What is a shard?

A shard is a separate server or node in a MongoDB cluster that holds only part of a collection's data. Together, all the shards form the complete store, but each one is responsible only for its own range of data.

#Key characteristics

• a shard holds not the whole collection, just a fragment of it
• distribution happens by shard key
• queries can reach a single shard, or run in parallel across several

#Why a shard is needed

• to split the load
• to scale data volume
• to speed up writes and reads through servers working in parallel

#Summary

A shard = a separate node holding part of the data, used in a sharded cluster for MongoDB's horizontal scaling.