Build the production reference architecture for a 1:1 + group messaging service at hyperscale (think WhatsApp / Messenger / Discord / Signal). The shape: hundreds of millions of long-lived TLS sockets, sub-second 1:1 + group delivery, end-to-end encrypted with multi-device fanout, multi-region active-active with regional-failover RPO=0 on metadata. Every component below is specified well enough that a staff SRE could file the implementation tickets — and would defend the choices in a 5-year incident retro.