between them, teams can detect breaking changes early - without relying on full end-to-end tests.

This practice is essential in distributed systems and microservice environments, where tight coupling and late discovery of integration issues can derail releases. Contract testing reduces integration risk, improves team autonomy, and enables safer, faster deployments.

Description of the Practice

  • A contract describes expected request/response formats and interaction rules between services.
  • Consumer-driven contract testing tools (e.g. Pact, Spring Cloud Contract) allow consumers to define expectations.
  • Providers validate contracts as part of their test pipeline to ensure compatibility.
  • Contracts are versioned, tested in CI/CD, and shared between teams or stored in a broker.
  • Breaking changes are caught early through test failures - not in production.

How to Practise It (Playbook)

1. Getting Started

  • Identify service-to-service or API interactions in your system.
  • Use a contract testing tool like Pact or Spring Cloud Contract to define consumer expectations.
  • Integrate contract verification into provider pipelines.
  • Share contract artifacts in a registry, repo, or contract broker accessible to both parties.

2. Scaling and Maturing

  • Automate publishing and verification of contracts across all environments.
  • Use semantic versioning and test matrices to validate compatibility across versions.
  • Enforce contract testing as a precondition for deployment in integration pipelines.
  • Make contracts visible and part of architectural governance.
  • Integrate contract changes into product planning to manage breaking changes collaboratively.

3. Team Behaviours to Encourage

  • Collaborate early between consumer and provider teams on contract design.
  • Treat contracts as living artefacts - versioned, reviewed, and evolved.
  • Use contract failures as learning opportunities, not blockers.
  • Keep contract scenarios small, focused, and reflective of real business flows.

4. Watch Out For…

  • Contracts that drift from actual system behaviour due to lack of maintenance.
  • Over-relying on contract tests while ignoring functional or end-to-end coverage.
  • Poor communication between teams about expected changes.
  • Complex broker setups that add friction to development.

5. Signals of Success

  • Breaking changes are caught before deployment, not in production.
  • Teams deploy independently while maintaining interoperability.
  • Contracts are trusted as accurate specifications of service behaviour.
  • Consumer and provider teams share a strong feedback loop.
  • Service integrations are more stable and require fewer coordination meetings.