---
title: Custom Scaling Policies
---

# Custom Scaling Policies

The default elastic policy scales on **CPU**. A **custom policy** lets the Scaler make scaling decisions based on any metric you choose — memory usage, network traffic, disk I/O, request count, queue depth, or any application-specific signal.

With a custom policy, **you are responsible for publishing the metric value to each group node.** The Scaler reads that value from every node, averages it across the group, and applies your scale-up and scale-down thresholds to the average.

Read [Auto Scaling Concepts](/docs/myaccount/auto-scaling/getting-started/auto-scaling-concepts) and [Create a Scale Group](/docs/myaccount/auto-scaling/getting-started/create-scale-group#elastic-policy) first — this page builds on the elastic-policy fields described there.

---

## How a Custom Policy Works

1. You choose a **custom policy name** (the custom attribute), for example `CUSTOM_ATTR` or `NETWORK_TRAFFIC`.
2. On each node, a script measures the metric and writes the value into that attribute, refreshed periodically by a cron job.
3. The Scaler reads the attribute from every node, averages it, and compares it against your scale-up and scale-down thresholds.
4. The attribute must be configured on the node **before** you capture the saved image, so that every node the Scaler launches publishes the metric automatically.

:::info
The custom attribute must be configured on the **image** used to create the scale group. The default value of a custom attribute is `0` until your script writes to it.
:::

### Custom Policy Name

Enter the attribute name in the **Policy parameter name** field when you create or edit an elastic policy. Use a descriptive, uppercase name (letters, digits, and underscores; it must start and end with a letter or digit) — for example `MEMORY` for memory usage, `NETTX` for transmitted network traffic, or `DISKWRIOPS` for disk write operations.

:::note
Some attributes are reported automatically by the platform and need **no** script: `MEMORY`, `NETTX`, `NETRX`, `DISKWRIOPS`, `DISKRDIOPS`, and `DISKWRBYTES`. If your custom policy name is one of these, the value is already populated for you. For any other attribute, follow the scripting steps below.
:::

### Node Utilization Thresholds

As with the default policy, you set the value that triggers a scale-up (increase cardinality) and the value that triggers a scale-down (decrease cardinality), plus the **watch period**, **period duration**, and **cooldown**. Scaling is evaluated on the **average** value of the attribute across the group.

---

## Set a Custom Attribute on a Node

Suppose you set the custom policy name to `CUSTOM_ATTR`, with a scale-up threshold of 60 and a scale-down threshold of 30. The Scaler adds a node when the average `CUSTOM_ATTR` exceeds 60 and removes one when it drops below 30. The example below tracks memory-utilization percentage in `CUSTOM_ATTR`.

1. Create a new node and connect to it over SSH.
2. Add a measurement script (below) to the node.
3. Schedule the script with cron so the attribute refreshes continuously.

The script needs three values, all found in `/var/run/one-context/one_env`:

- `ONEGATE_ENDPOINT`
- `TOKENTXT`
- `VMID`

### Create the Script

Create a file such as `file_name.sh`. Use **either** of the following two approaches.

#### Option 1 — publish via the OneGate endpoint

```bash
TMP_DIR=`mktemp -d`
echo "" > $TMP_DIR/metrics

MEM_TOTAL=`grep MemTotal: /proc/meminfo | awk '{print $2}'`
MEM_FREE=`grep MemFree: /proc/meminfo | awk '{print $2}'`
MEM_USED=$(($MEM_TOTAL-$MEM_FREE))
MEM_USED_PERC="0"

if ! [ -z $MEM_TOTAL ] && [ $MEM_TOTAL -gt 0 ]; then
    MEM_USED_PERC=`echo "$MEM_USED $MEM_TOTAL" | \
        awk '{ printf "%.2f", 100 * $1 / $2 }'`
fi

CUSTOM_ATTR=$MEM_USED_PERC

echo "CUSTOM_ATTR = $CUSTOM_ATTR" >> $TMP_DIR/metrics

VMID=$(source /var/run/one-context/one_env; echo $VMID)
ONEGATE_ENDPOINT=$(source /var/run/one-context/one_env; echo $ONEGATE_ENDPOINT)
ONEGATE_TOKEN=$(source /var/run/one-context/one_env; echo $TOKENTXT)

curl -X "PUT" $ONEGATE_ENDPOINT/vm \
    --header "X-ONEGATE-TOKEN: $ONEGATE_TOKEN" \
    --header "X-ONEGATE-VMID: $VMID" \
    --data-binary @$TMP_DIR/metrics
```

#### Option 2 — publish with the `onegate` CLI

```bash
MEM_TOTAL=`grep MemTotal: /proc/meminfo | awk '{print $2}'`
MEM_FREE=`grep MemFree: /proc/meminfo | awk '{print $2}'`
MEM_USED=$(($MEM_TOTAL-$MEM_FREE))
MEM_USED_PERC="0"

if ! [ -z $MEM_TOTAL ] && [ $MEM_TOTAL -gt 0 ]; then
    MEM_USED_PERC=`echo "$MEM_USED $MEM_TOTAL" | \
        awk '{ printf "%.2f", 100 * $1 / $2 }'`
fi

VMID=$(source /var/run/one-context/one_env; echo $VMID)

onegate vm update $VMID --data CUSTOM_ATTR=$MEM_USED_PERC
```

### Make the Script Executable and Run It

```bash
chmod +x file_name.sh
./file_name.sh
```

### Schedule It with Cron

For continuous monitoring, run the script on a schedule:

```bash
crontab -e
```

Add a line that runs the script at the interval you want (here, every minute):

```bash
* * * * * /root/file_name.sh
```

---

## Build the Image and Launch the Scale Group

1. After the script and cron are in place and working, [save an image](/docs/myaccount/node/features/images) from the node.
2. [Create a scale group](/docs/myaccount/auto-scaling/getting-started/create-scale-group) from that image, choosing **Custom** as the elastic policy parameter type and entering **the same attribute name** (`CUSTOM_ATTR`) you used in the script.

The Scaler will now watch the average `CUSTOM_ATTR` across the group and scale according to the increment and decrement thresholds you defined.

---

## Update a Custom Value with cURL

Each scale group with a custom policy exposes a cURL command you can call from scripts, cron jobs, or webhooks to push a metric value. Open the [Scale Group Details](/docs/myaccount/auto-scaling/manage/scale-group-details#elastic-policy-panel) tab and select **Get cURL** in the Elastic Policy panel.

---

## Inspect the Attribute on a Node

To confirm an attribute is set, run on the node:

```bash
onegate vm show VMID --json
```

The output includes your attribute under `USER_TEMPLATE`, for example:

```bash
{
  "VM": {
    "NAME": "machine_name",
    "ID": "machine_id",
    "STATE": "machine_state",
    "LCM_STATE": "machine_lcm_state",
    "USER_TEMPLATE": {
      "CUSTOM_ATTR": "set_attribute",
      ...
    },
    ...
  }
}
```

To find the VM ID, run:

```bash
onegate vm show
```

```bash
VM 8
NAME              : web_0_(service_1)
STATE             : RUNNING
IP                : 192.168.122.23
```

---

## Related Resources

| Resource                                                                                               | Use it for                                         |
| ------------------------------------------------------------------------------------------------------ | -------------------------------------------------- |
| [Auto Scaling Concepts](/docs/myaccount/auto-scaling/getting-started/auto-scaling-concepts)            | Policy terminology and how thresholds work.        |
| [Create a Scale Group](/docs/myaccount/auto-scaling/getting-started/create-scale-group#elastic-policy) | Where to enter the custom policy name.             |
| [Scale Group Details and Editing](/docs/myaccount/auto-scaling/manage/scale-group-details)             | Get the cURL command and edit the policy.          |
| [Node Images](/docs/myaccount/node/features/images)                                                    | Capture the saved image with your script baked in. |

---