Sharded Cluster Components

Three kinds of process make up a sharded cluster: shards store the documents, config servers store the map of where those documents are, and mongos routers carry requests between the application and the shards. The clearest way to see what each one does is to follow a single request. We will use the store.users collection again, sharded on user_id, with 100,000 users.

A production layout could look like this. Each shard and the config servers run as a three-member replica set:

Shard 1: shardRS1            Shard 2: shardRS2            Shard 3: shardRS3