Kubernetes How-Tos

Kubernetes has been taking over the container orchesteration stage for sometime now, it has a large community and a great set of tooling and contributers backing it. Along those lines, picking up kubernetes skills can be a little daunting at first, but fear not, as documentation is rich and concepts are well elaborated in not one but many places.

In this post, I won’t attempt to re-explain kubernetes concepts (pods, deployments, services, replicasets, …etc) since as I said, a lot of great people did that job already, and did it very well (if you still have doubts about some of the concepts, I would be glad to help clear smoke up). Instead, I will be adding a set of How-Tos for common tasks that you might encounter when working with kubernetes, the post might be updated every now and then to include more. Shall we begin?!

N.B: The How-Tos take the form of Q&A.

How to access K8s API from the command line

# First Get the Token and the API server address
APISERVER=$(kubectl config view | grep server | cut -f 2- -d ":" | tr -d " ")
TOKEN=$(kubectl describe secret $(kubectl get secrets | grep default | cut -f1 -d ' ') | grep -E '^token' | cut -f2 -d':' | tr -d '\t')

# Now use the token and the address to call the API
curl $APISERVER/api --header "Authorization: Bearer $TOKEN" --insecure
{
  "kind": "APIVersions",
  "versions": [
    "v1"
  ],
  "serverAddressByClientCIDRs": [
    {
      "clientCIDR": "0.0.0.0/0",
      "serverAddress": "10.0.1.149:443"
    }
  ]
}

How to get access to the dashboard

• First get the token.

  APISERVER=$(kubectl config view | grep server | cut -f 2- -d ":" | tr -d " ")
  TOKEN=$(kubectl describe secret $(kubectl get secrets | grep default | cut -f1 -d ' ') | grep -E '^token' | cut -f2 -d':' | tr -d '\t')
  

• Then create a secure channel to your kubernetes cluster using kubectl proxy
• Finally, visit http://localhost:8001/api/v1/namespaces/kube-system/services/https:kubernetes-dashboard:/proxy/ and use the token as an input when prompted.

For more Info check the documentation and the repo here

If you want to access the dashboard from outside, you need to modify the service with a NodePort

kubectl -n kube-system edit service kubernetes-dashboard

if you want to get access without a token or kube-config for the dashboard, apply the following yaml to give the cluster-admin role:

apiVersion: rbac.authorization.k8s.io/v1beta1
kind: ClusterRoleBinding
metadata:
  name: kubernetes-dashboard
  labels:
    k8s-app: kubernetes-dashboard
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: cluster-admin
subjects:
- kind: ServiceAccount
  name: kubernetes-dashboard
  namespace: kube-system

How to do health-checks with kubernetes

There are two options to do health checks:

• Liveness Probes: Many applications running for long periods of time eventually transition to broken states, and cannot recover except by being restarted. Kubernetes provides liveness probes to detect and remedy such situations.
• Readiness Probes: it’s used to check the start-up phase of a container in the pod. Imagine a container that loads some data from external storage such as S3 or a database that needs to initialize some tables. In this case you want to signal when the container is ready to serve traffic.

For more information check documentation

What kind of selectors can be applied

How to delete replicationsets without deleting pods

This can be achieved by setting the --cascade option to false at the end of the delete command, example:

kubectl delete rs rs_name --cascade=false

How to list pod names only

kubectl get pods -o go-template --template '{{range .items}}{{.metadata.name}}{{"\n"}}{{end}}'

OR

kubectl get pods -o jsonpath --template '{.items[*].metadata.name}'

if you want to delete all pods you can do it in a loop:

for pod in $(kubectl get pods -o go-template --template '{{range .items}}{{.metadata.name}}{{"\n"}}{{end}}'); do kubectl delete pod $pod; done

OR

for pod in $(kubectl get pods -o jsonpath --template '{.items[*].metadata.name}'); do kubectl delete pod $pod; sleep 60; done