API Contract

The contract explicitly defines all available endpoints (URL paths) and the HTTP methods (GET, POST, PUT, DELETE) they support. For each operation, it specifies the intended action, such as retrieving a resource or submitting data. This is the foundational routing map for the API.

• Example: GET /users/{id} is defined as an operation to retrieve a specific user.
• Standard: Defined in the paths object of an OpenAPI Specification.

This component defines the exact structure, data types, and constraints for all request bodies, query parameters, path parameters, headers, and response payloads. It uses schema languages like JSON Schema to enforce correctness.

• Key Elements: Required vs. optional fields, data types (string, integer, array), allowed values (enums), and format patterns (email, date-time).
• Purpose: Enables static validation during development and dynamic validation at runtime to prevent malformed data.

The contract specifies the security mechanisms required to access the API. This defines how to authenticate (prove identity) and the scopes or permissions needed for specific operations.

• Common Schemes: API Keys, OAuth 2.0 flows, Bearer tokens (JWT), and mutual TLS (mTLS).
• Security Object: In OpenAPI, this is defined in a global securitySchemes object and applied to individual operations.

A robust contract pre-defines the structure and meaning of error responses. This allows clients to handle failures programmatically. It standardizes HTTP status codes (4xx for client errors, 5xx for server errors) and the format of the error payload.

• Typical Error Payload: Includes a machine-readable error code, a human-readable message, and sometimes a trace ID for debugging.
• Benefit: Enables clients to implement consistent retry logic and user feedback based on error type.

This component documents the operational limits imposed on API consumers to ensure system stability and fair usage. It defines thresholds for request volume within specific time windows.

• Common Limits: Requests per second, requests per day, or concurrent connections.
• Communication: Often communicated via HTTP response headers like X-RateLimit-Limit, X-RateLimit-Remaining, and X-RateLimit-Reset.

Beyond strict operational rules, the contract includes essential metadata that describes the API's purpose, ownership, and lifecycle. This provides context for developers and tools.

• Includes: API title, version, description, terms of service, contact information, and server URLs (e.g., production vs. staging).
• Tooling: This metadata is used by API gateways, developer portals, and SDK generators.

Schema enforcement is the runtime application of validation rules, defined in a schema language like JSON Schema, to guarantee that data structures strictly conform to a predefined model.

• Runtime Guarantee: Performs dynamic validation on live data as it enters or leaves the system.
• Implemented via validation middleware (e.g., in Express.js, FastAPI) or library-based validation (Pydantic, Zod).
• Ensures schema conformance, catching malformed payloads before they reach core business logic.

These are two layers of validation applied to data, often in sequence.

• Syntactic Validation: Checks if data conforms to format and grammar rules. This is type checking and regex pattern validation (e.g., "Is this a valid email string?").
• Semantic Validation: Checks if syntactically correct data is meaningful within the business context. This involves constraint checking across fields (e.g., "Is the start_date before the end_date?").

An API contract defines the rules for both layers.

Payload verification is the comprehensive validation of an HTTP request or response body. It is the practical execution of checks defined by the API contract and related schemas.

• Scope: Includes validating structure (JSON/XML), content-type headers, encoding, data types, and all defined constraints.
• Integrates multiple specific validations: null checks, JWT claim validation for authenticated endpoints, and idempotency key validation for POST/PATCH operations.
• Critical for security, as it includes data sanitization to prevent injection attacks and XML External Entity (XXE) prevention.