Design an Instagram Stories feature
Company: Snapchat
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Technical Screen
Design a backend system for an **Instagram Stories–like** feature.
Users can post short-lived stories (photos/videos) that are visible to followers and automatically expire after 24 hours.
### Core requirements
- **Post story**: user uploads media and publishes a story.
- **View stories**: followers can see a tray/list of accounts with active stories and play story segments.
- **Expiration**: stories disappear after 24 hours (should not be viewable).
- **Privacy**: stories can be shared with either (a) all followers or (b) a restricted audience (e.g., “Close Friends”).
- **Seen state**: track whether a viewer has seen a given story segment (for UI ordering and analytics).
### Non-functional requirements
- Very low latency for story playback (global audience).
- High availability.
- Large scale (tens/hundreds of millions of users).
### Deliverables
- Propose APIs, data model, storage choices, and major services.
- Explain how you handle upload/transcoding/CDN delivery.
- Explain feed/tray generation (fanout strategies), caching, and expiration.
- Discuss consistency trade-offs for “seen” state and counters.
- Outline monitoring/metrics and failure modes.
Quick Answer: This question evaluates backend system design competencies including distributed systems, scalability, data modeling, API design, media storage/transcoding, CDN delivery, privacy controls, caching, and consistency for seen-state and counters.