What is database sharding and what are its strategies?

Sharding horizontally partitions data across multiple database instances for scalability. Strategies: Range-based (shard by value ranges - e.g., user IDs 1-1M on shard1), Hash-based (consistent hashing of key - even distribution but no range queries), Directory-based (lookup service maps keys to shards - flexible but lookup overhead), Geographic (by user location). Challenges: cross-shard queries, transactions across shards, rebalancing. Often used with application-level routing.