Stateless vs Stateful: Remember the User, or Hand Them a Ticket?
Two ways to handle client state between requests: a stateful server keeps your session in its own memory so every request must return to the same machine, while a stateless server keeps nothing — each request carries what it needs, so any instance can handle any request.
A stateful server keeps session state in its own memory, so requests must stick to the same server and a crash loses the session. A stateless server keeps nothing between requests — a self-contained token or a lookup into a shared store any server can read, so any instance handles any request. That is what makes horizontal scaling, load balancing, and autoscaling work by default; the cost is pushing state out into a token or a real database on purpose.
Transcript
Two ways to build a service. One remembers you between requests. The other remembers nothing — and just hands you a ticket. That tiny difference decides whether you can add servers on demand, or not. So which do you build: memory, or a ticket?
A stateful server keeps your session in its own memory. So every request has to come back to the SAME server — a sticky session. That works until you need to grow. A load balancer can't freely spread you around, and if that one server dies, your session dies with it.
A stateless server keeps NOTHING between requests. Each one carries what it needs — a self-contained token, or a lookup into a shared store any server can read. It's a coat-check ticket. Carry it, and ANY attendant can fetch your coat — no single one has to remember you.
That's the payoff. Any instance can handle any request — so you can add servers, remove them, or lose one, and it just works. Load-balance freely, autoscale on traffic, fail over instantly. Stateless services scale horizontally by default.
The cost: you push the state OUT — into the client's token, or a shared database — which adds a hop, or a little size. Stateful is simpler and faster locally, but it chains each client to one server and can't scale elastically — usually not worth it.
So keep your servers stateless — carry the state in a token or a shared store — and let only your real databases and caches hold state on purpose. It comes down to one choice: make a server remember the user, or hand the user a ticket?