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title: Auto Scaling Concepts
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import { Server, Layers, Sliders, Image as ImageIcon, Cpu, Clock, Share2 } from 'react-feather';

# Auto Scaling Concepts

E2E Auto Scaling lets you grow and shrink a pool of compute nodes automatically, based on a policy you define. It keeps your application responsive during periods of high demand and reduces your spend when demand drops — without manual intervention. Most workflows start from **Compute > Auto Scaling** in the MyAccount left navigation.

This page explains the concepts and terminology used throughout the Auto Scaling documentation. Read it once before you [create your first scale group](/docs/myaccount/auto-scaling/getting-started/create-scale-group).

<div style={{marginBottom:'32px',marginTop:'8px'}}>
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    <a href="#scaler" style={{display:'inline-flex',alignItems:'center',gap:'6px',padding:'6px 14px',background:'var(--ifm-color-emphasis-100)',border:'1px solid var(--ifm-color-emphasis-300)',borderRadius:'20px',fontSize:'13px',color:'var(--ifm-color-emphasis-700)',textDecoration:'none'}}><Server size={13} /> Scaler &amp; Scale Group</a>
    <a href="#group-nodes" style={{display:'inline-flex',alignItems:'center',gap:'6px',padding:'6px 14px',background:'var(--ifm-color-emphasis-100)',border:'1px solid var(--ifm-color-emphasis-300)',borderRadius:'20px',fontSize:'13px',color:'var(--ifm-color-emphasis-700)',textDecoration:'none'}}><Layers size={13} /> Group Nodes</a>
    <a href="#saved-image" style={{display:'inline-flex',alignItems:'center',gap:'6px',padding:'6px 14px',background:'var(--ifm-color-emphasis-100)',border:'1px solid var(--ifm-color-emphasis-300)',borderRadius:'20px',fontSize:'13px',color:'var(--ifm-color-emphasis-700)',textDecoration:'none'}}><ImageIcon size={13} /> Saved Image</a>
    <a href="#compute-plan" style={{display:'inline-flex',alignItems:'center',gap:'6px',padding:'6px 14px',background:'var(--ifm-color-emphasis-100)',border:'1px solid var(--ifm-color-emphasis-300)',borderRadius:'20px',fontSize:'13px',color:'var(--ifm-color-emphasis-700)',textDecoration:'none'}}><Cpu size={13} /> Compute Plan</a>
    <a href="#scaling-policy" style={{display:'inline-flex',alignItems:'center',gap:'6px',padding:'6px 14px',background:'var(--ifm-color-emphasis-100)',border:'1px solid var(--ifm-color-emphasis-300)',borderRadius:'20px',fontSize:'13px',color:'var(--ifm-color-emphasis-700)',textDecoration:'none'}}><Sliders size={13} /> Scaling Policy</a>
    <a href="#watch-period-and-cooldown" style={{display:'inline-flex',alignItems:'center',gap:'6px',padding:'6px 14px',background:'var(--ifm-color-emphasis-100)',border:'1px solid var(--ifm-color-emphasis-300)',borderRadius:'20px',fontSize:'13px',color:'var(--ifm-color-emphasis-700)',textDecoration:'none'}}><Clock size={13} /> Watch Period &amp; Cooldown</a>
    <a href="#load-balancer" style={{display:'inline-flex',alignItems:'center',gap:'6px',padding:'6px 14px',background:'var(--ifm-color-emphasis-100)',border:'1px solid var(--ifm-color-emphasis-300)',borderRadius:'20px',fontSize:'13px',color:'var(--ifm-color-emphasis-700)',textDecoration:'none'}}><Share2 size={13} /> Load Balancer</a>
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## Why Use Auto Scaling

| Benefit                | What it gives you                                                                                         |
| ---------------------- | --------------------------------------------------------------------------------------------------------- |
| Cost efficiency        | Nodes are added only when demand rises and removed when it falls, so you avoid over-provisioning.         |
| Consistent performance | Capacity tracks real-time load, so users see steady response times during traffic spikes.                 |
| Less manual work       | Node addition, removal, and load-balancer registration happen automatically.                              |
| Control                | Custom policies let you scale on CPU or any metric you define (memory, network, disk I/O, request count). |
| Higher availability    | Combined with a load balancer, traffic is spread across healthy nodes as the pool changes.                |
| Predictable schedules  | Scheduled policies pre-scale capacity for known peak and off-peak windows.                                |

---

## Scaler

The **Scaler** is the E2E service that manages all Auto Scaling functionality. It watches your metrics, applies your policy, and launches or terminates nodes on your behalf.

## Scale Group

A **scale group** is the core unit of Auto Scaling — a pool of compute nodes that the Scaler manages against a single policy (for example, "add a node when CPU stays above 60% for 30 seconds"). A scale group has a name, a saved image, a compute plan, a minimum/maximum/desired node count, one or more policies, and an attached network and security group.

## Group Nodes

The nodes inside a scale group are added and removed dynamically by the Scaler. These are called **group nodes** (or just **nodes**) throughout this documentation.

The lifecycle of a group node begins when the Scaler launches it and ends when the Scaler terminates it. **You are billed for the time between a node's start and its termination**, on an hourly basis.

## Saved Image

Because group nodes are created on demand, you need a way to bring your application up automatically at boot. That is what a **saved image** provides.

A [saved image](/docs/myaccount/node/features/images) is an image captured from a compute node — ideally one configured to launch your application at startup. Every scale group is built from a saved image, and **only images in the `Ready` state can be selected**.

:::tip
Configure the node so your application starts on boot (for example, with a systemd service or a startup script), then save the image. New group nodes will then come up serving traffic with no manual steps.
:::

## Compute Plan

The **compute plan** (or just **plan**) defines the hardware — vCPU, RAM, and storage — for your group nodes. It does **not** have to match the plan used to create your saved image.

#### Example plan sequence

1. Create a node on a small, economical plan and configure your application's launch sequence.
2. Save an image from that node.
3. Create a scale group from that image but choose the plan you actually need in production.

## Scaling Policy

A **scaling policy** drives the lifecycle of group nodes. The portal supports three policy types, and you can use one or combine them:

| Policy type                      | When the Scaler acts                                                                                     | Documented in                                                                                         |
| -------------------------------- | -------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------- |
| **Elastic Policy**               | When a monitored metric (CPU or a custom attribute) crosses your threshold for a sustained watch period. | [ Elastic Policy](/docs/myaccount/auto-scaling/getting-started/create-scale-group/#elastic-policy)    |
| **Scheduled Policy**             | At specific times you define with cron expressions (for example, scale up at 9 AM on weekdays).          | [Scheduled Policy](/docs/myaccount/auto-scaling/getting-started/create-scale-group/#scheduled-policy) |
| **Elastic and Scheduled Policy** | Both of the above together. This is the portal's default selection.                                      | [Elastic and Scheduled Policy](/docs/myaccount/auto-scaling/features/custom-policy/)                  |

A policy is built from these settings:

- **Minimum nodes** and **Maximum nodes**
- **Desired nodes** (cardinality)
- **Policy parameter** (the metric being watched)
- **Scale-up and scale-down thresholds**
- **Watch period** and **period duration**
- **Cooldown**

When you set an elastic upscale rule, the Scaler automatically creates the matching **downscale (negative) rule**. For example, an upscale expression of `CPU > 60` produces a downscale expression of `CPU < 30`.

### Minimum and Maximum Nodes

These set the floor and ceiling for the scale group. The Scaler never drops below the minimum or rises above the maximum. The portal default is a **minimum of 2** and a **maximum of 5**, and the maximum cannot exceed **50**.

### Desired Nodes (Cardinality)

The desired node count is normally determined by the policy, but you can adjust it manually — for example, to pre-warm extra capacity before a code or image update. See [Resize a scale group](/docs/myaccount/auto-scaling/manage/actions#resize-service).

:::tip
Start with 2 nodes and let the scale group take over from there.
:::

### Policy Parameter (Target Metric)

The **default** elastic policy parameter is **CPU utilization**. To scale on any other signal — memory, network traffic, disk I/O, request count, and so on — use a **custom policy**, which lets you publish your own metric to the node. See [Custom Scaling Policies](/docs/myaccount/auto-scaling/features/custom-policy).

## Watch Period and Cooldown

### Watch Period

A **watch period** has two parts — **Watch Period** (the number of consecutive periods) and **Period Duration** (the length of each period in seconds). The monitored metric must stay over the threshold for the full watch period before the Scaler acts. This prevents brief spikes from triggering a scale operation.

**Example**

- Expression: `CPU > 75`
- Watch Period: `2`
- Period Duration: `10` seconds

The Scaler watches two consecutive 10-second periods. If CPU stays above 75% for both, the scale-up begins.

### Cooldown

The **cooldown** is a pause after a scaling action during which no further scaling occurs, giving the system time to absorb the effect of the previous action. The portal default is **150 seconds**.

## Load Balancer

A [load balancer](/docs/myaccount/load-balancer) is the stable entry point for a scale group. As group nodes (and their IP addresses) come and go, the load balancer keeps a single, consistent address for your users and automatically lists or de-lists backend nodes as the Scaler changes the pool.

:::tip
Always pair a scale group with a load balancer. Attach the scale group as an **Auto Scale Group** backend — see [Backend Mapping](/docs/myaccount/load-balancer/manage/backends).
:::

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## Related Resources

| Resource                                                                                | Use it for                                            |
| --------------------------------------------------------------------------------------- | ----------------------------------------------------- |
| [Create a Scale Group](/docs/myaccount/auto-scaling/getting-started/create-scale-group) | Step-by-step creation flow.                           |
| [Custom Scaling Policies](/docs/myaccount/auto-scaling/features/custom-policy)          | Scale on memory, network, disk, or any custom metric. |
| [Auto Scale Encryption](/docs/myaccount/auto-scaling/features/encryption)               | Encrypt group nodes at rest.                          |
| [Autoscale for E1 Series](/docs/myaccount/auto-scaling/features/e1-series)              | Storage, IOPS, and billing for E1-series images.      |
| [Manage Scale Groups](/docs/myaccount/auto-scaling/manage)                              | Operate, edit, and monitor an existing scale group.   |
| [Auto Scaling API](/api/myaccount/compute/auto-scaling/get-scaler-scalegroups)          | Automate scale groups.                                |

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