Real-time channels: WebSockets and SSE at scale

Discord's 25 March 2026 reconnect storm, and the in-app tier that survives one: four ways to push to an online user and when SSE or WebSockets fits; HTTP/2 and HTTP/3; a separate gateway tier sized by connections, blast radius and reconnect admission (10M connections, 200K per node, 75 nodes); a connection registry with per-gateway leases and epochs; one pub/sub channel per gateway; the inbox as the record and resume by sequence number; heartbeats under the shortest idle timeout; full-jitter reconnects, admission control, bounded queues and slow drains; the fallback to mobile push; and auth on a long-lived socket.