Replica Set vs Sharding

Feature	Replica Set	Sharding
Purpose	High availability, failover, redundancy	Horizontal scaling, split data across machines
Data	Every node has ALL the data	Each shard has ONLY PART of the data
Writes	Only the primary can write	Writes go to whichever shard owns the data
Reads	Primary or secondaries	Across multiple shards
Storage	Duplicated on all nodes	Split across nodes
Improves write throughput?	❌ No	✅ Yes
Improves data capacity?	❌ No	✅ Yes

Key point

A replica set = same data, multiple copies
A sharded cluster = data split into many replica sets
In fact

A Sharded cluster is made up of multiple replica sets. Each shard is a replica set !

Example:

Shard 1 → Replica Set (Primary + Secondaries)
Shard 2 → Replica Set (Primary + Secondaries)
Shard 3 → Replica Set (Primary + Secondaries)