Question - Machine scale set configuration

Question

We currently have a machine scale set configured for the worker nodes of an ARO OpenShift cluster. This infrastructure is also mapped to a UCMDB platform to manage the various infrastructure assets at the application level. We've observed that when new machines are created within the machine scale set, the name changes dynamically, particularly the last few letters. Our specific question is whether it's possible to configure the machine set so that when new machines are created or deleted, they are assigned the exact same name, thus avoiding the need to modify the UCMDB entry.

Answer 1

Hey David, it looks like you’re hitting a common limitation: Azure VM Scale Sets (and by extension the ARO OpenShift worker nodes running on them) always generate instance names with a dynamic suffix by design. There isn’t a built-in way to force each new or replacement VM to use the exact same hostname as its predecessor. Here are a few workarounds you can consider so you don’t have to manually update your UCMDB entries each time:

1. Monitor the Scale Set resource, not individual VMs Instead of treating each VM as a separate CI, consider tracking the VMSS or OpenShift MachineSet object in UCMDB. That resource name (for example, aro-<clustername>-worker-<region>) is stable and doesn’t change when you scale in or out.
2. Use Azure tags for a stable identifier During provisioning (via an ARM template, Azure CLI, or an Automation runbook), apply a fixed tag like logicalName = worker-node-01 or appRole = aro-workernode Then configure UCMDB discovery to key off the tag value instead of the VM name. Tags survive reimages and replacements.
3. Automate UCMDB updates with Event Grid + Runbooks Subscribe to VMSS scale-in/scale-out events in Azure Event Grid, then fire an Azure Logic App or Automation Runbook that calls the UCMDB REST API to add or remove CI records automatically. This eliminates manual intervention entirely.
4. Leverage scale-in policies to reuse instance IDs more predictably While you can’t guarantee names, you can pick a scale-in policy (Default, OldestVM, NewestVM) so that instance IDs roll over more consistently. In many steady environments, a replacement VM will end up with the same numeric suffix (for example, vmssname_2), which may reduce CI drift.

Note: With ARO, don’t modify the underlying VMSS directly—always scale via the OpenShift Machine API (oc scale machineset) or the OpenShift console. Direct edits to the VMSS can conflict with the ARO operator.

Hope that helps!

Reference list

• Troubleshoot scale-in policy https://docs.microsoft.com/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-scale-in-policy
• Overview of scale-in policy https://docs.microsoft.com/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-scale-in-policy
• Enabling scale-in policies https://docs.microsoft.com/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-scale-in-policy#enabling-scale-in-policy
• Troubleshoot common VMSS errors https://docs.microsoft.com/troubleshoot/azure/virtual-machine-scale-sets/welcome-virtual-machine-scale-sets
• Cannot update my scale set https://supportabilityhub.microsoft.com/solutions/apollosolutions/8fa1fc3d-3144-d930-c57a-25ba33c01444/apollo-684a161f-1c5f-4d33-bd7f-4807e09eab6c
• Configure Cluster Autoscaling (CycleCloud) https://docs.microsoft.com/azure/cyclecloud/how-to/configure-autoscaling?view=cyclecloud-7
• Use the CycleCloud REST API https://docs.microsoft.com/azure/cyclecloud/how-to/use-rest-api?view=cyclecloud-7
• Get started with Azure Monitor autoscale https://docs.microsoft.com/azure/azure-monitor/platform/autoscale-get-started
• Troubleshoot Azure Monitor autoscale https://docs.microsoft.com/azure/azure-monitor/platform/autoscale-troubleshoot
• VMSS FAQ https://docs.microsoft.com/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-faq
• Best practices for Azure Autoscale https://docs.microsoft.com/azure/monitoring-and-diagnostics/insights-autoscale-best-practices
• Design considerations for Scale Sets https://docs.microsoft.com/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-design-overview?wt.mc_id=knowledgesearch_inproduct_azure-cxp-community-insider
• Scaling issues workflow (placement constraints) https://supportability.visualstudio.com/AzureIaaSVM/_wiki/wikis/AzureIaaSVM/496449
• VMSS SME landing page https://supportability.visualstudio.com/AzureIaaSVM/_wiki/wikis/AzureIaaSVM/500164/SME%20Topics/Virtual%20Machine%20Scale%20Sets%20(VMSS)
• VMSS scoping questions https://supportability.visualstudio.com/AzureChatWiki/_wiki/wikis/AzureChatWiki/1266702/Scoping%20Questions%20By%20DU/Compute%20Storage%20(aka%20VM)/Virtual%20Machine%20Scale%20Sets

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