Modern web applications rely heavily on APIs — the bridges between systems. When building these APIs, following established design principles ensures they remain predictable, reliable, and maintainable over time.
This article breaks down three key aspects of high-quality API design: REST, OpenAPI 3.1, Semantic Versioning, and Idempotent Design.

REST — The Foundation of Modern APIs

REST (Representational State Transfer) is an architectural style for designing networked applications.
It uses standard HTTP methods and focuses on stateless communication (each request stands alone) — each request from the client to the server must contain all the information needed to understand and process it, typically use JSON for request/response bodies.

Common RESTful methods:
- GET → read data
- POST → create data
– PUT / PATCH → update data
– DELETE → remove data

Example:
GET /api/v1/users/123

Returns the details of user 123.

OpenAPI 3.1 — The API Blueprint

The OpenAPI Specification (OAS) is a standard for describing REST APIs.
It acts as a contract between API providers and consumers — defining endpoints, request/response formats, authentication, and examples in a machine-readable format (usually YAML or JSON).

Example (OpenAPI 3.1 snippet):

openapi: 3.1.0
info:
  title: User Management API
  version: 1.0.0
paths:
  /users:
    get:
      summary: Retrieve all users
      responses:
        '200':
          description: List of users

Why use OpenAPI 3.1?

• Fully compatible with JSON Schema for data validation
• Easier integration with tools like Swagger UI, Postman, and Redoc
• Enables auto-generated documentation and client SDKs

Semantic Versioning — Clarity in Evolution

As APIs grow, change is inevitable. Semantic Versioning (SemVer) provides a clear and predictable way to communicate those changes.

The format follows:
MAJOR.MINOR.PATCH

MAJOR → Breaking changes (incompatible with previous versions)
MINOR → Backward-compatible feature additions
PATCH → Backward-compatible bug fixes

Example:

• /api/v1/users — initial release
• /api/v2/users — introduces breaking changes

Versioning your API (e.g., /v1, /v2) makes transitions smoother for developers consuming your API.

Idempotent Design — Safety and Predictability

Idempotent operations produce the same result no matter how many times they are performed.
This is critical for network reliability — if a request is retried due to timeout or failure, it shouldn’t create duplicate or inconsistent results.

Example:

• PUT /users/123 with { "name": "Alice" }
  → Sets the name to “Alice” every time
• POST /users
  → Creates a new user each time

Designing idempotent endpoints helps ensure system behaves predictably and is easier to recover from errors.

Combining REST principles, OpenAPI 3.1 documentation, semantic versioning, and idempotent design leads to well-structured, maintainable, and developer-friendly APIs.

By applying these consistently, we can make API more robust, self-explanatory, and future-proof — essential traits for modern software systems.