Supabase Realtime
Written in Elixir (Phoenix) — listens to the Postgres WAL (Write-Ahead Log) and broadcasts database changes as WebSocket events to clients.
Three channels: Postgres Changes (DB events filtered per RLS), Broadcast (client-to-client via server), Presence (who's online, typing indicators). Powers all of Supabase's realtime features (subscriptions, collaborative documents). Scale test: 1M+ concurrent connections per node. Competitors: Firebase Realtime DB, Pusher, Ably, Convex, native Postgres LISTEN/NOTIFY (less scalable).