Sharding was one of the features that MongoDB offered from an early stage, since version 1.6 was released in August 2010. Sharding is the ability to horizontally scale out our database by partitioning our datasets across different servers—the shards. Foursquare and Bitly are two of the most famous early customers for MongoDB that were also using sharding from its inception all the way to the general availability release.

In this chapter, we will learn how to design a sharding cluster and how to make the single most important decision around it—choosing the shard key. We will also learn about different sharding techniques and how to monitor and administrate sharded clusters. We will discuss the mongos router and how it is being used to route our queries across different shards. Finally, we will learn how we can recover from errors in our shard.