##### Question Design an Instagram-style photo and video sharing service with a personalized home feed (similar to Instagram). **Functional requirements:** 1. Users can create accounts, view profile pages, and follow/unfollow other users. 2. Users can create posts containing media (photos and short videos) plus a caption. Media storage is handled separately from post metadata. 3. Users can view a personalized home feed of recent or ranked posts from accounts they follow (with optional recommendations), using infinite scroll. 4. Users can like, comment on, and delete posts, and view post-detail pages. 5. Media should be uploaded from mobile and web clients and served globally with low latency through a CDN. 6. Support basic privacy (e.g. private accounts) so non-followers cannot see protected content. **Non-functional requirements:** 1. High read availability and low feed latency (feeds are far more read-heavy than write-heavy). 2. The system should be highly available and horizontally scalable, with freshness (new posts appear quickly). 3. Eventual consistency is acceptable where appropriate (feed ordering, counters); stronger consistency is needed for authorization, privacy, and deletes. 4. The design must survive sudden traffic spikes caused by celebrity accounts, viral posts, or very popular comments. **Discuss:** APIs, data models, storage choices, media upload and processing, feed generation, caching/CDN, asynchronous processing, rate limiting, reliability/monitoring, and strategies for hot accounts and hot posts. **Deep-dive follow-up (upload security):** The upload flow uses object-storage pre-signed URLs so clients can upload media directly to object storage. How can the system prevent or mitigate malicious uploads? Specifically, explain why size limits and server-side validation help, what they do *not* solve, and what additional controls are required. *Out of scope (optional): full recommendation ML, stories, ads.*