Design Instagram Stories

Design the stories feature of a photo and video app: users post photos or short videos that their followers can view for 24 hours, after which they disappear. At the top of the app each user sees a tray of followed accounts with stories they have not watched yet, and taps through them in order.

Cover upload and media storage, building the tray quickly, tracking what each viewer has seen, expiry after 24 hours, viewer lists for the author, and scale for hundreds of millions of daily users.