Designing a Real-Time Chat System at Scale (opens in new tab)

Most developers underestimate how hard messaging systems really are. A basic chat demo with WebSockets is easy to build. A production-grade messaging platform like WhatsApp, Telegram, Discord, or Slack is a completely different engineering problem. The hard part is not rendering messages in the UI. The hard parts are keeping millions of persistent connections alive, delivering messages reliably, preserving ordering, handling offline sync, scaling group chats, surviving partial failures, and k...