Design an Instagram Stories feature

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.