Design a Notification System

This is one of the recurring Airbnb design prompts, usually framed as a multi-channel platform spanning push and email, SMS and an in-app inbox. It looks simple. Something happens, you send a message. The depth shows up when one platform has to serve a booking confirmation that lands in seconds and a marketing campaign aimed at a hundred million people. It also has to respect a guest who muted everything except messages from their host. The interviewer lets you pick the scope, then pushes on the part that hurts. You cannot send the same thing twice. You have to respect what each user asked for. And a promotional blast can never crowd out a time-sensitive alert.

Anchor on volume first, because it sorts the whole design into two regimes. Across roughly 150 million guests and hosts, the triggered traffic runs on the order of 100 million notifications a day: every booking request, every host-guest message, every payment receipt and trip reminder. Spread evenly that is only about 1,200 a second, and evenings and weekends push the peak to something like 10,000 a second. Then a single marketing campaign can target 100 million users at once, and you should say those two numbers next to each other early. The steady stream is small and latency-sensitive; the campaign is a huge burst that nobody is waiting on. Almost every later decision is about keeping the burst from starving the stream.

The Airbnb angle

This is a product-driven round, so frame it around the guest and the host. You sketch on a shared whiteboard (Miro or Excalidraw in most reports) and you drive: pick the scope, name the channels, then go deep. Everything good here comes from reasoning off the trip. A new booking request the host has 24 hours to accept is urgent and can't be silenced. A wishlist price drop can wait for tomorrow's digest, but a login from a new device is a security message the user doesn't get to mute. Treat all three as one undifferentiated "notification" and you've flattened the whole product into a sender loop, the junior tell.