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In this lesson, we’ll deep-dive into the Kubernetes Service of type ClusterIP—the default Service type for in-cluster communication. In a typical multi-tier application (front-end, back-end, in-memory cache like Redis, and a database such as MySQL), each component lives in its own set of Pods. Since Pod IPs are ephemeral, you need a stable endpoint for reliable, load-balanced communication between tiers. A ClusterIP Service assigns a virtual IP and DNS name inside the cluster. Pods can address the Service by name (e.g., back-end), and Kubernetes will distribute traffic across the matching Pods.
The image is a diagram of a Kubernetes ClusterIP setup, showing a network of pods organized into front-end, back-end, and Redis layers, each with specific IP addresses.

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Why Use a ClusterIP Service?

BenefitDescription
Stable in-cluster addressPods reference a single virtual IP and DNS name instead of changing Pod IPs
Built-in load balancingDistributes traffic evenly across all healthy Pods
Decoupling componentsFront-end, back-end, cache, and DB tiers communicate via service names

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Defining a ClusterIP Service

Create service-definition.yml to expose your back-end Pods: 
# service-definition.yml
apiVersion: v1
kind: Service
metadata:
  name: back-end
spec:
  selector:
    app: myapp
    tier: back-end
  ports:
    - port: 80        # Service port exposed inside the cluster
      targetPort: 80  # Port on the container
      protocol: TCP
By default, spec.type is ClusterIP. If you omit it, Kubernetes will still create a ClusterIP Service unless you specify another type.

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Key Fields

FieldDescription
metadata.nameName of the Service (DNS name within cluster)
spec.selectorLabels used to identify the target Pods
spec.portsList of ports the Service exposes and routes to

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Pod Definition with Matching Labels

Ensure your Pods carry labels that match the Service’s selector: 
# pod-definition.yml
apiVersion: v1
kind: Pod
metadata:
  name: myapp-backend-pod
  labels:
    app: myapp
    tier: back-end
spec:
  containers:
    - name: nginx-container
      image: nginx
      ports:
        - containerPort: 80

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Deploying and Verifying

Apply the Service and check its status: 
kubectl apply -f service-definition.yml
kubectl get services
Example output: 
NAME       TYPE        CLUSTER-IP     EXTERNAL-IP   PORT(S)   AGE
back-end   ClusterIP   10.96.123.45   <none>        80/TCP    30s
Now, any Pod in the cluster can reach the back-end tier by calling: 
http://back-end:80
Kubernetes will automatically load-balance requests across all matching Pods.
Avoid mapping Service ports directly to host ports in production; use ClusterIP for secure, in-cluster traffic and consider Ingress or LoadBalancer types for external access.

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