Design a Story feature with offline support

Design a “Story” feature (similar to a social app’s stories) with the following capabilities:

Core user features

• Users can create a story (photo/video + optional caption).
• Followers/friends can view a user’s story.
• Stories expire after a fixed TTL (e.g., 24 hours).

Additional requirements

• Favorite/Bookmark : user can mark a story as “favorite” for quick access.
• Save for offline : user can explicitly save stories for offline viewing (device-side download + server-side state).
• Offline mode: if the device has no network, the app should still display previously saved stories.

Non-functional requirements

• Very large scale (tens/hundreds of millions of users, heavy read traffic).
• Low latency story viewing.
• Privacy/ACLs (e.g., friends-only, custom lists, blocks).
• Handle deletes/expiration correctly across caches and offline copies.

Deliverables:

• APIs (create story, fetch feed, view story, favorite/unfavorite, save/unsave)
• High-level architecture (storage, cache/CDN, fanout strategy)
• Data model
• Offline sync strategy and edge cases (expiration, revocation, device storage pressure)
• Metrics and reliability considerations