Course outline

01 · HTTP & REST APIs

02 · Databases & Persistence

03 · Authentication & Authorization

04 · API Design & gRPC

05 · Caching, Queues & Background Jobs

06 · Production-Ready Services

07 · Advanced Auth & Tenant Security

08 · Performance & Caching at Scale

09 · Testing & Release Confidence

Stage 3 · Build

Testing & Release Confidence

Contract Testing

Validate API compatibility with OpenAPI schemas, Pact contracts, generated clients, and backward-compatible fixtures.

8 min readBuilding Backend ApplicationsBuild

Why Contract Testing

APIs are consumed by clients. If you change the API shape, clients break. Contract tests verify that the API matches its specification. They catch breaking changes before deployment.

OpenAPI Validation

Govalidate-responses-against-openapi-spec.go

41 linesLn 1, Col 1Go

Validate actual API responses against the OpenAPI schema. This catches missing fields, wrong types, and unexpected response shapes.

Pact Contracts

Gopact-provider-verification.go

16 linesLn 1, Col 1Go

Pact contracts are generated by consumer tests. The provider verifies that it satisfies all contracts. This catches breaking changes between services.

Generated Clients

Gotest-with-generated-client.go

30 linesLn 1, Col 1Go

Generated clients are type-safe. If the API shape changes, the generated code fails to compile. This catches contract violations at build time.

Backward-Compatible Fixtures

Gosnapshot-testing-for-api-responses.go

27 linesLn 1, Col 1Go

Snapshot tests capture the exact API response shape. If any field changes, the test fails. Update snapshots when intentional changes are made.

Breaking Change Detection

buf breaking change detection


# Compare current proto against main branch
buf breaking --against ".git#branch=main"

# Compare OpenAPI specs
oapi-diff old-spec.yaml new-spec.yaml

Automated breaking change detection catches incompatible changes in CI. buf checks proto files. oapi-diff checks OpenAPI specs. Both prevent accidental breaking changes.

Test consumer and provider separately

Contract testing separates consumer expectations from provider implementation. Consumers define what they need. Providers verify they deliver it. This decouples deployment cycles.

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