HATEOAS และ Richardson Maturity Model คืออะไร?

Level 0 — "The Swamp of POX": one URI, one method (RPC over HTTP).
          POST /api  { "action": "getUser", "id": 42 }
Level 1 — Resources: many URIs, but still one verb.
          POST /users/42  ,  POST /users/42/delete
Level 2 — HTTP Verbs: proper methods + status codes (where most "REST" APIs live).
          GET /users/42 → 200 ,  DELETE /users/42 → 204
Level 3 — Hypermedia Controls (HATEOAS): responses link to what you can do next.