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Behavior in the Event of Zone 2 Going Down
In case, you have configured a fixed single instance, and it is running in Zone 2. If Zone 2 goes down, that specific instance will become unavailable. The instance will not automatically come up in Zone 1 or Zone 3. The VMSS does not have the capability to relocate or recreate the instance in another zone when only one instance is specified. You will effectively see the VMSS as down because there are no operational instances available to handle requests. The scale set cannot provide redundancy or failover capabilities with a single instance.
Here is a more detailed explanation of what happens when a zone goes down:
- Azure monitors the health of all zones and VMs in a VMSS.
- If a zone goes down, Azure will automatically detect the failure.
- Azure will then attempt to restart the VM in the failed zone.
- If Azure is unable to restart the VM in the failed zone, it will create a new VM in another available zone.
- Azure will then copy the state of the old VM to the new VM.
- Once the new VM is ready, Azure will switch traffic to the new VM.
To minimize the impact of a zone failure, you can use the following best practices:
- Use a zone redundant VMSS. This will ensure that your VMs are distributed across multiple zones, so that if one zone goes down, your VMs will still be available in other zones.
- Use auto scaling. This will ensure that your VMSS automatically scales up to meet demand, even if a zone goes down.
- Use a load balancer. This will distribute traffic across your VMs, so that if one VM goes down, traffic will be automatically routed to other VMs. https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-use-availability-zones?tabs=portal-2
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