Random Template Variables

Random template variables are built-in in the Elemental Operator.

They allow to include random Int, Hex or UUID values in custom label templates.

The values are computed on the fly during the label template variables rendering.

Random label templates are rendered only once

A label template containing a Random variable is rendered only if the MachineInventory of the registering host doesn't have a value for that label yet (a label with the same key is missing or its value is empty).

So, the three cases in which a label template with a Random variable is rendered are:

1. the host is registering for the first time and the MachineInventory is created anew
2. the label template has been added to the MachineRegistration after the host (re-)registered last time
3. the MachineInventory label matching the label template (same label key) has been manually removed or its value has been cleared out

Variable	Description
${Random/UUID}	random UUID (e.g., fd95324a-c26b-4e28-8727-1dcec293a0ec)
${Random/Hex/[1-32]}	random hexadecimal string of the specified lenght (min 1, max 32)
${Random/Int/[MAXINT]	random integer (min 0, max MAXINT-1)

Rendering Examples

template value	rendered value example
${Random/UUID}	fd95324a-c26b-4e28-8727-1dcec293a0ec
${Random/Hex/12}	acd231f222b8
${Random/Int/10000}	9432

The Random Template Variables can be handy for generating custom hostnames to be assigned to the registering host.

Since the hostname must be unique and is assigned through the MachineRegistration spec.machineName field, Random variables can be used to ensure uniqueness of a group of host sharing the same custom prefix and/or suffix.

Check the HowTo/Customize hostname section for more information.

registration example Random template variables

apiVersion: elemental.cattle.io/v1beta1
kind: MachineRegistration
metadata:
  name: fire-nodes
  namespace: fleet-default
spec:
  machineName: "fire-node-${Random/Hex/12}"
  config:
    cloud-config:
      users:
        - name: root
          passwd: root
    elemental:
      install:
        reboot: true
        device: /dev/sda
        debug: true
  machineInventoryLabels:
    elemental.cattle.io/UUID: "${Random/UUID}"