AWS Auto Scaling group - price-capacity-optimized for spot instances - replacement process

Question

Hello,

Let's suppose we have Auto Scaling groups that has 2 desired capacity and configured to launch them as spot instances with price-capacity-optimized strategy

We configured the ASG to launch two different instance types:  **t3.medium,  t3.large**

At first run, the running spot instacnes are as the following depending on the allocation strategy:
**2 with t3.medium**

Now let's suppose that, the other instance type t3.large got better allocation strategy (price-capacity-optimized),

**Will the ASG switch the running spot instances from t3.medium to be t3.large?**

Could you please advise?

Thanks

Accepted Answer

Hello

AutoScaling will *not* rebalance existing instances based on changes to the allocation strategy.  Additionally, its impossible to change the instance type of a running EC2 instance (inside of AutoScaling or not).

You can enable [Capacity Rebalancing](https://docs.aws.amazon.com/autoscaling/ec2/userguide/capacity-rebalance.html) on the group, and AutoScaling will try to proactively replace instances which are going to be terminated by spot soon, but this isn't constantly monitoring price/capacity, it only happens when Spot sends a notification to AutoScaling.

AWS Auto Scaling group - price-capacity-optimized for spot instances - replacement process

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