ASG: Inconsistent Handling of Decimal Places in Spot Instance Prices

Question

An attempt to launch a new EC2 spot instance from launch template fails when the spot instance price is formatted with three decimal places followed by zeros (e.g., "0.000700"). However, the price format with three significant digits followed by zeros (e.g., "0.007000") is accepted without issues. This inconsistency in decimal place validation is causing confusion and preventing successful instance launches.
During launch template creation, there are no errors or warnings related to this.

Expected Behavior
The EC2 spot instance should launch successfully regardless of the decimal place format, as long as the numerical value is valid.
If value is not considered correct, it should give a warning/error message.

Current Behavior
The EC2 launch process from an auto scaling group fails when the price is formatted with three decimal places followed by zeros (e.g., "0.000700"), but succeeds when formatted with three significant digits followed by zeros (e.g., "0.007000").

```
aws autoscaling describe-scaling-activities --auto-scaling-group-name "spot-t3-nano-asg" --region=ap-south-1
{
    "Activities": [
        {
            "ActivityId": "6ae64ee1-d80e-4e0a-bbd3-e5bc7c522af6",
            "AutoScalingGroupName": "spot-t3-nano-asg",
            "Description": "Launching a new EC2 instance.  Status Reason: Value (0.000700) for parameter price is invalid. \"0.000700\" is an invalid spot instance price. Launching EC2 instance failed.",
            "Cause": "At 2024-12-12T01:22:40Z an instance was started in response to a difference between desired and actual capacity, increasing the capacity from 0 to 1.",
            "StartTime": "2024-12-12T01:22:42.195000+00:00",
            "EndTime": "2024-12-12T01:22:42+00:00",
            "StatusCode": "Failed",
            "StatusMessage": "Value (0.000700) for parameter price is invalid. \"0.000700\" is an invalid spot instance price. Launching EC2 instance failed.",
            "Progress": 100,
            "Details": "{\"Subnet ID\":\"subnet-059e739f688e00000\",\"Availability Zone\":\"ap-south-1a\"}",
            "AutoScalingGroupARN": "arn:aws:autoscaling:ap-south-1:559050245391:autoScalingGroup:ac27ab00-3fa7-4d0f-a12b-9753a8500000:autoScalingGroupName/spot-t3-nano-asg"
        }
```

Reproduction Steps
Create a launch template. Specify the spot instance with max price as "0.000100".
Scale desired or max capacity in the auto scaling group to attempt to launch a new EC2 spot instance.
Observe the failure message indicating that the price is invalid with aws autoscaling describe-scaling-activities or from console > ec2> auto scaling groups > select group > acivities.

To make it work:
Change the spot instance max price to "0.001000" from the launch template.
Update ASG.
Observe that the instance launches successfully from ASG.

Question:
**Is this an API bug?**

Thanks!

re:Post Agent · Answer

Based on the information provided, this behavior does appear to be inconsistent and could potentially be considered an API bug or at least an inconsistency in the way AWS handles decimal places for spot instance prices.

The fact that the price format "0.000700" is rejected while "0.007000" is accepted, despite both representing valid numerical values, suggests an issue with the input validation or parsing of spot instance prices in the EC2 Auto Scaling API.

This inconsistency in handling decimal places is not expected behavior. Typically, AWS services should be able to handle various decimal representations of the same numerical value, especially when it comes to pricing information.

The error message "Value (0.000700) for parameter price is invalid. '0.000700' is an invalid spot instance price" further indicates that there might be an overly strict or improperly implemented validation check in the API.

Given that this issue prevents successful instance launches and causes confusion for users, it would be advisable to report this to AWS support. They can investigate further and potentially address it as a bug in their API handling of spot instance prices.

In the meantime, as a workaround, users should format their spot instance prices using three significant digits followed by zeros (e.g., "0.007000") to avoid this issue when creating launch templates or configuring Auto Scaling groups with spot instances.  
**Sources**  
[Keeping Unhealthy Auto Scaling (ASG) Instances (Detaching Unhealthy ASG Instance instead of Terminating) | AWS re:Post](https://repost.aws/articles/ARxDaRTv7ZRsKPxyg19uOl1Q/keeping-unhealthy-auto-scaling-asg-instances-detaching-unhealthy-asg-instance-instead-of-terminating)  
[Keeping Unhealthy Auto Scaling (ASG) Instances (Detaching Unhealthy ASG Instance instead of Terminating) | AWS re:Post](https://repost.aws/articles/ARxDaRTv7ZRsKPxyg19uOl1Q/detaching-unhealthy-asg-instance-instead-of-terminating-preserving-unhealthy-instances-in-aws-auto-scaling-groups)

ShahadC · Answer

Hi,

This is because you can't currently set a value lower than 0.001 for the Max Price.  I'm guessing the extra 0's added at the end are just being ignored.  Keep in mind that this value could change in the future, but for now you'll need to set a value of 0.001 or higher.

Also something to note is that this parameter is rarely used anymore.  It was designed back when Spot had Bidding, but that went away in 2017, and now Spot prices can never be higher than the OnDemand price, and are changed gradually based on long term trends vs bidding for capacity with other AWS customers:
https://aws.amazon.com/blogs/compute/new-amazon-ec2-spot-pricing/

ASG: Inconsistent Handling of Decimal Places in Spot Instance Prices

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