MongoDB - Sharding

One of the central reasons for Mongo to exist is to safely and quickly handle
very large datasets. The clearest method of achieving this is through horizontal
sharding by value ranges—or just sharding for brevity. Rather than a single
server hosting all values in a collection, some range of values are split (or in
other words, sharded) onto other servers. For example, in our phone numbers
collection, we may put all phone numbers less than 1-500-000-0000 onto
Mongo server A and put numbers greater than or equal to 1-500-000-0001
onto a server B. Mongo makes this easier by autosharding, managing this
division for you.

Sharded cluster picture :