Kubernetes API Fundamentals

Step 1 of 5

Kubernetes Manifests

Let's begin my creating a new project called myproject:

oc new-project myproject

Create a new pod manifest that specifies two containers:

cat > pod-multi-container.yaml <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: my-two-container-pod
  namespace: myproject
  labels:
    environment: dev
spec:
  containers:
    - name: server
      image: nginx:1.13-alpine
      ports:
        - containerPort: 80
          protocol: TCP
    - name: side-car
      image: alpine:latest
      command: ["/usr/bin/tail", "-f", "/dev/null"]
  restartPolicy: Never
EOF

Create the pod by specifying the manifest:

oc create -f pod-multi-container.yaml

View the detail for the pod and look at the events:

oc describe pod my-two-container-pod

Let's first execute a shell session inside the server container by using the -c flag:

oc exec -it my-two-container-pod -c server -- /bin/sh

Run some commands inside the server container:

ip address
netstat -ntlp
hostname
ps
exit

Let's now execute a shell session inside the side-car container:

oc exec -it my-two-container-pod -c side-car -- /bin/sh

Run the same commands in side-car container. Each container within a pod runs it's own cgroup, but shares IPC, Network, and UTC (hostname) namespaces:

ip address
netstat -ntlp
hostname
exit

Basic Operations with the Kubernetes API

Verify the currently available Kubernetes API versions:

oc api-versions

Use the --v flag to set a verbosity level. This will allow you to see the request/responses against the Kubernetes API:

oc get pods --v=8

Use the oc proxy command to proxy local requests on port 8001 to the Kubernetes API:

oc proxy --port=8001

Open up another terminal by clicking the + button and select Open New Terminal.

Send a GET request to the Kubernetes API using curl:

curl -X GET http://localhost:8001

We can explore the OpenAPI definition file to see complete API details.

curl localhost:8001/openapi/v2

Send a GET request to list all pods in the environment:

curl -X GET http://localhost:8001/api/v1/pods

Use jq to parse the json response:

curl -X GET http://localhost:8001/api/v1/pods | jq .items[].metadata.name

We can scope the response by only viewing all pods in a particular namespace:

curl -X GET http://localhost:8001/api/v1/namespaces/myproject/pods

Get more details on a particular pod within the myproject namespace:

curl -X GET http://localhost:8001/api/v1/namespaces/myproject/pods/my-two-container-pod

Export the manifest associated with my-two-container-pod in json format:

oc get pods my-two-container-pod --export -o json > podmanifest.json

Within the manifest, replace the 1.13 version of alpine with 1.14:

sed -i 's|nginx:1.13-alpine|nginx:1.14-alpine|g' podmanifest.json

Update/Replace the current pod manifest with the newly updated manifest:

curl -X PUT http://localhost:8001/api/v1/namespaces/myproject/pods/my-two-container-pod -H "Content-type: application/json" -d @podmanifest.json

Patch the current pod with a newer container image (1.15):

curl -X PATCH http://localhost:8001/api/v1/namespaces/myproject/pods/my-two-container-pod -H "Content-type: application/strategic-merge-patch+json" -d '{"spec":{"containers":[{"name": "server","image":"nginx:1.15-alpine"}]}}'

Delete the current pod by sending the DELETE request method:

curl -X DELETE http://localhost:8001/api/v1/namespaces/myproject/pods/my-two-container-pod

Verify the pod is in Terminating status:

oc get pods

Verify the pod no longer exists:

curl -X GET http://localhost:8001/api/v1/namespaces/myproject/pods/my-two-container-pod

Replica Sets

Get a list of all pods in the myproject Namespace:

oc get pods -n myproject

Create a ReplicaSet object manifest file:

cat > replica-set.yaml <<EOF
apiVersion: extensions/v1beta1
kind: ReplicaSet
metadata:
  name: myfirstreplicaset
  namespace: myproject
spec:
  selector:
    matchLabels:
     app: myfirstapp
  replicas: 3
  template:
    metadata:
      labels:
        app: myfirstapp
    spec:
      containers:
        - name: nodejs
          image: openshiftkatacoda/blog-django-py
EOF

Create the ReplicaSet:

oc apply -f replica-set.yaml

In a new terminal window, select all pods that match app=myfirstapp:

oc get pods -l app=myfirstapp --show-labels -w

Delete the pods and watch new ones spawn:

oc delete pod -l app=myfirstapp

Imperatively scale the ReplicaSet to 6 replicas:

oc scale replicaset myfirstreplicaset --replicas=6

Imperatively scale down the ReplicaSet to 3 replicas:

oc scale replicaset myfirstreplicaset --replicas=3

The oc scale command interacts with the /scale endpoint:

curl -X GET http://localhost:8001/apis/extensions/v1beta1/namespaces/myproject/replicasets/myfirstreplicaset/scale

Use the PUT method against the /scale endpoint to change the number of replicas to 5:

curl  -X PUT localhost:8001/apis/extensions/v1beta1/namespaces/myproject/replicasets/myfirstreplicaset/scale -H "Content-type: application/json" -d '{"kind":"Scale","apiVersion":"extensions/v1beta1","metadata":{"name":"myfirstreplicaset","namespace":"myproject"},"spec":{"replicas":5}}'

You can also get information regarding the pod by using the GET method against the /status endpoint

curl -X GET http://localhost:8001/apis/extensions/v1beta1/namespaces/myproject/replicasets/myfirstreplicaset/status

The status endpoint's primary purpose is to allow a controller (with proper RBAC permissions) to send a PUT method along with the desired status.

Deployments/Finalizers

Create a manifest for a Deployment with a Finalizer:

cat > finalizer-test.yaml<<EOF
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: finalizer-test
  namespace: myproject
  labels:
    app: finalizer-test
  finalizers:
    - finalizer.extensions/v1beta1  
spec:
  replicas: 3
  template:
    metadata:
      labels:
        app: finalizer-test
    spec:
      containers:
        - name: hieveryone
          image: openshiftkatacoda/blog-django-py
          imagePullPolicy: Always
          ports:
            - name: helloworldport
              containerPort: 8080
EOF

Create the Deployment.

oc create -f finalizer-test.yaml

Verify the Deployment has been created.

oc get deploy

Verify the ReplicaSet has been created:

oc get replicaset

Verify the pods are running:

oc get pods

Attempt to delete the Deployment.

oc delete deployment finalizer-test

Open up another terminal by clicking the + button and select Open New Terminal.

Observe the Deployment still exits and has been updated with the deletionGracePeriodSeconds and deletionTimestamp fields.

oc get deployment finalizer-test -o yaml | grep 'deletionGracePeriodSeconds\|deletionTimestamp'

Attempt to scale the Deployment up and down. Although status is updated, pods will not be created/deleted:

oc scale deploy finalizer-test --replicas=5
oc scale deploy finalizer-test --replicas=1

oc get deploy
oc get pods

Update the Deployment with the Finalizer value unset.

cat > finalizer-test-remove.yaml<<EOF
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: finalizer-test
  namespace: myproject
  labels:
    app: finalizer-test
  finalizers:
spec:
  replicas: 3
  template:
    metadata:
      labels:
        app: finalizer-test
    spec:
      containers:
        - name: hieveryone
          image: openshiftkatacoda/blog-django-py
          imagePullPolicy: Always
          ports:
            - name: helloworldport
              containerPort: 8080
EOF

Replace the Deployment.

oc replace -f finalizer-test-remove.yaml

The Deployment will now be deleted.

oc get deploy
oc get pods

See the following:

Deployment Controller (DeletionTimestamp != nil)

SyncStatusOnly Method

Custom Resource Definitions

Begin by running a proxy to the Kubernetes API server:

oc proxy --port=8001

Open up another terminal by clicking the + button and select Open New Terminal.

Let's create a new Custom Resource Definition (CRD) object manifest for Postgres:

cat >> postgres-crd.yaml <<EOF
apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: postgreses.rd.example.com
spec:
  group: rd.example.com
  names:
    kind: Postgres
    listKind: PostgresList
    plural: postgreses
    singular: postgres
    shortNames:
    - pg
  scope: Namespaced
  version: v1alpha1
EOF

Create the CRD resource object:

oc create -f postgres-crd.yaml

You should now see the Kubernetes API reflect a brand new api group called rd.example.com:

curl http://localhost:8001/apis | jq .groups[].name

This will also be reflected in the oc api-versions command:

oc api-versions

Within the rd.example.com group there will be an api version v1alpha1 (per our CRD resource object). The database resource resides here.

curl http://localhost:8001/apis/rd.example.com/v1alpha1 | jq

Notice how oc now recognize postgres as a present resource (although there will be no current resource objects at this time).

oc get postgres

Let's create a new Custom Resource (CR) object manifest for the database:

cat >> wordpress-database.yaml <<EOF
apiVersion: "rd.example.com/v1alpha1"
kind: Postgres
metadata:
  name: wordpressdb
spec:
  user: postgres
  password: postgres
  database: primarydb
  nodes: 3
EOF

Create the new object:

oc create -f wordpress-database.yaml

Verify the resource was created:

oc get postgres

View the details about the wordpressdb object:

oc get postgres wordpressdb -o yaml

cd <directory>	Change directory
ls	List directory
echo 'contents' > <file>	Write contents to a file
cat <file>	Output contents of file

i	In Command Mode	Change into Insert Mode, you can now insert and edit text in the file
esc	In Insert Mode	Change into Command Mode, you can now execute commands
:wq	In Command Mode	Save and Exit

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Kubernetes API Fundamentals

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