How to place spot instances in the cheapest AZ without hard-coding it or writing complex logic?

Question

One of my regions has a specific AZ that's consistently almost 50% cheaper than the others, for my instance type.
Yet my placement strategy consistently picks one of the more expensive AZs.
Here's the relevant code snippet:

```
			InstancesDistribution: &awsautoscaling.InstancesDistribution{
				SpotAllocationStrategy:              awsautoscaling.SpotAllocationStrategy_LOWEST_PRICE,
				OnDemandAllocationStrategy:          awsautoscaling.OnDemandAllocationStrategy_LOWEST_PRICE,
				OnDemandBaseCapacity:                jsii.Number(0),
				OnDemandPercentageAboveBaseCapacity: jsii.Number(0),
				SpotInstancePools:                   jsii.Number(2),
			},

```
Is there a simple configuration fix for this? I'd like to avoid having to write a complicated Lambda to figure out the lowest price and dynamically change configurations.

ShahadC · Answer

Based on your snippit and tags on this question, I assume you're talking about launching Spot instances in an ASG?

Currently there's no way to do what you're asking.  The ASG will always first try to balance work across all available AZs.  However, the AutoScaling team is always working on new options to meet usecases that aren't currently possible, so I've passed on this feedback to them for future feature decisions.

Currently you have 3 options
- Leave it as-is, and let ASG launch across all the AZs
- Lambda functions, as you suggested (periodically checking prices and swapping the single AZ for the ASG) - note that this is much more likely to have availability issues from launch failures
- Create 2 ASGs; 1 in your preferred AZ and another with all the other regions AZs enabled in it.  Set the scaling metrics such that the "Primary" ASG scales first.  This could lead to a bit of oscillation/churn between ASGs, especially if its a smaller environment and/or the metric is spiky
- directly launch capacity via `aws ec2 create-fleet` instead of using an ASG (but, you'll lose out on all the other ASG features if you do this)

Remember that Spot prices can and will change over time, so even if that 1 AZ has been cheaper in the short/medium term doesn't mean it'll always be that way, or that it'll have enough Spot capacity to consistently meet your needs

TheQuietBuilder · Answer

You're absolutely right to expect SpotAllocationStrategy: LOWEST_PRICE to favor the cheapest AZ automatically—but unfortunately, that’s not how it currently behaves in Auto Scaling Groups (ASG). The LOWEST_PRICE strategy selects the lowest price among the allowed Availability Zones (AZs), but it still tries to balance across AZs, rather than locking to the absolute cheapest one.

Short Answer:
There’s no current way to guarantee the ASG always picks the cheapest AZ without introducing custom logic.

🔧 Workaround Options:
Custom Lambda Script (Your original idea):
Periodically check Spot pricing and update the ASG’s AZs to include only the cheapest one. Yes, it's more work—but it's the only path to true price targeting.

Use Two ASGs:

One ASG limited to the historically cheapest AZ.

One ASG spanning other AZs (as backup).

Configure ECS Capacity Providers to prefer the primary ASG.

Use EC2 Fleet directly (if you're not dependent on ECS capacity providers):
EC2 Fleet gives you more control over instance placement/pricing—but loses ASG features like health checks, scaling policies, etc.

Final Thoughts:
Spot pricing trends are not guaranteed, so today's cheapest AZ might not stay that way.

I agree: AWS should allow a true "lowest price region-wide" strategy. Your feedback helps.

Let me know if you’d like help writing the Lambda logic or Terraform/CDK setup to implement the lowest-cost-AZ filter.

How to place spot instances in the cheapest AZ without hard-coding it or writing complex logic?

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