What Is Database Sharding?

Q: What is sharding?

A: Sharding is a technique for horizontally partitioning a large dataset across multiple database instances ("shards"), where each shard holds a subset of the overall data.

Key points to cover:

• A shard key (or partition key) determines which shard a given row/document lives on; common strategies include range-based, hash-based, and directory-based sharding.
• Benefits: distributes read/write load across machines, allows storage to scale beyond a single node's capacity, and can improve query latency by keeping working sets smaller per shard.
• Trade-offs: cross-shard joins and transactions become expensive or require special handling; rebalancing shards (e.g., after adding nodes) can be operationally complex; a poorly chosen shard key can create hot shards.
• Contrast with replication: sharding splits data (each shard has different data), while replication copies the same data across nodes for availability/read-scaling.