Designing WhatsApp's Typing Indicator: The Question That Tests Your Real-Time Skills

Designing WhatsApp's typing indicator is a complex task that tests real-time skills, requiring consideration of pubsub vs persistent connection, transport choice, ephemeral vs durable storage, and backpressure. The indicator should be per-pair, short-lived, and have no value if it arrives late. Designers should separate ephemeral signals from durable messages, using different storage and delivery guarantees. A common mistake is conflating these two types of signals.