For greater efficiency, the MongoDB sharded cluster balancer (the sharding mechanism) doesn't operate at the document level. Rather, documents are grouped into chunks. Which documents are placed into a given chunk is determined by the shard key range. Mongo shell helper methods exist that let you view the size of individual shards and manually move one or more chunks from one shard to another. This process is known as chunk migration (covered in the next chapter, Chapter 16, Sharded Cluster Management and Development).
Here is a diagram that illustrates the shard key's distributional role:
The overall diagram represents a sharded cluster. In this diagram, we're assuming the shard key is a field named X. Within each shard is a distribution of two chunks. Within each chunk is a group of documents. The key space for X ranges from its smallest value, labeled X:minKey,to its largest, labelled X:maxKey. The...
