Real-time UIs are easy to demo; production is harder. Every WebSocket event has to reconcile with PostgreSQL so reconnects, duplicate deliveries, and late joins still converge on the same truth users see in the database.
On the stack We use NestJS gateways with explicit room/subscription rules so a user only receives streams they are authorized for–same user id checks as the REST layer, not a separate informal policy.
Stripe idempotency keys are part of that story for premium flows: the UI can retry, the network can duplicate, and the ledger still records a single successful charge tied to the right entitlement.
The payoff is an app that feels instant without being fragile: live presence and messaging, durable history after refresh, and payments that behave under real-world network conditions.
