API Definition

Overview

API Definition is the formal description of an API’s available operations, data models, parameters, responses, security, and protocol details. It is used to provide a shared representation that tools and teams can use to implement and govern the interface. It differs from an API contract, which also includes behavioral promises and organizational obligations beyond the formal description.

A typical implementation works as follows: The definition is authored in OpenAPI, AsyncAPI, protocol buffers, GraphQL schema language, or another format and published with the service version. Testing API Definition should cover malformed input, denied access, rate limits, timeouts, duplicates, asynchronous completion, and version changes.

Common risks include missing operations and undocumented extensions. Important failure modes include missing operations, undocumented extensions, inconsistent examples, and divergence from runtime behavior. These failures can mislead generated clients and tests.

Core controls require teams to treat it as source-controlled code, validate it, generate artifacts reproducibly, review changes, and compare against deployed behavior. The API Definition contract should define identifiers, validation, authorization, status semantics, errors, limits, and version behavior at the same boundary.

Operational evidence should include definition format, revision, service version, validation result, generated artifact versions, and owner. API Definition should be documented alongside API Contract, API Schema, and OpenAPI Specification. Changes to API Definition should be checked against older clients, stored payloads, enum growth, default behavior, and retry assumptions.

The API Definition owner should publish stable error semantics and guidance on whether correction, retry, status lookup, or escalation is appropriate. Data handled by API Definition should use documented types, units, timestamp rules, nullable fields, and enum behavior to prevent silent interpretation errors.