Kubernetes - Headless Service 🧟

Description 👀

A headless service is a service that does not have a ClusterIP. This means that the service will not have a virtual IP address. Instead, the service will return the Pod IP addresses. This is useful for stateful applications that require a stable IP address for each Pod.

Examples 🧩

• headless service definition

    apiVersion: v1
    kind: Service
    metadata:
        name: mysql-h
    spec:
        ports:
        - port: 3306
        selector:
            app: mysql
        clusterIP: None
  

  • headless service does not have a ClusterIP, must set clusterIP to None

• pod using headless service

    apiVersion: v1
    kind: Pod
    metadata:
        name: myapp-pod
        labels:
            app: mysql
    spec:
        containers:
          - name: mysql
            image: mysql
        # pod uses headless service
        subdomain: mysql-h
        hostname: mysql-pod
  

• stateful set using headless service

    apiVersion: apps/v1
    kind: StatefulSet
    metadata:
        name: mysql-deployment
        labels:
            app: mysql
    spec:
        serviceName: mysql-h
        replicas: 3
        selector:
            matchLabels:
                app: mysql
        template:
            metadata:
                name: myapp-pod
                labels:
                    app: mysql
            spec:
                containers:
                  - name: mysql
                    image: mysql
  

  • serviceName is the name of the headless service that the stateful set will use
  • the stateful set does not need to specify the subdomain and hostname because the stateful set will automatically create a headless service for each pod in the stateful set

• stateful set using headless service with pvc for each pod

    apiVersion: apps/v1
    kind: StatefulSet
    metadata:
        name: mysql
        labels:
            app: mysql
    spec:
        replicas: 3
        selector:
            matchLabels:
                app: mysql
        template:
            metadata:
                labels:
                    app: mysql
            spec:
                containers:
                  - name: mysql
                    image: mysql
                    volumeMounts:
                      - name: data-volume
                        mountPath: /var/lib/mysql
        volumeClaimTemplates:
          - metadata:
                name: data-volume
            spec:
                accessModes:
                    - ReadWriteOnce
                storageClassName: google-storage
                resources:
                    requests:
                        storage: 500Mi
  

  • volumeClaimTemplates is used to create a pvc for each pod in the stateful set
  • stateful sets do not automatically delete pvc when pods fail or are deleted, instead they ensure that when the pod is recreated, the pvc is bound back to the pod that was recreated.

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