Optional checks or targets

[nicoche]

Hey!

Have you considered implementing optional checks or targets?

Here is an example:

• I have a check Idle that will decide on scaling action when my workload has between 0 and 1 replicas
• I have a check Active that will decide on scaling action when my workload has between 1 and x replicas

I don't want my check Idle to be even considered if I have, say, 5 replicas. I only want to make my ScaleIn / ScaleOut / ScaleNone decision based on the Active check when I have between 2 and x replicas.

I have been playing with group, but it is really difficult to build something that behaves like this, especially when you have more than 2 checks. I think that being able to enable/disable checks or targets based on some criteria would bring a lot of flexibility.

An example API could be:

check "target-value-check" {
  # ...
  evaluation_criterias { # No criteria means that the check is always evaluated
    criteria "count-allocations" { # Either min or max must be defined
      min = 2
      max = 10
    }

  strategy "target-value" {
    target    = 20
    threshold = 0.0001
  }
  # ...
}

What do you think? I'm happy to help implement this.

[nicoche]

I've drafted a PR with a similar approach, but a simpler API: #1020

Let me know what you think