Documentation: add documentation to explain the compatibility configuration between SlimFaas and Horizontal Pod Autoscaler

[antoineblancke]

❗Potential Conflict Between HPA and SlimFaas/ReplicasAtStart

When using Horizontal Pod Autoscaler (HPA) together with SlimFaas, the minReplicas value defined in the HPA must be greater than or equal to the SlimFaas annotation SlimFaas/ReplicasAtStart.

Failing to respect this rule may result in unexpected behavior:
the HPA will enforce its minimum replicas based on CPU utilization, while SlimFaas will attempt to set its initial replica count independently, potentially causing conflicting scaling decisions or oscillations.

---

✅ Example Configuration

HPA

apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: my-hpa
spec:
  maxReplicas: 8
  minReplicas: 2 # Must be >= SlimFaas/ReplicasAtStart
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: receiver
  metrics:
    - type: Resource
      resource:
        name: cpu
        target:
          type: Utilization
          averageUtilization: 65

SlimFaas

template:
  metadata:
    labels:
      app: my-service
    annotations:
      SlimFaas/Function: "true"
      SlimFaas/NumberParallelRequest: "2"
      SlimFaas/ReplicasAtStart: "2"

---

✅ Recommendation

Ensure that:

HPA.minReplicas >= SlimFaas/ReplicasAtStart

This guarantees consistent autoscaling behavior between SlimFaas and the Kubernetes HPA.