Over time, shards become unbalanced. Some grow faster. Some become hotspots. You need to rebalance.
Manual rebalancing: Decide which keys to move. Simple but requires human attention.
Automatic rebalancing: The system detects imbalance and moves data. More complex but hands-off.
Rebalancing while the system is running is hard. You're moving data while reads and writes continue.
Two-phase approach: Copy data to new shard while old shard stays active. Switch reads. Then switch writes. Then delete from old shard.