https://kubernetes.io/docs/tasks/debug-application-cluster/troubleshooting/
Check cluster info and status of components:
kubectl cluster-info
kubectl get componentstatuses
Check nodes:
kubectl get nodes
kubectl describe node <name>
Check validating webhooks and API services:
kubectl get validatingwebhookconfigurations
kubectl get apiservices
The API server may time out or forbid requests if an API service or the service backing a validating webhook is down.
List pods and check their status: kubectl get pods --all-namespaces. Are they all Running? (Jobs can be Completed also.)
Describe failed pods: kubectl -n <namespace> describe pod <name>.
Check logs for a pod: kubectl -n <namespace> logs <name>
Are pods missing? Check deployments and replicasets to see if they have trouble creating pods:
kubectl get deployment --all-namespaces
kubectl -n <namespace> describe deployment <name>
kubectl -n <namespace> describe replicaset <name>
Check statefulsets and daemonsets:
kubectl get statefulsets --all-namespaces
kubectl -n <namespace> describe statefulset <name>
kubectl get daemonsets --all-namespaces
kubectl -n <namespace> describe daemonset <name>
Check that services exist and have endpoints:
kubectl get svc --all-namespaces
kubectl -n <namespace> describe svc <name>
Check ingresses:
kubectl get ingress --all-namespaces
kubectl -n <namespace> describe ingress <name>
Make sure ingresses have the correct IP and the DNS is configured for them.
Check certificates:
kubectl get certificates --all-namespaces
kubectl -n <namespace> describe certificate <name>
Compare also with the secrets that are used to store the certificates:
kubectl -n <namespace> describe secret <name>
Check resource usage in terminal for nodes and pods:
kubectl top nodes
kubectl top pods --all-namespaces
In Grafana, start from the cluster level and "drill down". The dashboards "Kubernetes / Compute Resources / Cluster" and "Kubernetes / USE Method / Cluster" show cluster level metrics.