MongoDB Documentation, Release 3.0.4 Sharded cluster has the following components: shards , query routers and config servers . Shards store the data. To provide high availability and data consistency, in a production sharded cluster, each shard is a replica set $^{1}$. For more information on replica sets, see Replica Sets (page 563). Query Routers , or mongos instances, interface with client applications and direct operations to the appropriate shard or shards. The query router processes and targets operations to shards and then returns results to the clients. A sharded cluster can contain more than one query router to divide the client request load. A client sends requests to one query router. Most sharded clusters have many query routers. Config servers store the cluster’s metadata. This data contains a mapping of the cluster’s data set to the shards. The query router uses this metadata to target operations to specific shards. Production sharded clusters have exactly 3 config servers. To shard a collection, you need to select a shard key . A shard key is either an indexed field or an indexed compound field that exists in every document in the collection. MongoDB divides the shard key values into chunks and distributes MongoDB distributes data, or shards, at the collection level. Sharding partitions a collection’s data by the shard key . 10.1.3 Data Partitioning Shard Keys 10.1. Sharding Introduction 663 1 For development and testing purposes only, each shard can be a single mongod instead of a replica set. Do not deploy production clusters without 3 config servers.