ECS + Spot Integration - Multiple ASGs vs SpotFleet

EC2 Spot Fleet supports a static amount of OD and a scalable target capacity for Spot capacity. They should start there.

The customer should focus on diversifying their Spot requests across AZs and instance types/sizes (super simple with Spot Fleet and containers) to reduce the likelihood of not being able to provision Spot capacity. Keep in mind that Spot and OD pull from the same capacity pool. If they cannot get a Spot Instance in a given pool, they very well might not be able to get an OD instance in the same pool. The reaction to not getting a Spot Instance should be to try a different pool (either another AZ of same instance type/size, or a different instance type/size in the same AZ)- Spot Fleet will handle this for them automatically by specifying multiple launch specifications or launch templates.

If they still want to have an ASG with OD as a "backup", they could consider keeping an ASG that scales based on an alarm that triggers when FulfilledCapacity is < TargetCapacity for the Spot Fleet for a given period of time, and join this ASG to the same ECS cluster. Critical is having the OD target pull from a different pool of capacity than defined in the Spot Fleet.