What is Kubernetes Headless Service?

Question

Accepted Answer

**Headless Service** — direct Pod DNS without load balancing.

```yaml
apiVersion: v1
kind: Service
metadata:
  name: postgres-headless
spec:
  clusterIP: None  # Headless!
  selector:
    app: postgres
  ports:
  - port: 5432
    targetPort: 5432
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: postgres
spec:
  serviceName: postgres-headless
  replicas: 3
  selector:
    matchLabels:
      app: postgres
  template:
    metadata:
      labels:
        app: postgres
    spec:
      containers:
      - name: postgres
        image: postgres:15
        ports:
        - containerPort: 5432
```

```bash
# DNS resolution
# Headless: postgres-0.postgres-headless.ns.svc.cluster.local
#          postgres-1.postgres-headless.ns.svc.cluster.local
#          postgres-2.postgres-headless.ns.svc.cluster.local

# Regular ClusterIP: postgres.ns.svc.cluster.local (round-robin)

# Test DNS
kubectl exec -it my-pod -- nslookup postgres-headless
kubectl exec -it my-pod -- nslookup postgres-0.postgres-headless

# Check endpoints
kubectl get endpoints postgres-headless
kubectl describe endpoints postgres-headless

# Application connection string
postgresql://postgres-0.postgres-headless:5432/mydb
postgresql://postgres-1.postgres-headless:5432/mydb
postgresql://postgres-2.postgres-headless:5432/mydb
```

**Use cases:**
- StatefulSets (databases, queues).
- Direct Pod addressing.
- Client-side load balancing.
- Stateful applications.

DevOps: What is Kubernetes Headless Service?