9. Labels and seleectors
Labels are key-value pairs used to append metadata to a K8s object.
First, let’s view the labels for an internal K8s service:
$ kubectl get svc kube-dns -n kube-system -o yaml | yq '.metadata.labels'
k8s-app: kube-dns
kubernetes.io/cluster-service: "true"
kubernetes.io/name: CoreDNS
Now we know that k8s-app is a label key commonly used by internal K8s objects, let us observe which objects also contain this key:
$ kubectl get svc,ds,rs,pods -A -L k8s-app
NAMESPACE NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE K8S-APP
default service/kubernetes ClusterIP 10.96.0.1 <none> 443/TCP 9m17s
kube-system service/kube-dns ClusterIP 10.96.0.10 <none> 53/UDP,53/TCP,9153/TCP 9m16s kube-dns
kube-system service/metrics-server ClusterIP 10.96.1.164 <none> 443/TCP 9m9s metrics-server
kubernetes-dashboard service/dashboard-metrics-scraper ClusterIP 10.102.193.25 <none> 8000/TCP 9m4s dashboard-metrics-scraper
kubernetes-dashboard service/kubernetes-dashboard ClusterIP 10.99.142.68 <none> 80/TCP 9m4s kubernetes-dashboard
NAMESPACE NAME DESIRED CURRENT READY UP-TO-DATE AVAILABLE NODE SELECTOR AGE K8S-APP
kube-system daemonset.apps/kube-proxy 1 1 1 1 1 kubernetes.io/os=linux 9m16s kube-proxy
NAMESPACE NAME DESIRED CURRENT READY AGE K8S-APP
kube-system replicaset.apps/coredns-7db6d8ff4d 1 1 1 9m2s kube-dns
kube-system replicaset.apps/metrics-server-c59844bb4 1 1 1 9m2s metrics-server
kubernetes-dashboard replicaset.apps/dashboard-metrics-scraper-b5fc48f67 1 1 1 9m2s dashboard-metrics-scraper
kubernetes-dashboard replicaset.apps/kubernetes-dashboard-779776cb65 1 1 1 9m2s kubernetes-dashboard
NAMESPACE NAME READY STATUS RESTARTS AGE K8S-APP
kube-system pod/coredns-7db6d8ff4d-kbfcv 1/1 Running 0 9m2s kube-dns
kube-system pod/etcd-minikube 1/1 Running 0 9m16s
kube-system pod/kube-apiserver-minikube 1/1 Running 0 9m16s
kube-system pod/kube-controller-manager-minikube 1/1 Running 0 9m16s
kube-system pod/kube-proxy-rcrrb 1/1 Running 0 9m2s kube-proxy
kube-system pod/kube-scheduler-minikube 1/1 Running 0 9m16s
kube-system pod/metrics-server-c59844bb4-pnl69 1/1 Running 0 9m2s metrics-server
kube-system pod/storage-provisioner 1/1 Running 0 9m15s
kubernetes-dashboard pod/dashboard-metrics-scraper-b5fc48f67-nmncw 1/1 Running 0 9m2s dashboard-metrics-scraper
kubernetes-dashboard pod/kubernetes-dashboard-779776cb65-p6qms 1/1 Running 0 9m2s kubernetes-dashboard
Change the -L to -l (lowercase) to filter out a certain label value:
$ kubectl get svc,ds,rs,pods -A -l k8s-app=kube-dns
NAMESPACE NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
kube-system service/kube-dns ClusterIP 10.96.0.10 <none> 53/UDP,53/TCP,9153/TCP 11m
NAMESPACE NAME DESIRED CURRENT READY AGE
kube-system replicaset.apps/coredns-7db6d8ff4d 1 1 1 11m
NAMESPACE NAME READY STATUS RESTARTS AGE
kube-system pod/coredns-7db6d8ff4d-kbfcv 1/1 Running 0 11m
For more information, see https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/#label-selectors.