TL;DR — WhatsApp Messaging

Design an end-to-end encrypted messaging platform like WhatsApp that serves 2 billion monthly active users. The system must guarantee message delivery even when recipients are offline, support group chats of up to 1024 members, and maintain real-time presence and typing indicators across all conversations. Key features: 1-on-1 and group messaging with E2E encryption. Group chats supporting up to 1024 members.