Redundancy and Replication

These two words appear together in almost every system design answer. They are not the same thing, and it helps to separate them early.

Redundancy means keeping a spare copy of a part of the system. If that part stops, another one is already running and can do its work.

Replication means keeping copies of data in step. When the data changes in one place, the same change is applied to the other copies.

So redundancy is about the machines, and replication is about the data. You usually need both