Sharding is a load management technique used to split databases across multiple servers. Most sharding works on the primary key or some other index being modulus with the number of shards in order to decide where o tore that information.

For example, the primary key may be 7 and there may be three shards. 7 % 3 = 1 (when seven is divided by three there is a remainder of one?) so this record would be stored on the first shard.

Sharding can boost performance dramatically but is heavily disadvantaged when writing complex queries as these may need to have aggregate results from multiple servers. Applications should be designed so that they shard across dimensions that don’t usual mix (e.g. Users of an e-mail system).