Design a Photo Sharing Service

Functional Requirements:

• Users can register and log in to the service.
• Users can upload photos (and optionally short videos).
• Users can view their own photos and profiles.
• Users can follow and unfollow other users.
• Users can see a home feed showing recent posts from users they follow.
• Users can like and comment on posts.
• Users receive notifications for relevant events (e.g., new follower, likes, comments).

Non-Functional Requirements:

• The system should support a large number of users (millions to billions).
• Home feed should load quickly (target: < 1 second per request).
• Photo uploads should be reliable and reasonably fast.
• High availability: the service should remain operational even if some components fail.
• Data consistency: eventual consistency is acceptable for feed and notifications, but actions like follow/unfollow should be reflected promptly.
• Scalability: the system must handle increasing load and data volume without major redesign.
• Media files (photos/videos) should be delivered efficiently worldwide (low latency for global users).
• User data and media should be stored securely and reliably.