A hotspot is a shard that receives disproportionate traffic.
Causes:
- Celebrity user with million followers on one shard
- Time-based keys where recent data is hot
- Poorly chosen shard key
Solutions:
- Add a random suffix to hot keys (spread Justin Bieber across shards)
- Use composite shard keys
- Monitor and rebalance
In interviews, when you propose a sharding strategy, ask yourself: "What could become a hotspot?" and discuss mitigation.