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Introduction to pods

From container to pods

Kubernete pods schema

Let's see how we can deploy a simple container using kubernetes. To do this we need to introduce two k8s resources:

Pod:
- A pod is a collection of containers sharing a network and mount namespace and is the basic unit of deployment in Kubernetes. All containers in a pod are scheduled on the same node.
Deployment:
- A deployment is a supervisor for pods, giving you fine-grained control over how and when a new pod version is rolled out as well as rolled back to a previous state.

Before continuing

The docker images used in this demo can be found in:

lookup
probe

Create a pod with kubectl

To run a pod all you need is:

kubectl run --generator=run-pod/v1  lookup-pod --image=dciangot/lookup --port=80

If everything went well you can then see your pod running after a while:

 kubectl get pods

NAME                      READY     STATUS    RESTARTS   AGE
lookup-pod    1/1       Running   0          1m

Inspect resources

Every k8s resource can be inspected with the command describe. This will provide you with a variety of useful information on the state and cofiguration of the resource. For instance you can get the internal pod network IP of the running container in the pod.

kubectl describe pod lookup-pod | grep IP:
IP:                     172.17.0.3

Delete

Delete the two pods and pay attention to what happens.

kubectl delete pod lookup-pod

Using manifest files

Any resource can also be created from a yaml or json template. In case of a pod you can find the configuration file in templates/pods.yml:

apiVersion: v1
kind: Pod
metadata:
  name: lookup-manifest-pod
spec:
  containers:
  - name: lookup-container 
    image: dciangot/lookup
    ports:
    - containerPort: 80
    resources:
      limits:
        memory: "64Mi"
        cpu: "500m"

As you can see, is also possible to set limits for the resource to be reserved for the pod.

Now the syntax for creating a resource from a manifest file is:

kubectl create -f templates/k8s/pods.yml

And then take a look to the pod details using describe command:

kubectl describe pod lookup-manifest-pod

And finally remove the pod with:

kubectl delete lookup-manifest-pod

Create a deployment from configuration file

You can find the configuration file for a deployment in templates/deployments.yml:

apiVersion: apps/v1beta1
kind: Deployment
metadata:
  name: lookup-deployment
spec:
  replicas: 1
  template:
    metadata:
      labels:
        app: myapp
    spec:
      containers:
      - name: lookup-container-deployment
        image: dciangot/lookup 
        ports:
        - containerPort: 80
        env:
        - name: SIMPLE_SERVICE_VERSION
          value: "1.0"
        resources:
          limits:
            memory: "64Mi"
            cpu: "500m"
      - name: probe-container
        image: dciangot/probe

Here you can see the addition of the replica field where you can specifiy how many replicas of the same file you want. Also the label metadata is important to organize the application management as we will see next on the k8s service section.

kubectl create -f templates/k8s/deployments.yml

Also, let's try to see the environment variable set inside the container with the kubectl executility.

First of all take and note the pod name:

kubectl get pod -l app=myapp

NAME                                    READY   STATUS    RESTARTS   AGE
lookup-deployment-77d5748999-tcbhj   2/2     Running   0          4m11s

Then execute env command inside the container with:

kubectl exec -ti lookup-deployment-77d5748999-tcbhj  env -c lookup-container-deployment | grep SIMPLE_SERVICE_VERSION

We can also get to the endpoint from the probe contianer directly on localhost with:

$ kubectl exec -ti  lookup-deployment-85b549f8b6-4lgls -c probe-container curl localhost
<h3>Hello World!</h3><b>Hostname:</b> lookup-deployment-85b549f8b6-4lgls<br/>

Update a pod

Try to modify the manifest and then run:

kubectl apply -f templates/deployments.yml

Delete a pod inside a deployment

If you try to remove the pod under the deployment control, what happens?

Before continuing: can I connect outside the pod?

Let's finally create another probe container outside the pod:

kubectl run --generator=run-pod/v1  probe-cluster-pod --image=dciangot/probe

Run curl in the container and check that indeed you can access the endpoint of the pods at:

$ kubectl describe pod lookup-deployment-85b549f8b6-4lgls | grep IP:
IP:                 10.244.3.6

$ kubectl exec -ti  probe-cluster-pod  curl 10.244.3.6
<h3>Hello World!</h3><b>Hostname:</b> lookup-deployment-85b549f8b6-4lgls<br/>