Designing a Chat System: Storing History, Read Receipts, and Online Status (Presence)

How do chat apps earn user trust in just three seconds? This piece breaks down the hidden architecture behind modern messaging—durable writes, read receipts at scale, and real-time presence—drawing lessons from systems like WhatsApp, Slack, and Discord. You’ll learn why naïve designs fail, how platforms like Telegram and Signal reduce write amplification, and what it really takes to deliver messages reliably at global scale.