Stateful or stateless? The difference is where your session lives.
STATEFUL
The server keeps your session in its own memory. Log in, and Server A knows
who you are — but only Server A. Your next request has to come back to that
same server, which is why stateful setups often need sticky sessions.
STATELESS
The server keeps nothing locally between requests. Each request carries what
it needs — typically a signed token the server validates. Server A, B or C
can all handle it, because none of them are holding anything the others lack.
WHY IT MATTERS
Stateless scales out with less coordination: add a server, send it traffic,
done. Stateful can absolutely scale too — it just needs shared session
storage (Redis, a database) or session affinity at the load balancer. It's
a difference in coordination cost, not a hard limit.
TWO THINGS WORTH BEING PRECISE ABOUT
1. Stateless doesn't mean "stores no data." A stateless service still reads
and writes databases and caches constantly. What it avoids is depending
on local, per-server session state between requests.
2. A token in a request isn't security by itself. It has to be verified —
signature, expiry, issuer, audience — on every request. An unvalidated
token is just a string the client sent you.
IN PRACTICE
Most APIs go stateless and push shared state behind the app servers, into
Redis or the database. That's the pattern in the diagram: identical Rails
API servers, no local sessions, shared services behind them.
Pick what fits your use case.
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