API Schema

Overview

API Schema is the formal definition of the structure, types, required fields, constraints, and relationships in API messages. It is used to enable validation, documentation, code generation, and compatible data evolution. It differs from an API contract, which also defines behavior, permissions, timing, errors, and side effects.

A typical implementation works as follows: A schema describes requests and responses in JSON Schema, OpenAPI, protocol buffers, GraphQL, or another type system and is validated during development or runtime. Testing API Schema should cover malformed input, denied access, rate limits, timeouts, duplicates, asynchronous completion, and version changes.

Common risks include loosely defined types and unbounded values. Important failure modes include loosely defined types, unbounded values, precision loss, undocumented nullability, and incompatible changes. These failures can create security or data defects.

Core controls require teams to use explicit formats and limits, version schemas, reject ambiguous input, test boundary values, and apply compatibility checks. The API Schema contract should define identifiers, validation, authorization, status semantics, errors, limits, and version behavior at the same boundary.

Operational evidence should include schema ID, format version, message type, validation result, field path, constraint, and service release. API Schema should be documented alongside API Definition, API Contract, and API Response Envelope. Changes to API Schema should be checked against older clients, stored payloads, enum growth, default behavior, and retry assumptions.

Observability for API Schema should record operation, version, principal, request ID, latency, retry count, error code, and final state without exposing secrets. The API Schema owner should publish stable error semantics and guidance on whether correction, retry, status lookup, or escalation is appropriate.