I Built a Real-Time Multiplayer Bingo Engine with Next.js, Supabase, and Ably

[ BingWow ]( https://bingwow.com ) is a free multiplayer bingo platform. You type a topic, AI generates a card, and up to 20 people play together in the browser. No downloads, no accounts. Teachers use it for classroom review games, event planners use it for baby showers and team building, and watch party hosts use it for live TV events. Here's how the real-time multiplayer works under the hood. ## The Stack - **Next.js 16** (App Router) on Vercel - **Supabase** (PostgreSQL + Auth + Storage) - **Ably** for real-time pub/sub - **Tailwind v4** for styling ## The Hard Problem: Simultaneous Bingo Claims Most of the game is simple. A player taps a cell, the client marks it optimistically, and a fire-and-forget POST goes to the server. No waiting, no blocking. The moment it gets complicated is bingo. When a claim completes a line, you need to atomically determine who won. Two players might complete their lines within milliseconds of each other. The server has to pick one winner and broadcast