Services Overview

Services — Networking in Kubernetes

A Service is an abstraction that defines a logical set of pods and a policy for accessing them. Since pods are ephemeral and their IPs change constantly, Services provide a stable endpoint.

Without Services, you would have no reliable way to connect to your pods.

Why Services Exist

  Problem: Pod IPs are temporary
  ┌─────────────────────────────────────┐
  │  Pod A: 10.1.1.5 (running)         │
  │  Pod A: 10.1.2.8 (restarted!)      │
  │  Pod A: 10.1.3.2 (rescheduled!)    │
  │                                     │
  │  How do other apps find Pod A?      │
  └─────────────────────────────────────┘

  Solution: Services give a stable address
  ┌─────────────────────────────────────┐
  │  Service: my-app (10.0.0.100)      │
  │       │                             │
  │       ├──▶ Pod A (10.1.1.5)        │
  │       ├──▶ Pod A (10.1.2.8)        │
  │       └──▶ Pod A (10.1.3.2)        │
  │                                     │
  │  Traffic always goes to 10.0.0.100  │
  │  Service load-balances across pods  │
  └─────────────────────────────────────┘

Service Types

ClusterIP (default) — Exposes the Service only within the cluster. Internal traffic only.

NodePort — Exposes the Service on each node's IP at a static port. Accessible from outside the cluster.

LoadBalancer — Provisions an external load balancer (cloud providers). Exposes the Service externally.

ExternalName — Maps a Service to an external DNS name. No proxying involved.

Creating a Service

# Quick way: expose a deployment
kubectl expose deployment nginx --port=80 --type=ClusterIP

# Full YAML definition
apiVersion: v1
kind: Service
metadata:
  name: nginx-service
spec:
  type: ClusterIP
  selector:
    app: nginx
  ports:
    - protocol: TCP
      port: 80
      targetPort: 80

selector — Which pods to route traffic to. Must match pod labels.

port — The port the Service listens on.

targetPort — The port on the pod that receives traffic.

Service Discovery

# DNS-based discovery (automatic)
# Pods can reach a Service at:
#   ..svc.cluster.local

# Example: access "nginx-service" in "default" namespace
curl http://nginx-service.default.svc.cluster.local

# Short form (same namespace only)
curl http://nginx-service

# Environment variables (legacy approach)
# K8s injects env vars for each Service into new pods