# Vertical and horizontal scaling

> Make one machine bigger, or run more machines, and know which one to reach for first.

Scaling vertically means a bigger machine, and scaling horizontally means more machines sharing the work.

## Use it when

- Vertical: one box is the limit, the code keeps state in memory, and a restart is acceptable.
- Horizontal: you need to survive losing a machine, or one box of the biggest size is not enough.
- Either: a load bar is red on a single tier, which tells you what to grow.

## How it works

Going vertical changes one number, and nothing else in the system notices. A 4 vCPU server becomes an 8 vCPU one. The ceiling is the largest machine you can rent, and the price per core climbs near the top.

Going horizontal puts a load balancer in front of several copies. Two 2 vCPU servers hold as much CPU as one 4 vCPU server, but they need a place to share state, usually the database, because a user's next request may land on the other copy.

## What it costs

Vertical scaling has a hard ceiling and one point of failure, and resizing a box usually means a restart. Horizontal scaling adds a balancer, health checks, and the rule that servers hold no local state. It also multiplies database connections, which is why people add a [connection pool](https://wiki.playquorum.dev/patterns/connection-pooling.md) soon after.

Start vertical because it is cheap in engineering time, then go horizontal when you need failure tolerance or run out of box.

## In Quorum

The two Designs serve the same Users with the same total CPU. The first has one larger [App Server](https://wiki.playquorum.dev/components/server.md), and the second has two smaller ones behind a [Load Balancer](https://wiki.playquorum.dev/components/lb.md). Compare the Burn and the caption of each.





The [Scale up first](https://wiki.playquorum.dev/patterns/scale-up-first.md) and [Several app servers behind a load balancer](https://wiki.playquorum.dev/patterns/several-app-servers-behind-a-load-balancer.md) rungs cover each step. The glossary defines [vertical and horizontal scaling](https://wiki.playquorum.dev/glossary/vertical-and-horizontal-scaling.md), and [Survive a traffic spike](https://wiki.playquorum.dev/guides/survive-a-traffic-spike.md) uses both.

Source: https://wiki.playquorum.dev/patterns/vertical-and-horizontal-scaling