Vertical vs horizontal scaling?

Definition:

Vertical scaling adds resources to one machine, while horizontal scaling adds machines and distributes requests or data. Horizontal scaling improves capacity only if work can be shared without losing correct state coordination. Stateless request handlers are easier to spread, but databases, sessions and background work may still need special handling.

Key points:

• Vertical: give one machine more CPU or memory.

• Horizontal: add instances and distribute work.

• Keep request-serving instances stateless; move shared durable state to suitable storage.

• Doubling app instances also increases possible database connections and dependency traffic; budget shared resources across the fleet.

Interview point:

• State which tier is being scaled and whether the bottleneck moves to another tier afterward.