Concept

![[images/截屏2026-03-24 14.50.56.png]]

A Service provides a single point of entry for accessing one or more Pods.

Can’t rely on Pod IP

Pods are motral and may only live a short time. (ephemeral)
Pod IP address may not stay the same.
Pods can be horizontally scaled so each Pod gets its own IP address.
A Pod gets an IP address after it has been scheduled (no way for clients to know IP ahead of time).

Service Do

Services abstract Pod IP addresses from consumers.
Load balances between Pods.
Relies on labels to associate a Service with a Pod.
Node’s kube-proxy creates a virtual IP for Services
Layer 4 (TCP/UDP over IP)
Services are not ephemeral.
Creates endpoints which sit betwwen a Service and Pod.

Calling Service

![[images/截屏2026-03-24 15.06.21.png]]

Service uses label to associate with the Pods.

Service Types

Services can be defined in different ways:

ClusterIP: Expose the service on a cluster-internal IP (default) （微服务之间调用）
NodePort: Expose the service on each Node’s IP at a static port. （本地学习、测试）
LoadBalancer: Provision an external IP to act as a load balancer for the service. （云环境，基本由云服务厂商提供）
ExternalName: Maps a service to a DNS name. （几乎不用）

ClusterIP Service

Serivce IP is exposed internally within the cluster.
Only Pods within the cluster can talk to the Service.
Allows Pods to talk to other Pods.

NodePort Service

Exposes the Service on each Node’s IP at a static port.
Allocates a port from a range (default is 30000-32767).
Each Node proxies the allocated port.

LoadBalancer Service

Exposes a Service externally.
Useful when combined with a cloud provider’s load balancer.
NodePort and ClusterIP Services are created.
Each Node proxies the allocated port.

![[images/截屏2026-03-24 15.16.27.png]]

ExternalName Service

Service that acts as an alias for an external service.
Allows a Service to act as the proxy for an external service.
External service details are hidden from cluster (easier to change).
- Only to change the ExternalName Service but not need to change the Pods in cluster.

![[images/截屏2026-03-24 15.19.11.png]]

Create Service

Port Forward

Use kubectl port-forward to forward a local port to a Pod port.

# Listen on port 8080 locally and forward to port 80 in Pod
kubectl port-forward pod/[pod_name] 8080:80
# Listen on port 8080 locally and forward to Deployment's Pod
kubectl port-forward deployment/[deployment_name] 8080
# Listen on port 8080 locally and forward to Service's Pod
kubectl port-forward service/[service_name] 8080

kubectl port-forward is used on local test or debugging. So Service is needed to forward external requests.

YAML

![[images/截屏2026-03-24 16.03.03.png]]

Connect

metadata:
	name: frontend		# name of Service. Each Service gets a DNS entry

...
metadata:
	name: backend

A frontend Pod can access a backend Pod using backend:port

NodePort

apiVersion: v1
kind: Service
metadata:
 name: nginx-nodeport
spec:
 type: NodePort
 selector:
    app: my-nginx
 ports:
 - port: 80
   targetPort: 80
   nodePort: 31000	# optionally set NodePort value (defaults between 30000-32767)

ClusterIP

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

LoadBalancer

apiVersion: v1
kind: Service
metadata:
 name: nginx-loadbalancer
spec:
 type: LoadBalancer
 selector:
    app: my-nginx
 ports:
  - name: "80"
    port: 80
    targetPort: 80

  - name: "443"
    port: 443
    targetPort: 443

ExternalName

apiVersion: v1
kind: Service
metadata:
 name: external-service		# Other Pods can use this FQDN to access the external service
spec:
 type: ExternalName			# Set type to ExternalName
 externalName: api.musuyin.com	# Service will proxy to FQDN
 ports:
 - port: 9000		# can be called with external-service:9000

kubectl

kubectl create -f [yaml_name.yaml]
kubectl apply -f [yaml_name.yaml]
kubectl delete -f [yaml_name.yaml]

# Shell into a Pod and test a URL. Add -c [containerID] in cases where multiple containers are running in the Pod
kubectl exec [pod_name] -- curl -s http://podIP:port