Horizontal Pod Autoscaler

Horizontal Pod Autoscaler (HPA)

The Horizontal Pod Autoscaler automatically scales the number of pods in a Deployment or ReplicaSet based on observed CPU utilization or custom metrics.

HPA is how Kubernetes achieves elastic scaling — your application scales up during traffic spikes and scales down during quiet periods, saving resources.

How HPA Works

  Traffic Increases
       │
       ▼
  ┌──────────────┐
  │  HPA polls   │  (every 15 seconds by default)
  │  metrics     │
  └──────┬───────┘
         │
         ▼
  CPU usage > target?  ──Yes──▶  Scale up pods
         │
         No
         │
         ▼
  CPU usage < target?  ──Yes──▶  Scale down pods
         │
         No
         │
         ▼
  Do nothing

Creating an HPA

# Quick way
kubectl autoscale deployment my-app --min=2 --max=10 --cpu-percent=70

# Full YAML definition
apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: my-app-hpa
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: my-app
  minReplicas: 2
  maxReplicas: 10
  metrics:
    - type: Resource
      resource:
        name: cpu
        target:
          type: Utilization
          averageUtilization: 70
    - type: Resource
      resource:
        name: memory
        target:
          type: Utilization
          averageUtilization: 80
  behavior:
    scaleUp:
      stabilizationWindowSeconds: 60
      policies:
        - type: Percent
          value: 100
          periodSeconds: 60
    scaleDown:
      stabilizationWindowSeconds: 300
      policies:
        - type: Percent
          value: 10
          periodSeconds: 60

scaleTargetRef — Which Deployment to scale.

metrics — What to measure (CPU, memory, or custom metrics).

behavior — How fast to scale up and down. Scaling down is slower to avoid flapping.

Prerequisites for HPA

Resource requests must be set — HPA calculates utilization as a percentage of requested resources. Without requests, HPA cannot work.

Metrics server required — Install the metrics server for CPU/memory metrics:

# Install metrics server (Minikube)
minikube addons enable metrics-server

# Install metrics server (standard)
kubectl apply -f https://github.com/kubernetes-sigs/metrics-server/releases/latest/download/components.yaml

Monitoring HPA

# Check HPA status
kubectl get hpa

# Detailed info
kubectl describe hpa my-app-hpa

# Watch in real-time
kubectl get hpa -w