What are Kubernetes Resource Metrics and Metrics Server?

Question

Accepted Answer

**Metrics Server** — source of metrics for `kubectl top`, HPA, VPA.

**Installation:**
```bash
kubectl apply -f https://github.com/kubernetes-sigs/metrics-server/releases/latest/download/components.yaml

# For self-signed cert
kubectl patch deployment metrics-server -n kube-system --type='json' -p='[
  {"op":"add","path":"/spec/template/spec/containers/0/args/-","value":"--kubelet-insecure-tls"}
]'
```

**Verification:**
```bash
kubectl top nodes
kubectl top pods --sort-by=memory
kubectl top pods -n production --containers

# Raw metrics API
kubectl get --raw "/apis/metrics.k8s.io/v1beta1/pods" | jq
kubectl get --raw "/apis/metrics.k8s.io/v1beta1/nodes" | jq
```

**Metrics Server vs Prometheus:**
- Metrics Server: current values, for HPA/VPA, no history stored.
- Prometheus: historical data, alerts, dashboards.

**HPA with custom metrics:**
```yaml
apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
spec:
  metrics:
    - type: Pods
      pods:
        metric:
          name: http_requests_per_second
        target:
          type: AverageValue
          averageValue: "100"
    - type: Object
      object:
        describedObject:
          apiVersion: networking.k8s.io/v1
          kind: Ingress
          name: myapp
        metric:
          name: requests-per-second
        target:
          type: Value
          value: "1k"
```

**Prometheus Adapter** — bridge between Prometheus and K8s custom metrics API.

DevOps: What are Kubernetes Resource Metrics and Metrics Server?