Database Sharding

How it works

Sharding partitions data horizontally across multiple database nodes by a shard key, so each holds only a slice. It scales writes and storage past a single machine's limits — at the cost of cross-shard queries and rebalancing complexity.

Mental models

• The shard key determines which node owns each row — choose it carefully.
• Range, hash, and directory-based sharding trade locality for balance.
• Cross-shard joins and transactions are the expensive parts.