Contract-first API design defines the API specification before writing implementation code, enabling parallel frontend and backend development. OpenAPI Generator creates server stubs from specs, Schemathesis detects implementation drift in CI, and AsyncAPI extends contract-first principles to event-driven architectures.
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What does contract-first API design mean, and how does it differ from code-first?
In contract-first design, teams define the API specification (OpenAPI, AsyncAPI, Protobuf) before writing any implementation code. This enables parallel development: frontend teams mock from the spec while backend teams implement it. Code-first generates the spec from existing code, which often results in poorly designed APIs.
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A team adopts contract-first with OpenAPI. Which tool can generate server stubs in multiple languages from the spec?
OpenAPI Generator takes an OpenAPI spec and generates server stubs (controller interfaces and models) in 50+ languages/frameworks and client SDKs in 40+ languages. Teams implement the generated interfaces, ensuring the implementation stays aligned with the agreed spec.
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In contract-first API design, what is a breaking change to an API contract?
A breaking change modifies the contract in a way that breaks existing consumers: removing fields, changing field types, making optional fields required, or removing endpoints. Adding optional fields, new optional parameters, or new endpoints are non-breaking (backward-compatible) changes.
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Which practice ensures that the production code remains aligned with the API contract over time in a contract-first workflow?
Running contract tests in CI (e.g., using Dredd, Schemathesis, or custom spec validation) automatically detects implementation drift — when the code diverges from the agreed spec. Tools like Schemathesis generate test cases from the OpenAPI spec and run them against the live server to catch inconsistencies.
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A team uses AsyncAPI instead of OpenAPI for their event-driven microservices. What does AsyncAPI describe that OpenAPI cannot?
AsyncAPI is the contract-first standard for event-driven APIs: it describes Kafka topics, AMQP queues, WebSocket channels, and MQTT subscriptions — asynchronous patterns where messages flow in multiple directions. OpenAPI is designed for synchronous request-response HTTP APIs and cannot adequately describe pub/sub or streaming patterns.