Why is replicaSet considered as Highly available but not fault tolerant?

i read documentation showing , replicaset maintains high availability.
Why there is no mention of replicaset being fault-tolerant. if i run a 5 node replicaset and 1 node goes down, still my application will be up and running. How this doesnot prove that replicaset provides both highavailability and fault-tolerance?


Fault tolerance, high availability, and automatic failover broadly describe the same behaviour although they may have slightly different connotations. The concept of High Availability includes durability, redundancy, and automatic failover with minimal or no downtime.

Fault tolerance is more specifically the number of members that can be unavailable in a deployment while still maintaining full availability of read/write behaviour. For example, assuming all members of your 5-member replica set are voting, the fault tolerance would be 2 members.

Scroll down to the next paragraph on the documentation page you referenced:

Replication provides redundancy and increases data availability. With multiple copies of data on different database servers, replication provides a level of fault tolerance against the loss of a single database server.

Consider Fault Tolerance is also one of the sections on the Replica Set Architecture documentation. Your deployment can be highly available with a fault tolerance of 1, or you can increase the fault tolerance by adding additional members.


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