What is Kubernetes Custom Metrics API?

Question

Accepted Answer

**Custom Metrics** — application-specific metrics for HPA scaling.

```yaml
# Install Prometheus Adapter
apiVersion: apps/v1
kind: Deployment
metadata:
  name: prometheus-adapter
  namespace: custom-metrics
spec:
  replicas: 1
  template:
    spec:
      containers:
      - name: prometheus-adapter
        image: registry.k8s.io/prometheus-adapter/prometheus-adapter:v0.11.1
        args:
        - --prometheus-url=http://prometheus:9090
        - --metrics-relist-interval=30s
        - --config=/etc/adapter/config.yaml
        volumeMounts:
        - name: config
          mountPath: /etc/adapter
      volumes:
      - name: config
        configMap:
          name: prometheus-adapter-config
---
# HPA using custom metrics
apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: myapp-hpa
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: myapp
  minReplicas: 2
  maxReplicas: 20
  metrics:
  - type: Pods
    pods:
      metric:
        name: http_requests_per_second
      target:
        type: AverageValue
        averageValue: "1000"
  - type: Object
    object:
      describedObject:
        apiVersion: networking.k8s.io/v1
        kind: Ingress
        name: myapp
      metric:
        name: requests_per_second
      target:
        type: Value
        value: "10k"
  - type: External
    external:
      metric:
        name: sqs_queue_length
      target:
        type: Value
        value: "30"
```

```bash
# Commands
kubectl get --raw "/apis/custom.metrics.k8s.io/v1beta1" | jq
kubectl get --raw "/apis/custom.metrics.k8s.io/v1beta1/namespaces/default/pods/*/http_requests_per_second" | jq

# Check HPA metrics
kubectl get hpa myapp-hpa -o yaml | grep -A 20 metrics
kubectl describe hpa myapp-hpa
```

**Metric Types:**
- Pods: per-Pod metrics.
- Object: metrics from another resource.
- External: metrics from outside cluster.

DevOps: What is Kubernetes Custom Metrics API?