Start by clarifying requirements and scale, then design a data model for users, friendships, and activity events. Explain a fan-out strategy (push vs. pull) for delivering events to followers, and cover ranking, caching, and privacy enforcement. Trade-offs should be justified based on Roblox's social and gaming context.
Pro tip: Emphasize that privacy rules must be enforced at multiple layers (event creation, fan-out, and read time) to prevent leaks, and discuss how to handle edge cases like unfriending or blocking. Also, mention that ranking should balance recency, engagement, and social proximity to avoid noise.
Ask about scale (DAU, events per user), latency requirements, and what types of activities to surface. Confirm privacy expectations and whether real-time updates are needed.
Define entities: User, Friendship (graph), ActivityEvent (type, timestamp, payload), and Follow (if asymmetric). Consider storage choices (e.g., SQL for friendships, NoSQL for events) and indexing for efficient queries.
Decide between push (write to followers' feeds on event creation) and pull (read from friends' events on demand). Discuss hybrid approaches and trade-offs in latency, storage, and consistency.
Describe how to rank events (e.g., by recency, engagement, affinity) and cache feeds (e.g., Redis) to reduce latency. Mention precomputation and invalidation strategies.
Explain how privacy settings (public, friends-only, custom) are enforced at event creation, fan-out, and read time. Cover handling of unfriending, blocking, and deleted content.
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