Design: Watched-Video and Resume Playback Service

Context

Design a backend service that records each user’s watched video list and lets them resume any title from the last watched timestamp across devices. Assume a large-scale, consumer-facing video platform with millions of users and multiple device types (mobile, web, TV).

Requirements

1. Core features
   • Store a per-user list of watched videos with last-watched timestamp and status (playing/paused/completed).
   • Resume playback on any device from the last saved timestamp.
2. APIs
   • Define read/write APIs for single and batch operations, idempotent progress updates, and cross-device sync notifications.
3. Data model
   • Schema for progress per (user, video) and a browsable watch history list.
4. Idempotent updates
   • Handle network retries and out-of-order events from multiple devices/sessions.
5. Cross-device synchronization
   • Near-real-time updates when a user pauses/stops on one device and resumes on another.
6. Privacy and controls
   • Support “hide from history,” data minimization, deletion/export, encryption, and access control.
7. Scale
   • State read and write access patterns and how to scale storage, throughput, and latency.

What to deliver

• High-level architecture.
• API definitions (endpoints, request/response shapes, idempotency semantics).
• Data model (including indexes/keys) and storage choices.
• Update semantics and conflict resolution across devices.
• Privacy controls and compliance considerations.
• Read/write scale estimates and capacity strategies.