Roblox has a marketplace with millions of items. A user can flag/unflag any item as a favorite (similar to Like/Bookmark).
Goal: Design a system that can show the number of favorites each item has.
User Stories / QPS targets
- As a user, I want to know if this item is favorited by me — 1M QPS
- As a user, I want to favorite/unfavorite this item — 100K QPS
- As a user, I want to see the total favorite count for this item — 1M QPS
Extra:
– As a user, I want to list all items I have favorited — 50K QPS
(Insert diagram here: request flow with API → cache → counters store → database; write path & read path.)
Design for read-heavy workload:
- Data model:
UserFavorites(userId → itemIds set),ItemCounters(itemId → favCount); optionalUser→Item bitset/bloom. - Reads: Item count &“did I favorite?”served from Redis/Memory cache with short TTL + hot-key protection; cache keys sharded by
itemId. - Writes: Idempotent toggle API → write to UserFavorites (NoSQL/Cassandra/DynamoDB) + atomic counter (Redis INCR / DynamoDB counter) with outbox/stream to reconcile source of truth.
- Consistency: Eventual for counts; strong for per-user state (read-your-writes via write-through cache).
- List my favorites: paginate from
UserFavorites(store as sorted set for recency). - Ops: rate limiting, dedup, retries, TTL backfill job, cold-key preload, metrics.
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