Add protect min on demand option

[clayrisser]

It would be nice to have an option that automatically enables instance protection on the minimum number of required on-demand instances. Theoretically, when this number is decreased, it should deactivate instance protection on the respective instances.

[xlr-8]

Hello, could you clarify a bit the need of this, I'm not sure to follow. Thanks!

[clayrisser]

Instance protection should be enabled on the minimum number of required on-demand instances to prevent the on-demand instances from rotating.

[cristim]

In a mixed group, where we need to keep a number of running on-demand nodes, the idea is to protect the on-demand instances from scale-in in order for AutoScaling to first terminate the spot instances.

If we use percentages, the number of protected instances should go up and down according to the desired capacity.

[clayrisser]

That's ok, that just means you might have to mark some as unprotected when you scale down

[cristim]

I'm actually thinking that it may cause replacement churn if we do this:

let's say you protect N given as 50% percentage, then the group scales down terminating a spot instance and then you will no longer need to protect N on demand but say N-1.

The group will now need to replace one of those on-demand nodes with spot, instead of just terminating one spot node in the first place, that gets us to the same result.

So I think this only makes sense if we're using the min_on_demand_number.

[clayrisser]

That's fine, I would still find this feature useful for min on demand number

[cristim]

@clayrisser are you still interested in this functionality?