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). A 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.

The relationships between these elements is shown in the following diagram:

As of MongoDB 3.6, shards must be implemented as replica sets.

Development, continuous deployment, and staging environments

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

The simplest possible...