Headless Service

Headless Services are a special type of Kubernetes Service. They do not provide a cluster IP address or load balancing through a Service IP address. Instead, when a client uses the DNS name of a headless Service for query, Kubernetes directly returns the individual IP addresses of the pods selected by the Service. This design allows clients to connect directly to the pods themselves. Headless Services are ideal for certain applications such as database clusters and cache clusters.

Headless Services are typically used in:

StatefulSets: In StatefulSets such as MySQL, Kafka, and Elasticsearch, each pod has unique responsibilities and does not require load balancing. A headless Service gives each pod its own access entry.
Applications that need custom load balancing: Some applications need to manage their own load balancing policies rather than relying on Kubernetes' default load balancing policy.

Creating a Headless Service

Use kubectl to access the cluster. For details, see Accessing a Cluster Using kubectl.

Create a StatefulSet and an associated headless Service.

vi headless.yaml

File content:

apiVersion: v1
kind: Service       # Set the resource object type to Service.
metadata:
  name: nginx-headless
  labels:
    app: nginx
spec:
  ports:
    - name: web       # Name of the port for communications between pods
      port: 80        # Port number for communications between pods
  selector:
    app: nginx        # Select the pod labeled with app:nginx.
  clusterIP: None     # Set this parameter to None, indicating that a headless Service will be created.
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: web
spec:
  serviceName: nginx-headless   # Name of the headless Service associated with the StatefulSet
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:alpine
        ports:
        - containerPort: 80
          name: web
      imagePullSecrets:
        - name: default-secret

Create the StatefulSet and headless Service.
```
kubectl create -f headless.yaml
```

Check whether the pods have been created.

kubectl get pods -l app=nginx -o wide

Information similar to the following is displayed:

NAME     READY   STATUS    RESTARTS   AGE     IP          NODE            NOMINATED NODE   READINESS GATES
web-0    1/1     Running   0          41s     10.0.0.22   192.168.0.194   <none>           <none>
web-1    1/1     Running   0          38s     10.0.0.23   192.168.0.194   <none>           <none>
web-2    1/1     Running   0          37s     10.0.0.39   192.168.0.21    <none>           <none>

Check whether the headless Service has been created.

kubectl get svc nginx-headless

Information similar to the following is displayed:

NAME             TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)   AGE
nginx-headless   ClusterIP   None         <none>        80/TCP    2m20s

Create a temporary pod to obtain DNS.

kubectl run -it --rm --image=busybox:1.28 test-pod -- sh

The command for obtaining DNS is as follows:

nslookup nginx-headless

Information similar to the following is displayed:

Server:    10.247.3.10
Address 1: 10.247.3.10 coredns.kube-system.svc.cluster.local

Name:      nginx-headless
Address 1: 10.0.0.22 web-0.nginx-headless.default.svc.cluster.local
Address 2: 10.0.0.23 web-1.nginx-headless.default.svc.cluster.local
Address 3: 10.0.0.39 web-2.nginx-headless.default.svc.cluster.local

This indicates that each StatefulSet pod has an independent DNS record.