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Self Awareness

Any Kubernetes installation can be used for this example, as described in the INSTALL.

To focus on the usage of the Downward API, we create just a Pod which references its properties in the environment variables and mounts labels and annotations as files:

kubectl create -f https://k8spatterns.io/SelfAwareness/pod.yml

The sample random-generator REST service exposes an endpoint /info, which picks up this info and returns it in the response. For this simple example, we don’t provide a Service (but of course, feel free to add one). Instead, we are just using a port-forwarding reverse proxy to access the REST endpoint:

kubectl port-forward random-generator 8080:8080 &

We start it in the background to stay in the same terminal windows.

Next, let’s test this endpoint:

curl -s http://localhost:8080/info | jq .

results in

{
  "memory.free": 171,
  "NODE_NAME": "minikube",
  "memory.used": 189,
  "CPU.procs": 1,
  "memory.max": 3014,
  "pattern": "Self Awareness",
  "annotations": "kubernetes.io/config.seen=\"2023-03-21T17:49:51.553809009Z\"\nkubernetes.io/config.source=\"api\"",
  "id": "e79022b1-f586-44fb-8ecf-e1443f002441",
  "POD_IP": "10.244.0.6",
  "env": {
    "KUBERNETES_PORT_443_TCP": "tcp://10.96.0.1:443",
    "PATH": "/usr/local/openjdk-11/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
    "KUBERNETES_PORT_443_TCP_ADDR": "10.96.0.1",
    "KUBERNETES_PORT": "tcp://10.96.0.1:443",
    "JAVA_HOME": "/usr/local/openjdk-11",
    "KUBERNETES_PORT_443_TCP_PROTO": "tcp",
    "LANG": "C.UTF-8",
    "KUBERNETES_SERVICE_HOST": "10.96.0.1",
    "KUBERNETES_SERVICE_PORT": "443",
    "NODE_NAME": "minikube",
    "PATTERN": "Self Awareness",
    "HOSTNAME": "random-generator",
    "KUBERNETES_PORT_443_TCP_PORT": "443",
    "JAVA_VERSION": "11.0.16",
    "KUBERNETES_SERVICE_PORT_HTTPS": "443",
    "POD_IP": "10.244.0.6",
    "HOME": "/root"
  },
  "version": "1.0",
  "labels": "app=\"random-generator\""
}

Please note the entries for POD_INFO, NODE_NAME, annotations, and labels and how they refer to Pod internal data.

To verify that an update of the label is reflected in the mounted volumes, use

patch=$(cat <<EOT
[
  {
    "op": "add",
    "path": "/metadata/labels",
    "value": {
      "app": "random-generator-updated"
    }
  }
]
EOT
)
kubectl patch pod random-generator --type=json -p=$patch

When you now re-run the curl command, you will see how the labels entry in the response changes.