🔗 Kubernetes Services

A Service in Kubernetes is an abstraction that defines a logical set of Pods and a policy by which to access them. It provides a stable virtual IP (ClusterIP) even if the actual Pod IPs change.

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✅ Why Use Services?

• Expose applications to other pods or the outside world
• Load balance traffic across Pod replicas
• Decouple consumers from changing Pod IPs
• Enable stable DNS-based communication between workloads

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🧩 Service Types

1. ClusterIP (default)

• Internal access only
• Exposes service on a cluster-internal IP

2. NodePort

• Exposes service on each Node's IP at a static port
• Accessible outside the cluster via <NodeIP>:<NodePort>

3. LoadBalancer

• Cloud provider provisioned external IP
• Built on top of NodePort + external load balancer

4. ExternalName

• Maps service to an external DNS name (e.g., mydb.example.com)

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🛠️ Create a ClusterIP Service

From Pod (imperative)

kubectl expose pod redis --port=6379 --name=redis-service --type=ClusterIP

From YAML

apiVersion: v1
kind: Service
metadata:
  name: my-service
spec:
  selector:
    app: myapp
  ports:
    - port: 80
      targetPort: 8080
  type: ClusterIP

kubectl apply -f service-definition.yaml

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🌐 NodePort Service Example

apiVersion: v1
kind: Service
metadata:
  name: webapp-service
  namespace: default
spec:
  type: NodePort
  selector:
    name: simple-webapp
  ports:
    - port: 8080
      targetPort: 8080
      nodePort: 30080

Flow:

User -> NodeIP:30080 -> Service:8080 -> Pod:8080

Access Example:

curl http://<NodeIP>:30080

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🧪 Inspecting Services

kubectl get services
kubectl get svc <name> -o yaml
kubectl describe svc <name>
kubectl get endpoints <svc-name>

Sample output:

NAME         ENDPOINTS                        AGE
kubernetes   172.17.0.2:6443,172.17.0.3:6443   25m

Each IP:PORT = 1 Pod endpoint.

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🧠 Tips & Gotchas

• Endpoints must match Pods with the selector
• To specify a custom nodePort, use YAML, not kubectl expose
• A Service with no matching Pods still exists but has no endpoints

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🌐 Kubernetes DNS Access

If your Pod is in the marketing namespace and wants to reach a db-service in the same namespace:

Use:

• Short DNS: db-service
• Full DNS: db-service.marketing.svc.cluster.local

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✅ Summary

Service Type	Access Scope	Notes
ClusterIP	Internal only	Default
NodePort	External via NodeIP	Static port on every node
LoadBalancer	External LB	Cloud-provider only
ExternalName	External DNS	For services outside the cluster

• Use kubectl expose or YAML to define Services
• Check endpoints to verify Service → Pod linkage
• Pair Services with labels and selectors for clean communication