kubernetes - GET_HOSTS_FROM 变量的值是多少？

Question

我正在关注 Kubernetes 教程:https://kubernetes.io/docs/tutorials/stateless-application/guestbook/#creating-the-redis-master-service

但是有一条线我不明白。在前端部署中有 GET_HOSTS_FROM 变量。它的值为“dns”。是进一步评估还是保留为“dns”？

这是整个对应的yaml:

#application/guestbook/frontend-deployment.yaml 

apiVersion: apps/v1 # for versions before 1.9.0 use apps/v1beta2
kind: Deployment
metadata:
  name: frontend
  labels:
    app: guestbook
spec:
  selector:
    matchLabels:
      app: guestbook
      tier: frontend
  replicas: 3
  template:
    metadata:
      labels:
        app: guestbook
        tier: frontend
    spec:
      containers:
      - name: php-redis
        image: gcr.io/google-samples/gb-frontend:v4
        resources:
          requests:
            cpu: 100m
            memory: 100Mi
        env:
        - name: GET_HOSTS_FROM
          value: dns
          # Using `GET_HOSTS_FROM=dns` requires your cluster to
          # provide a dns service. As of Kubernetes 1.3, DNS is a built-in
          # service launched automatically. However, if the cluster you are using
          # does not have a built-in DNS service, you can instead
          # access an environment variable to find the master
          # service's host. To do so, comment out the 'value: dns' line above, and
          # uncomment the line below:
          # value: env
        ports:
        - containerPort: 80

Answer 1

GET_HOSTS_FROM 的值不会被进一步评估 - 它仍然是“dns”。

查看应用程序的源代码 here , GET_HOSTS_FROM 用于确定 Redis 主从主机是否来自环境，或者默认情况下是 primary 的 Kubernetes 服务名称和 slave :

  $host = 'redis-master';
  if (getenv('GET_HOSTS_FROM') == 'env') {
    $host = getenv('REDIS_MASTER_SERVICE_HOST');
  }

当主机名是 Kubernetes 服务名时，它们将使用集群的 DNS 进行解析.

值得一提的是 pod 可以如何引用相同命名空间和不同命名空间中的服务(摘录自上面给出的文档的链接):

...if you have a Service called "my-service" in a Kubernetes Namespace "my-ns", the control plane and the DNS Service acting together create a DNS record for "my-service.my-ns". Pods in the "my-ns" Namespace should be able to find it by simply doing a name lookup for my-service ("my-service.my-ns" would also work).