Integration Contract Testing Engineering

Guarantee that component boundaries remain compatible as the system evolves.

Decision Criteria

• Contract tests are required for all externally consumed boundaries: REST DTOs, websocket events, queue payloads.
• Integration scope must include actual adapters and middleware behaviors.
• Backward compatibility windows must be test-encoded for versioned contracts.
• High-change interfaces require schema snapshot and diff enforcement.

Principal Practices

• Assert response envelope invariants for all public endpoints.
• Validate domain error mapping to stable API error codes.
• Create queue payload schema tests for each job type and version.
• Include fallback-mode contract tests (Redis unavailable, queue lag, S3 temporary failure).

Failure Modes & Anti-Patterns

• Handler tests that bypass middleware and context setup.
• Queue consumers accepting malformed payloads silently.
• Endpoint tests that verify status code only, not payload contract.