A brief summary of MongoDB sharded clusters
In a sharded cluster, you partition a collection of data across multiple shards, where each shard runs on a separate set of host machines. You control how the system distributes the data by defining a shard key rule. Based on the shard key of each document, the system groups subsets of documents together into chunks, where a range of shard key values identifies each chunk. The cluster balances these chunks across its shards.
In addition to holding sharded collections in a database, you may also be storing unsharded collections in the same database. All a database's unsharded collections live on one specific shard in the cluster, designated as the primary shard for the database (not to be confused with a replica set's primary replica). Figure 5.1 shows the relationship between a database's collections and the shards in the cluster.
Figure 5.1: Correlation between a database's collections and...
