Built the real-time messaging layer

Built the real-time messaging layer for a production social platform: WebSocket chat with typing indicators, read receipts, reactions, edits, media, and live presence tracked authoritatively in Redis across a multi-worker cluster with no shared memory. Every message passes automated safety screening and encrypted storage on the hot path, with 100% delivery at p50 71ms / p99 326ms send-to-echo, and 400 concurrent sessions held with zero drops and zero errors on a single worker. Cut send latency ~3x by parallelizing ~25 serialized round-trips and moving link-preview generation off the hot path.