Unable to override instance type in ASG

Question

I have a launch template that specifies `t3.medium` as the instance to launch via SPOT.

I want to override this in ASG configuration. To do that I edit ASG config and do the following:

* Under instance type requirements, I choose "override launch template"
* I select 2 instance types that I want: `t3.medium` and `t3a.medium`
* Under instance purchase options, I choose 100% Spot, 0% On-Demand and disable "On Demand base capacity" option
* Under "Allocation strategies" choose "Lowest price"

When I click on `Update` I get the error message:

```
Incompatible launch template: You cannot use a launch template that is set to request Spot Instances (InstanceMarketOptions) when you configure an Auto Scaling group with a mixed instances policy. Add a different launch template to the group and try again.
```

Is this a bug or am I missing something?

Accepted Answer

Hello,

You need to remove 'spot' from the launch template options, since its incompatible with the multiple instance types feature in AutoScaling.  Since you're selecting "100% spot" in the overrides all your instances will be spot (even though you don't have it selected in the LT).

Basically, when you're using the overrides option you need to let the AutoScaling logic take over for the spot or not decisions (which AutoScaling then passes on to EC2 Fleet in a CreateFleet call if you ever want to look in cloudtrail at what happens under the hood)

This behavior is mentioned here: https://docs.aws.amazon.com/autoscaling/ec2/userguide/create-launch-template.html
But I can see how the wording is a bit ambiguous, so I've reached out to the AutoScaling team to have it updated to be more clear that you MUST leave the 'request spot instances' box un-checked

Thank you for choosing launch templates :)

Unable to override instance type in ASG

Relevant content