Kubernetes Session Two

Recap of Kubernetes Resource Object I

• What is a Pod?
  • Minimal unit for Kubermetes
• How to create a Pod?
  • kubectl run db --image mongo
  • Declarative way to create Pod
    • cat pod/db.yml
• Run multiple containers in a Pod
  • cat pod/go-demo-2.yml
• Monitor Pod health
  • cat pod/go-demo-2-health.yml
• Use ReplicaSet to scale Pod
  • cat rs/go-demo-2.yml

Kubernetes Resource Object II

Service

Service is an abstraction that defines a set of Pods and a policy to access them.

Service Types

• Cluster IP
  • Expose the service on a cluster-internal IP
• NodePort
  • Expose the service on each Node’s IP at a static port
  • <NodeIP>: <NodePort>
• LoadBalancer
  • Expose the service externally using a cloud provider’s load balancer
• ExternalName
  • Map the service to an external address(e.g. kubernetes.io)

Expose the service

Use kubectl expose to expose a resource as a new Kubernetes Service. That resource can be a Deployment, another Service, a ReplicaSet, a ReplicationController, or a Pod.

Define a new ReplicaSet:

cat svc/go-demo-2-rs.yml

Different with rs/go-demo-2.yml:

- name: db
  image: mongo:3.3
  command: ["mongod"]
  args: ["--rest", "--httpinterface"]
  ports:
	- containerPort: 28017
  protocol: TCP

Create ReplicaSet object and get state from Kubernetes:

kubectl create -f svc/go-demo-2-rs.yml
kubectl get -f svc/go-demo-2-rs.yml

Expose the ReplicaSet as a new Service:

kubectl expose rs go-demo-2 --name=go-demo-2-svc  --target-port=28017 --type=NodePort

Describe the service:

kubectl describe svc go-demo-2-svc

Service labels are inherited from Pods and selector matches the one from the RepicaSet.

PORT=$(kubectl get svc go-demo-2-svc -o jsonpath="{.spec.ports[0].nodePort}")
IP=$(minikube ip)
open "http://$IP:$PORT"
open "http://localhost:$PORT"

Process involved of above command:

Note: Kube-proxy added iptables rules which capture traffic to the Service port and redirect it to endpoints. For each endpoint object, it adds iptables rule which selects a Pod.

Delete the service:

kubectl delete svc go-demo-2-svc

Declarative way to create service

cat svc/go-demo-2-svc.yml

Create and get status of the service:

kubectl create -f svc/go-demo-2-svc.yml
kubectl get -f svc/go-demo-2-svc.yml

Access the service:

open "http://$IP:30012"
open "http://localhost:30012"

Get endpoint status and list the pods that should receive the requests:

kubectl get ep go-demo-2 -o yaml

Destroy the ReplicaSet and Service:

kubectl delete -f svc/go-demo-2-svc.yml
kubectl delete -f svc/go-demo-2-rs.yml

####Services can also abstract other kinds of backends

• You want to have an external database cluster in production, but in test you use your own databases.
• You want to point your service to a service in another Namespace or on another cluster.
• You are migrating your workload to Kubernetes and some of your backends run outside of Kubernetes.

Define a Service without a selector:

kind: Service
apiVersion: v1
metadata:
  name: my-service
spec:
  ports:
  - protocol: TCP
    port: 80
    targetPort: 9376

Because this service has no selector, the corresponding Endpoints object will not be created. You can manually map the service to your own specific endpoints:

kind: Endpoints
apiVersion: v1
metadata:
  name: my-service
subsets:
  - addresses:
      - ip: 1.2.3.4
    ports:
      - port: 9376

Accessing a Service without a selector works the same as if it had a selector. The traffic will be routed to endpoints defined by the user (1.2.3.4:9376 in this example).

###Split pods and establish communication via services

Best Practice: A Pod contains only one container.

Create db ReplicaSet:

cat svc/go-demo-2-db-rs.yml
kubectl create -f svc/go-demo-2-db-rs.yml

Create db Service:

cat svc/go-demo-2-db-svc.yml
kubectl create -f svc/go-demo-2-db-svc.yml

Create api ReplicaSet:

cat svc/go-demo-2-api-rs.yml
kubectl create -f svc/go-demo-2-api-rs.yml

Create api Service:

cat svc/go-demo-2-api-svc.yml
kubectl create -f svc/go-demo-2-api-svc.yml

List all the objects created:

kubectl get all

Visit the service:

PORT=$(kubectl get svc go-demo-2-api -o jsonpath="{.spec.ports[0].nodePort}")

curl -i "http://$IP:$PORT/demo/hello"
curl -i "http://localhost:$PORT/demo/hello"

Delete all the objects:

kubectl delete -f svc/go-demo-2-db-rs.yml

kubectl delete -f svc/go-demo-2-db-svc.yml

kubectl delete -f svc/go-demo-2-api-rs.yml

kubectl delete -f svc/go-demo-2-api-svc.yml

Define mutiple objects in one YAML file

cat svc/go-demo-2.yml

Create objects:

kubectl create -f svc/go-demo-2.yml

kubectl get -f svc/go-demo-2.yml

Visit the service:

PORT=$(kubectl get svc go-demo-2-api \
    -o jsonpath="{.spec.ports[0].nodePort}")

curl -i "http://$IP:$PORT/demo/hello"

Discover Services

Services can be dicovered in two modes:

• Environemtn variables
• DNS

A look at environment variables in one of the Pods:

POD_NAME=$(kubectl get pod \
    --no-headers \
    -o=custom-columns=NAME:.metadata.name \
    -l type=api,service=go-demo-2 \
    | tail -1)

kubectl exec $POD_NAME env

Describe the service:

kubectl describe svc go-demo-2-db

Process involved:

Deployment

ReplicaSet is used to scale Pod. But ReplicaSet are rarely used independently. Instead use Deployment to create ReplicaSet.

A Deployment controller provides declarative updates for Pods and ReplicaSets.

cat nginx-deployment.yaml

Create a deployment:

kubectl create -f nginx-deployment.yaml --record

Scale the deployment:

kubectl scale deployment nginx-deployment --replicas 10

Update image:

kubectl set image deployment/nginx-deployment nginx=nginx:1.9.1

Check update history:

kubectl rollout history deployment/nginx-deployment

Roll back:

kubectl rollout undo deployment/nginx-deployment

Summary

Kubernetes Cluster Overview

Involved Kubernetes Objects

Category	Object Name
Resource & Configuration	Pod, ReplicaSet, ReplicationController, Deployment, StatefulSet, DaemonSet, Job, CronJob
Storage	Secret, ConfigMap, LocalVolume, PersistentVolume
Network	Namespace, Service, Ingress

• Pod
• ReplicaSet(ReplicationController)
• Service
• Deployment

Application Type	Resource Object
Long-running	Deployment
Batch	Job/CronJob
Node-daemon	DaemonSet
Stateful Application	StatefulSet

References

• https://kubernetes.feisky.xyz/he-xin-yuan-li/index-2
• https://kubernetes.io/docs/concepts/services-networking/service/
• https://github.com/vfarcic/k8s-specs