Sharded Cluster

Typically use key-based hash or range partitioning to assign data to nodes in the cluster.

A MongoDB sharded cluster consists of the following components:

• shard: Each shard contains a subset of the sharded data. Each shard can be deployed as a replica set.

• mongos: The mongos acts as a query router, providing an interface between client applications and the sharded cluster.

• config servers: Config servers store metadata and configuration settings for the cluster. As of MongoDB 3.4, config servers must be deployed as a replica set (CSRS).

Shard Key

A good shard key has

• sufficient cardinality

• distributed writes (avoid hot spots on inserts)

• targeted reads (“query isolation”)

  Generally, the fastest queries in a sharded environment are those that mongos route to a single shard, using the shard key and the cluster meta data from config server.

Note: When choosing a shard key for time series data, consider compound shard key {arbitrary value, incrementing value}

Further Readings

• MongoDB for Time Series Data