What are "Current Replicas" in the Kubernetes HPA Calculation?

Question

In Kubernetes, the desired replica count for a Horizontal Pod Autoscaler (HPA) is defined as the following:

desiredReplicas = ceil[currentReplicas * ( currentMetricValue / desiredMetricValue )] Source

In our clusters, we have some HPA's that are configured with 5 periodSecond intervals.

behavior:
  scaleUp:
    policies:
    - type: Percent
      value: 100
      periodSeconds: 5

From my understanding, in English this means "double the number of pods every 5 seconds".

Now, let's imagine a scenario where the pod count needs to double from 100 to 200 replicas because a bunch of messages got dumped into a Queue. During this scale-out, let's imagine 10 of the pods get scheduled immediately, but the remaining 90 pods get stuck in a "Pending" state while they wait 1-2 minutes for cluster-autoscaler to provision new nodes.

When the next calculation is run in 5 seconds (because periodSeconds=5), will currentReplicas be:

a) 110 replicas

b) 200 replicas

c) Other

More specifically, how is currentReplicas defined? Are NotReady pods included in the currentReplica count?

Answer 1

In Kubernetes, the currentReplicas value used by the Horizontal Pod Autoscaler (HPA) refers to the number of pods that have been requested, regardless of whether they are in the Running, Pending, or NotReady states. So, NotReady and Pending pods are included in the currentReplicas count.

In your scenario:

• You have an HPA policy that doubles the pod count every 5 seconds (periodSeconds: 5), starting from 100 pods.
• After the first scale-up, 10 pods get scheduled and start running, but 90 pods remain in the "Pending" state waiting for new nodes to be provisioned by the cluster autoscaler.

When the next HPA calculation is run in 5 seconds, currentReplicas will be 200 replicas. This includes the 110 pods that are running and pending combined.

The HPA tracks the desired number of pods, which was set to 200 during the last scale-up. Pods in the Pending or NotReady state are counted toward the currentReplicas because the HPA bases its calculations on the number of pods requested, not just those in the Running state.

Behavior of HPA in this Scenario:

• The HPA will see that it already requested 200 replicas, so unless the metric values (e.g., CPU or memory utilization) change significantly, it will not request more replicas.
• The cluster autoscaler will continue to work in the background, provisioning new nodes to accommodate the remaining Pending pods.
• Eventually, once nodes are available, the Pending pods will move to the Running state.

Conclusion:

In this case, the currentReplicas during the next HPA calculation would be b) 200 replicas.