Design a photo-sharing social network with follows, media uploads and a home feed

Design a photo-sharing social network in the style of Instagram. Users upload photos and short videos with captions, follow other users, and scroll a home feed of posts from the accounts they follow. Posts can be liked and commented on, and each user has a profile that shows their posts. Interviewers in this loop cross-questioned design choices heavily, so be ready to justify each decision: why one data store rather than another, what breaks first as usage grows, how latency, cost and consistency trade off, and what you would change if usage patterns shifted. Rounds also set aside about ten minutes for behavioral questions, so plan to cover the core design in less than a full round. ```hint Fan-out timing Decide when a new post gets attached to followers' feeds, at write time or at read time, and what happens for accounts with enormous follower counts. ``` ```hint Keep bytes off the hot path Photos and videos dominate storage and bandwidth; think about how they travel separately from the post metadata. ``` ### Constraints and Clarifications - Assume a read-heavy workload: feeds are viewed far more often than posts are created. - Scale numbers are not given; state your own estimates. - Stories, direct messages, search and ads are out of scope unless the interviewer adds them. ### Clarifying Questions - Is the home feed chronological or ranked? - Are videos in scope, and how long can they be? - How quickly must a new post appear in followers' feeds? - What is the largest follower count a single account can have? - Is the user base global, and must the system serve several regions? ### What a Strong Answer Covers - Scope and estimates that justify a read-optimized design - A media path that keeps large uploads off application servers and serves media through a CDN - Feed generation that works for typical accounts and for accounts with enormous follower counts - A data model and store choices justified by access patterns, including the follow graph - Consistency choices per feature: the author's own posts, followers' feeds, counts - Failure handling, multi-region scaling, and a clear defense of trade-offs under follow-up questions ### Follow-up Questions - An account with 100 million followers posts. Walk through exactly what happens. - Product wants a ranked feed instead of a chronological one. What changes? - A user deletes a post that already sits in millions of cached feeds. How does it disappear? - Video uploads triple within a year. Which parts of the design, and of the bill, change most?