System Design: Hybrid Media Storage for Mobile Apps

You are asked to design a hybrid media storage system for a mobile social/content app that combines on-device (local) and remote (cloud) storage to handle images and videos.

Core content

You should explain how media is stored, retrieved, and synchronized across the device and the cloud. Cover upload flows (including compression, format handling, and resumable uploads), read flows (cached thumbnails, progressive loading, and CDN usage), and management operations (edit, delete, and versioning). Discuss policies that decide when to keep media only locally, when to push to cloud object storage, and when to evict local cache.

Flow / stages to cover in the interview

• Client-side: capture, local persistence, background upload, retry/backoff, and offline access.
• Network/edge: adaptive upload (cell vs Wi‑Fi), use of mobile CDN for reads, and upload endpoints with authentication.
• Backend: object store (S3-like), metadata DB (user/media indexes), background transcoding/thumbnailing, sync service, and notification for device reconciliation.

Skill signals to demonstrate

You should show understanding of data locality, cache strategies (LRU, TTL, prefetch), consistency models (eventual vs strong), conflict resolution for edits across devices, scalability (sharding, CDNs, multipart uploads), and cost trade-offs (cold vs hot storage, bandwidth vs latency). Be prepared to justify design choices for availability, durability, and offline-first user experience.