Metrics server

Learn that the metrics server is required for HPA.

Metrics server

The HPA gets CPU and memory metrics from metrics-server; metrics-server in turn collects them from the kubelet on each node. If it isn't installed, the HPA can do nothing and kubectl top doesn't work:

$ kubectl top pods
error: Metrics API not available

minikube addons enable metrics-server
# or
kubectl apply -f https://github.com/kubernetes-sigs/metrics-server/releases/latest/download/components.yaml

$ kubectl top pods -l app=api
NAME                  CPU(cores)   MEMORY(bytes)
api-7f8c9d6b5-abcde   142m         180Mi

Testing under load

To see the HPA, artificial load is generated and watched with -w. Because of the metric collection and decision intervals, the reaction can take a minute or two:

kubectl run yuk --image=busybox:1.36 --restart=Never -- \
  sh -c "while true; do wget -q -O- http://api:8000/; done"
kubectl get hpa api -w
NAME   TARGETS         REPLICAS
api    cpu: 38%/70%    2
api    cpu: 156%/70%   2
api    cpu: 156%/70%   5
api    cpu: 64%/70%    5
kubectl delete pod yuk

Exercise A load test

Write three commands: enable the metrics-server add-on in Minikube; show the resource usage of the pods labelled app=api; watch the api HPA live.

Show example solution

minikube addons enable metrics-server
kubectl top pods -l app=api
kubectl get hpa api -w

Test yourself

What is needed to scale on application metrics like request count?
- It can't be scaled
- A custom metrics provider like the Prometheus Adapter or KEDA
- Only metrics-server
- Bigger nodes
Show answer

A custom metrics provider like the Prometheus Adapter or KEDA — metrics-server only provides CPU and memory.

Show the pods' current resource usage.

kubectl ____ pods -l app=api

Show answer

kubectl top pods -l app=api

Match.

metrics-server

CPU/memory metrics
Metrics API not available

No metrics-server
KEDA

Event-driven scaling
hpa -w

Watch the scaling

Next lesson: Summary - HPA

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