fielding skill

---
name: fielding-rest
description: Design network-based software architectures using Roy Fielding's REST principles. Emphasizes the constraints of the web—statelessness, cacheability, uniform interfaces, and HATEOAS. Use when designing web APIs that must endure for decades.
---

# Roy Fielding Style Guide

## Overview

Roy Fielding (creator of REST and HTTP co-author) defines the architectural style that powers the World Wide Web. His philosophy is about **architectural constraints** that induce properties like scalability, visibility, and modifiability. True REST is not just JSON over HTTP; it is about decoupling the client from the server through a uniform interface and hypermedia.

> "A REST API should not be dependent on any single communication protocol, though its successful mapping to HTTP is why it is famous."

## Core Principles

1.  **Uniform Interface**: Resources are identified by URIs, and manipulation happens via standard representations (HTML, JSON).
2.  **Statelessness**: Every request must contain all context necessary to understand it. The server stores no session state.
3.  **Cacheability**: Responses must define themselves as cacheable or not to improve network efficiency.
4.  **HATEOAS (Hypermedia as the Engine of Application State)**: The client navigates the API via links provided dynamically by the server, not by hardcoding URL structures.
5.  **Layered System**: The client cannot tell if it is connected directly to the server or an intermediary (CDN, load balancer).

## Prompts

### Design a True REST API

> "Act as Roy Fielding. Critique this API design.
>
> Focus on:
> *   **Noun-Verbs**: Are they using POST to 'get' data? Are they tunnelling RPC through HTTP?
> *   **Hypermedia**: Does the response include links (`_links`, `Link` header) to related resources?
> *   **Caching**: Are `Cache-Control` headers used correctly?
> *   **Status Codes**: Are they using 200 OK for errors? Are they using 201 Created?"

### Architectural Review

> "Evaluate this system architecture against REST constraints.
>
> Questions to ask:
> 1.  Is the server storing session state (violates Statelessness)?
> 2.  Are the clients coupled to the internal DB ID structure (violates Uniform Interface)?
> 3.  Can we insert a caching proxy without breaking the client (Layered System)?
> 4.  If I change the URL of a resource, will the client break (Violates HATEOAS)?"

## Examples

### The Richardson Maturity Model (Level 3 - Glory of REST)

#### Request
```http
GET /accounts/12345 HTTP/1.1
Host: bank.example.com
Accept: application/vnd.bank.account+json
```

#### Response
```http
HTTP/1.1 200 OK
Content-Type: application/vnd.bank.account+json
Cache-Control: public, max-age=3600
Link: <https://bank.example.com/docs/account>; rel="profile"

{
  "account_number": "12345",
  "balance": {
    "currency": "USD",
    "value": 100.00
  },
  "status": "active",
  "_links": {
    "self": {
      "href": "/accounts/12345"
    },
    "deposit": {
      "href": "/accounts/12345/deposit",
      "method": "POST"
    },
    "withdraw": {
       "href": "/accounts/12345/withdraw",
       "method": "POST"
    },
    "details": {
       "href": "/accounts/12345/transactions",
       "method": "GET"
    }
  }
}
```
*Note the `_links` section. Ideally, the client only knows the entry point (`/`) and navigates entirely via these links.*

### Anti-Patterns (What NOT to do)

*   **RPC style**: `POST /updateUser?id=123`.
*   **Ignoring Semantics**: Using `GET` to delete data.
*   **API Versioning in URL**: `/v1/users` (Fielding argues versioning should be in the Media Type `Accept: application/vnd.myapi.v1+json`).
*   **No Hypermedia**: Returning just `{ "id": 1, "name": "bob" }` without context on what can be done with "bob".

## Resources

*   [Architectural Styles and the Design of Network-based Software Architectures (Fielding's Dissertation)](https://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm)
*   [REST APIs must be hypertext-driven (Fielding's Blog)](https://roy.gbiv.com/untangled/2008/rest-apis-must-be-hypertext-driven)