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LAB-K8S-03: PODs

Description: This is a lab dedicated to manipulation and exploration of POD objects, where participants will get familiar with PODs inside a cluster.

Duration: ±45m

Goals

At the end of this lab, each participant will have:

  • created, updated, deleted a POD using commands as well as YAML declarations..
  • debugged a common error

Prerequisites

Exploring a POD

The instructor has created a POD inside your namespace ! Your task is to list the pods running in your namespace:

kubectl get pods
  • ✅ What is the name of the pod that has been created?

POD Status

  • Pending: The Pod has been accepted by the Kubernetes system, but one or more of the Container images has not been created. This includes time before being scheduled as well as time spent downloading images over the network, which could take a while.
  • Running: The Pod has been bound to a node, and all of the Containers have been created. At least one Container is still running, or is in the process of starting or restarting.
  • Succeeded: All Containers in the Pod have terminated in success, and will not be restarted.
  • Failed: All Containers in the Pod have terminated, and at least one Container has terminated in failure. That is, the Container either exited with non-zero status or was terminated by the system.
  • Unknown: For some reason the state of the Pod could not be obtained, typically due to an error in communicating with the host of the Pod.
  • ✅ What is the pod status ?

POD node

Now run the same command with -o wide option:

kubectl get pods -o wide
  • ✅ On what node is this pod running ?

POD details

In order to to look more thoroughly at a POD's details we need to use the kubectl describe command for pods.

kubectl describe pod welcome
Name:         welcome
Namespace:    tatooine
Priority:     0
Node:         node66449-sokube-k8s-training.hidora.com/10.102.9.82
Start Time:   Sun, 07 Jun 2020 14:08:43 -0700
Labels:       org=sokube
Annotations:  <none>
Status:       Running
IP:           10.239.0.16
IPs:
  IP:  10.239.0.16
Containers:
  welcome:
    Container ID:  docker://0d8eaf9fc29d4a7d0a008439fef5588151633c9e706b79385b7fbfe41a023faa
    Image:         busybox
    Image ID:      docker-pullable://busybox@sha256:95cf004f559831017cdf4628aaf1bb30133677be8702a8c5f2994629f637a209
    Port:          <none>
    Host Port:     <none>
    Command:
      /bin/sh
      -c
    Args:
      echo "welcome on $PLANET" > /welcome.txt; sleep 3600
    State:          Running
      Started:      Sun, 07 Jun 2020 14:08:47 -0700
    Ready:          True
    Restart Count:  0
    Environment:
      PLANET:  tatooine
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-b2sfc (ro)
Conditions:
  Type              Status
  Initialized       True 
  Ready             True 
  ContainersReady   True 
  PodScheduled      True 
Volumes:
  default-token-b2sfc:
    Type:        Secret (a volume populated by a Secret)
    SecretName:  default-token-b2sfc
    Optional:    false
QoS Class:       BestEffort
Node-Selectors:  <none>
Tolerations:     node.kubernetes.io/not-ready:NoExecute for 300s
                 node.kubernetes.io/unreachable:NoExecute for 300s
Events:
  Type    Reason     Age        From                                               Message
  ----    ------     ----       ----                                               -------
  Normal  Scheduled  <unknown>  default-scheduler                                  Successfully assigned dagobah/welcome to node66449-sokube-k8s-training.hidora.com
  Normal  Pulling    13s        kubelet, node66449-sokube-k8s-training.hidora.com  Pulling image "busybox"
  Normal  Pulled     11s        kubelet, node66449-sokube-k8s-training.hidora.com  Successfully pulled image "busybox"
  Normal  Created    10s        kubelet, node66449-sokube-k8s-training.hidora.com  Created container welcome
  Normal  Started    10s        kubelet, node66449-sokube-k8s-training.hidora.com  Started container welcome
  • ✅ What is image used by the container in this pod ?
  • ✅ What does this container do exactly ? (check command)
  • ✅ Where does it output text ?

Executing commands in a POD container

With the kubectl exec command, we can execute commands remotely into the containers of specified pods. Here we run an interactive shell session (/bin/sh) into the unique container of the POD.

kubectl exec -it welcome -- /bin/sh
  • ✅ Can you show the content of the file ?
/ # ls
bin          etc          proc         sys          usr          welcome.txt
dev          home         root         tmp          var
/ # more welcome.txt
welcome on ...........
/ # exit

Restart Policy

A PodSpec has a restartPolicy field with possible values Always, OnFailure, and Never. The default value is Always. restartPolicy applies to all Containers in the Pod. restartPolicy only refers to restarts of the Containers by the kubelet on the same node. Exited Containers that are restarted by the kubelet are restarted with an exponential back-off delay (10s, 20s, 40s …) capped at five minutes, and is reset after ten minutes of successful execution.

  • ✅ According to the definition above, and to the field Restart Count in the pod describe result, what values could we potentially have in the welcome pod specification ?
  • ✅ Can you confirm ? How ?
  • ✅ Run kubectl get pod -o

POD as YAML

  • Extract the POD's in the YAML format, for future usage:

    kubectl get pod welcome -o=yaml > welcome.yaml

  • ✅ Have a look at welcome.yaml, do you think we have only the strict minimum content necessary to create a POD here?

  • ✅ Now that we have the pod spec, what was the value for restartPolicy ?

    grep -i restart welcome.yaml

Delete a POD

Now we can work on our own PODs

  • ✅ Delete the welcome pod without waiting 
    kubectl delete pod welcome --grace-period=0

Creating your first POD

Declarative vs Imperative

💡 Declarative vs Imperative

Declarative : Description of a desired state, the system (Docker, Kubernetes ...) will try to reach this state in time. Imperative/Procedural : Control of the final state. pod

  • ✅ Example of the imperative command use to deploy a pod :

    kubectl run --generator=run-pod/v1 my-app-example --image=nginx:latest

  • A declarative approach would be to describe the desired state in a file. :white_check_mark: Apply the desired state to the kubernetes cluster with the apply command:

    kubectl apply -f my-app-example-2.yml

  • Let's delete one of the pods, the other one will serve us in a future exercise.

    kubectl delete pods my-app-example

Explore your Landing Zone

Inside your namespace, the instructor has initialized a set of kubernetes objects that will allow you to see your namespace's applications using a browser. We won't have a look at them immediately in the labs, but it is important for you to not alter them before.

⚠️ DO NOT ALTER the service sokube-k8s-training-<YOURNAMESPACE>-landingzone-svc in your namespace or you might not be able to see your POD ⚠️

The current system setup for your is actually redirecting the URL of your landing zone to a POD with the following criteria:

  • Running on container port 8080
  • Having a label app with value simple-todo
  • ✅ Do we have such a POD in our namespace ?

Our application team has provided us at the last minute a pod declaration file for the application, given in the simple-todo-pod.yml file at the root of this lab.

  • ✅ Try to create the POD

    kubectl create -f simple-todo-pod.yml

  • ✅ Can you correct the errors in this file ? (solution here)

  • ✅ Reopen your langing zone and confirm you see the application

  • ✅ Explore the application and give a positive feedback to your application team for the nice job

Environment variables

The default landing zone todo application feels very generic and does not reflect your own landing zone.

Our application developers have included the possibility to customize the header to change the owner organization of the todo application (which is Star Wars by default), by using the environment variable ORG.

  • ✅ Read the documentation on environment variables

  • ✅ Modify the POD declaration file to include an environment variable ORG with the value of your namespace. You might have to delete the pod and recreate it.

  • ✅ Confirm you now have your own customized landing zone

Next lab