Database Sharding

Q&A: What is database sharding, and why/when would you use it?

Expected discussion: Horizontally partitioning a large dataset across multiple database instances ("shards") based on a shard key, to scale storage/writes beyond a single machine. Covers sharding strategies (range-based, hash-based, directory-based), rebalancing challenges when adding/removing shards, and the added complexity of cross-shard queries, joins, and transactions.