Services Basics

This section covers the everyday Service patterns you will use to give ephemeral pods a stable virtual IP and DNS name.

Each example is a small, complete manifest or command you can adapt directly.

Prerequisites

• A running cluster on Kubernetes 1.36.2 with kube-proxy or a CNI dataplane and CoreDNS installed.
• kubectl matching your control plane minor version.
• A Deployment whose pods carry a label you can select on, for example app: web.

Quick check that the pieces exist:

kubectl get pods -n kube-system -l k8s-app=kube-dns
kubectl get deploy -A | grep -i coredns || true

Basic Examples

1. A ClusterIP Service in front of a Deployment

The default Service type, reachable only inside the cluster.

apiVersion: v1
kind: Service
metadata:
  name: web
spec:
  selector:
    app: web
  ports:
    - name: http
      port: 80
      targetPort: 8080

• selector matches pods labeled app: web; those become the backends.
• port is the port the Service listens on; targetPort is the container port.
• No type means ClusterIP, so this VIP is cluster-internal only.
• Always name ports (name: http) so probes and other objects can reference them.

2. Resolve a Service by DNS

Every Service gets a DNS record from CoreDNS.

kubectl run tmp --rm -it --image=busybox:1.36 --restart=Never -- \
  nslookup web.default.svc.cluster.local

• The full name is <service>.<namespace>.svc.cluster.local.
• From the same namespace, web alone resolves thanks to the search domain.
• The record returns the ClusterIP, not pod IPs (for a normal Service).
• If resolution fails, check CoreDNS and the pod's /etc/resolv.conf.

3. Target a named port instead of a number

Referencing the port by name decouples the Service from container port numbers.

spec:
  selector:
    app: web
  ports:
    - name: http
      port: 80
      targetPort: http

• targetPort: http points at the container port named http.
• The container spec must declare ports: [{name: http, containerPort: 8080}].
• Renumbering the container port no longer requires editing the Service.
• This is a best practice for long-lived Services.

4. Expose more than one port

A Service can map several ports at once.

spec:
  selector:
    app: web
  ports:
    - name: http
      port: 80
      targetPort: 8080
    - name: metrics
      port: 9090
      targetPort: 9090

• Each entry needs a unique name when there is more than one port.
• Common split: http for traffic, metrics for Prometheus scraping.
• The names show up in EndpointSlices, aiding debugging.
• Keep protocol defaults (TCP) unless you truly need UDP or SCTP.

5. Inspect the endpoints behind a Service

Confirm which pods a Service is actually routing to.

kubectl get endpointslices -l kubernetes.io/service-name=web
kubectl describe service web

• EndpointSlices list the current Ready pod IPs and ports.
• An empty endpoint set almost always means the selector matches nothing or pods are not Ready.
• kubectl describe service summarizes the selector, ports, and endpoints.
• This is the first command to run when traffic disappears.

6. Quickly reach a Service with port-forward

Test a Service from your laptop without exposing it publicly.

kubectl port-forward service/web 8080:80

• Local port 8080 now tunnels to the Service port 80.
• Useful for smoke tests before wiring up Ingress or a LoadBalancer.
• The tunnel routes through the API server, so it is for debugging, not production traffic.
• Ctrl-C closes the tunnel.

7. Set readiness so traffic only hits healthy pods

Endpoints follow pod readiness, so define a probe.

readinessProbe:
  httpGet:
    path: /healthz
    port: http
  periodSeconds: 5

• Unready pods are pulled from EndpointSlices and stop receiving traffic.
• The pod is not killed - it just drains until it is Ready again.
• This is what makes rolling updates seamless for clients.
• Point the probe at a cheap, dependency-free endpoint.

Intermediate Examples

8. A headless Service for direct pod DNS

Skip the VIP and return pod IPs directly, which StatefulSets need.

apiVersion: v1
kind: Service
metadata:
  name: db
spec:
  clusterIP: None
  selector:
    app: db
  ports:
    - name: pg
      port: 5432

• clusterIP: None makes the Service headless.
• DNS returns one A record per Ready pod instead of a single VIP.
• Clients or the app do their own load balancing across those IPs.
• StatefulSet pods also get stable per-pod DNS like db-0.db.

9. Preserve client source IP on external traffic

Keep the real caller IP for a LoadBalancer Service.

spec:
  type: LoadBalancer
  externalTrafficPolicy: Local
  selector:
    app: web
  ports:
    - name: http
      port: 80
      targetPort: 8080

• Local routes only to pods on the receiving node, avoiding a second hop.
• The client source IP is preserved, which logging and rate limiting need.
• Nodes with no local pod fail the cloud LB health check and are skipped.
• The trade-off is less even spreading than the default Cluster policy.

10. Point a Service at an external address

Alias an out-of-cluster host behind a stable in-cluster name.

apiVersion: v1
kind: Service
metadata:
  name: payments-api
spec:
  type: ExternalName
  externalName: api.payments.example.com

• ExternalName returns a CNAME to the external host - no proxying or VIP.
• Handy for migrating a dependency into the cluster without changing callers.
• It resolves at DNS time only, so it does not do health checking or TLS.
• Swap the type later once the backend moves in-cluster.

11. Pin a client to one pod with session affinity

Keep a caller on the same backend for a window when you need stickiness.

spec:
  selector:
    app: web
  sessionAffinity: ClientIP
  sessionAffinityConfig:
    clientIP:
      timeoutSeconds: 10800
  ports:
    - name: http
      port: 80
      targetPort: 8080

• sessionAffinity: ClientIP keys routing on the source IP.
• The timeout controls how long the affinity lasts before rebalancing.
• Use it sparingly, since it works against even load spreading.
• It is L4 affinity only, not cookie-based stickiness.

Stack versions: This page was written for Kubernetes 1.36.2, Docker Engine 29.6.1 (BuildKit default), containerd (CRI runtime on nodes), Helm 3, Compose v2, Argo CD (latest - verify at build), and Gateway API (GA - verify controller support at build).

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