Architectural overview

A sharded cluster is comprised of the following elements:

• Two or more shards. Each shard must be a replica set.
• One or more query routers (mongos). mongos provides an interface between our application and the database.
• A replica set of config servers. Config servers store metadata and configuration settings for the entire cluster.

Reference: https://www.infoq.com/news/2010/08/MongoDB-1.6

We can't have a shard composed of a single server.

Development, continuous deployment, and staging environments

In preproduction environments it may be an overkill to use the full set of servers, and for efficiency reasons, we may opt to use a more simplified architecture.

The simplest possible configuration...