# Autoscaling, reactive and scheduled

> Add copies of a service when load rises and remove them when it falls, either after the load shows up or ahead of a peak you can predict.

An autoscaler changes the number of running copies to follow demand, so you pay for the peak only while it lasts.

## Use it when

- Traffic has a daily shape, so a fixed fleet is idle at night and short at noon.
- Spikes arrive faster than a person can add servers.
- The tier is stateless, so a new copy can take traffic as soon as it boots.

## How it works

A reactive policy reads a signal, such as CPU, requests per copy, or queue depth for workers. It compares the signal with a target and adds copies when it passes. A new copy takes time to boot and warm up, so a reactive policy always trails the load. A min keeps the floor high enough to survive a failure, and a max caps the bill.

A scheduled policy adds copies before a known peak, such as 9 a.m. on weekdays, so they are warm when users arrive. Reactive scaling then handles whatever the schedule missed.

## What it costs

Boot time is the weak point: scaling at the moment of a spike is too late for a slow-starting runtime. Aggressive scale-in causes flapping, and a missing max turns a bot flood into a large bill. Signal choice matters too. CPU misses a service that waits on a slow database.

## In Quorum

Link an [App Server](https://wiki.playquorum.dev/components/server.md) into an [Autoscaler](https://wiki.playquorum.dev/components/autoscaler.md) after researching [Autoscaling](https://wiki.playquorum.dev/research/autoscaling.md). The schedule switch needs [Scheduled scaling](https://wiki.playquorum.dev/research/scheduled-scaling.md). The Design pairs it with slow start on the [Load Balancer](https://wiki.playquorum.dev/components/lb.md), so a new copy takes traffic gradually.



The [JVM warmup under load](https://wiki.playquorum.dev/incidents/jvm-warmup.md) incident shows cold copies arriving mid-spike. The glossary defines [autoscaling](https://wiki.playquorum.dev/glossary/autoscaling.md) and [diurnal traffic](https://wiki.playquorum.dev/glossary/diurnal-traffic.md), and the [Autoscale the compute tier](https://wiki.playquorum.dev/patterns/autoscale-the-compute-tier.md) rung builds it step by step.

Source: https://wiki.playquorum.dev/patterns/autoscaling-reactive-and-scheduled