Casting your friend group as a K-Pop group without making a database the product

Try the demo: K-Saju Crew For fun only. K-Saju is an entertainment project. The K-Pop roles below are a playful interpretation of saju-inspired signals, not personality assessment or advice. A two-person compatibility page can stay stateless with almost no effort. Put both birth dates in a URL, render the result on the server, and the link is the record. No account, no database, no cleanup job. That was already a product rule in K-Saju. We do not retain personal inputs. A result is reproducible from its GET parameters. Then we built /crew : “What if your friend group debuted as a K-Pop group?” A creator makes a link, sends it to a group chat, and each friend enters their own birth date. At three to seven members, the app assigns distinct positions, shows pairwise chemistry, and creates a shareable poster. The fun part is the casting. The engineering problem is that the social flow needs a temporary shared state. A link cannot accumulate submissions by itself. This post is about the decisions behind that feature: where we allowed state, how we made the result durable without retaining a lobby forever, and how we kept the casting explainable instead of treating it as a black-box score. The conflict: a self-service group flow needs somewhere to collect data There were two clean but incomplete options. The first was to keep everything stateless. The creator would enter all members' dates at once, then receive a result URL. It matched our existing architecture, but it defeated the point of sharing a link. The person who starts the group often does not know everyone else's date, and asking them to collect it in a chat creates friction before the feature has started. The second was a conventional persistent group object. It would make joining easy, but it would turn a deliberately stateless service into one that keeps user-provided dates indefinitely unless we built retention and deletion policies around it. We chose a hybrid instead: The lobby is temporary state. It store