Kubernetes Services and Service Discovery

1. Exposing Kubernetes workloads

Kubernetes offers several types of Services to expose workloads:

• ClusterIP (Default): Exposes the Service only within the cluster. It’s not accessible from the outside.

• NodePort: Exposes the Service on each Node’s IP at a static port. It’s accessible from outside the cluster using <NodeIP>:<NodePort>.

• LoadBalancer: Exposes the Service externally using a cloud provider's load balancer. It’s accessible via the load balancer’s external IP.

• ExternalName: Maps the Service to the contents of the externalName field (e.g., a DNS name).

Using NodePort service

Note that the NodePort Service has a lot of downsides:

• you can only have one service per port

• you can only use ports 30000–32767,

• if your Node/VM IP address changes, you need to deal with that.

• Running nginx application using deployment.

• Deploy a simple Nginx application,

• Create POD using deployment.yml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx
spec:
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2
        ports:
        - containerPort: 80

• Apply deployment.yml file,

kubectl apply -f deployment.yml

• Create service for the application using service.yml file,

apiVersion: v1
kind: Service
metadata:
  name: nginx-service
spec:
  type: NodePort
  selector:
    app: nginx
  ports:
    - protocol: TCP
      port: 80
      targetPort: 80
      nodePort: 30001

• Apply services.yml file,

kubectl apply -f service.yml

• Verify service status,

kubectl get svc -o wide

• Expose port to access application on browser,

kubectl port-forward svc/nginx-service 8081:80 --address=0.0.0.0 &

• Now try to access,

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2. Discover Services and Pods within a Kubernetes cluster using DNS

Service Discovery Using DNS

Let’s start by deploying a simple Service,

• Create deployment and service and apply both of them,

# deployment.yml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: hello-world
  namespace: default
spec:
  replicas: 3
  selector:
    matchLabels:
      app: hello-world
  template:
    metadata:
      labels:
        app: hello-world
    spec:
      containers:
      - name: hello-world
        image: hashicorp/http-echo:0.2.3
        args:
        - "-text=Hello, Kubernetes!"
        ports:
        - containerPort: 5678

# service.yml
apiVersion: v1
kind: Service
metadata:
  name: hello-world-service
  namespace: default
spec:
  selector:
    app: hello-world
  ports:
    - protocol: TCP
      port: 80
      targetPort: 5678

• Apply both files,

kubectl apply -f deployment.yml
kubectl apply -f service.yml

• Test deployment and service at once,

kubectl get all

• Discover the Service Using DNS,

• You can resolve the Service DNS name from within any Pod in the same namespace. Deploy a test Pod,

kubectl run -it --rm --image=busybox dns-test -- /bin/sh

• Within this Pod, you can use the nslookup or dig command to resolve the Service name,

nslookup hello-world-service.default.svc.cluster.local

Pod Discovery Using DNS

• Step 1 : Enable Headless Service

To discover individual Pods, you should create a Headless Service by setting clusterIP: None,

apiVersion: v1
kind: Service
metadata:
  name: hello-world-headless
  namespace: default
spec:
  clusterIP: None
  selector:
    app: hello-world
  ports:
    - protocol: TCP
      port: 80
      targetPort: 5678

• Apply file,

kubectl apply -f dns-headless.yml

• Step 2 : Discover Pods Using DNS

Test DNS Resolution Inside a Pod,

Use a test Pod to check DNS resolution. Let’s use a busybox Pod for this,

kubectl run -it --rm --image=busybox dns-test -- /bin/sh

Inside the shell of the dns-test Pod, use the nslookup command to try to resolve the Headless Service and Pods,

nslookup hello-world-headless.default.svc.cluster.local

This will return a list of IP addresses, each corresponding to a Pod running under the hello-world deployment.

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Thank you for reading. I hope you were able to understand and learn something new from my blog.

Happy Learning!

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