UpdatePolicy Resets MinInstancesInService in an ASG

The MinInstancesInService property for UpdatePolicy is currently not capable of using the dynamic DesiredCapacity value of the AutoScalingGroup at the time of an update. If you have a stack that has a lower MinSize but dynamically scales up significantly to meet demand. If you set the MinInstancesInService property to 3 (for example) and start an update when the group is scaled to 30 instances, the first thing CloudFormation will do is terminate 27 instances leaving the ASG way under provisioned.

Start by reviewing the CloudFormation UpdatePolicy behavior for an AutoScalingGroup when MinInstancesInService is set and DesiredCapacity has scaled above MinSize. Reproduce the update scenario described, then verify that an update preserves the dynamically scaled capacity instead of terminating instances down to the configured minimum.

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