Design a cloud photo-storage service with upload, download, and multi-device synchronization. Store photo bytes separately from searchable metadata. Support resumable uploads, multiple derived sizes, deletion, and a device catching up after being offline. Discuss hot versus cold media, consistency, deduplication, and recovery. ### Constraints & Assumptions - A photo should not become visible in a library until its upload is complete. - Retried upload or delete requests must be idempotent. - Access control is evaluated before issuing media access. ### Clarifying Questions to Ask - Is deletion immediately permanent or recoverable for a retention period? - Are edits new immutable versions or in-place mutations? - Is deduplication scoped to one account or global? ```hint Synchronize metadata changes Devices can consume a per-account ordered change log and fetch blobs only when needed. ``` ### What a Strong Answer Covers - Multipart object upload, metadata transaction, finalization, and cleanup of abandoned uploads. - A versioned library/change-log model for offline device sync. - Object storage, derived assets, hot/cold tiers, caching, integrity, authorization, and deletion. ### Follow-up Questions - How would two devices edit the same photo concurrently? - How would you restore metadata if a database region is lost? - How do you prevent a signed download URL from outliving revoked access?