DATA DISTRIBUTION
Three servers each.
Only one adds room.
BOTH SPREAD YOUR DATA
Both put your data on three boxes, for two opposite reasons.
REPLICATION COPY TO ALL
WRITE
to all three
A–Z
A–Z
full copy
A–Z
Every node holds the whole set, so three nodes still store one set.
SHARDING ROUTE TO ONE
WRITE
picks one
A–H
I–P
one slice
Q–Z
Each node holds one slice, so the three hold three times as much.
REPLICATION — the same data on
every node. Buys availability.
SHARDING — different data on
each one. Buys capacity.
Three servers each.
Only one adds room.
You'll want this one
before you shard.