స్టేట్‌ఫుల్ మరియు స్టేట్‌లెస్ సర్వీసుల మధ్య తేడా ఏమిటి?

STATELESS → the service keeps NO client state between requests:
  → each request contains all needed info; any server can handle any request
  → state lives ELSEWHERE (database, cache, client, token) if needed
STATEFUL → the service MAINTAINS state across requests:
  → a specific server holds a client's state (session in memory, etc.)
  → requests must go to the SAME server (or state must be shared/synchronized)