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Predictable Demands

We recommend using a Minikube installation for this example. Please refer to the installation instructions for details.

To play with memory limits, we assume you use the default 2GB memory for your Minikube VM. You can adjust this value with the --memory flag. ; To access the PersistentVolume used in this demo, mount a local directory logs/ into the Minikube VM with the following command:

minikube start --mount --mount-string="$(pwd)/logs:/tmp/example" --memory 2G

The directory is now available within the Minikube VM at the path /tmp/example.

For this example, we will use the random number service, a simple REST service that returns just a random number. This service is contained in the image k8spatterns/random-generator:1.0 from Docker Hub.

Hard requirements

We will set up this image that it will write a log file in a PersistentVolume and get some configuration data from a ConfigMap.

Now lets try to create the Pod

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

The Pod will not start immediately because the PersistentVolumeClaim (PVC) is missing. Check the status with:

kubectl get pods
NAME               READY   STATUS    RESTARTS   AGE
random-generator   0/1     Pending   0          3s

When you describe the pod, you can see that the PVC is missing:

kubectl describe pod random-generator
...
Events:
  Type     Reason            Age                From               Message
  ----     ------            ----               ----               -------
  Warning  FailedScheduling  44s (x2 over 44s)  default-scheduler  persistentvolumeclaim "random-generator-log" not found

Let’s now create the PersistentVolume and PVC:

kubectl apply -f https://k8spatterns.io/PredictableDemands/pv-and-pvc.yml

When we create the PV and PVC we are already one step further:

kubectl describe pod random-generator
...
Events:
  Type     Reason            Age                From               Message
  ----     ------            ----               ----               -------
  Warning  FailedScheduling  32s (x5 over 7m44s)  default-scheduler  persistentvolumeclaim "random-generator-log" not found
  Warning  FailedScheduling  15s (x6 over 15s)    default-scheduler  pod has unbound immediate PersistentVolumeClaims
  Normal   Scheduled         15s                  default-scheduler  Successfully assigned default/random-generator to minikube
  Normal   Pulled            3s (x3 over 14s)      Pulling image "k8spatterns/random-generator:1.0"
  Warning  Failed            3s (x3 over 14s)     kubelet, minikube  Error: configmap "random-generator-config" not found

The PVC could be bound, but the ConfigMap is still missing.

Now you can create the ConfigMap:

kubectl apply -f https://k8spatterns.io/PredictableDemands/configmap.yml

and the pod will be finally up:

kubectl get pods
NAME               READY   STATUS    RESTARTS   AGE
random-generator   1/1     Running   0          3s

You can send an HTTP request to the service to check if everything is working correctly. As we haven’t installed a Service, let’s run curl within the cluster to check the output:

POD_IP=$(kubectl get pod -l app=random-generator -o jsonpath='{.items[0].status.podIP}')
kubectl run -itq --rm --image=k8spatterns/curl-jq --restart=Never curl -- http://$POD_IP:8080 | jq .

This will result in an output like this:

{
  "random": 79003229,
  "pattern": "Predictable Demands",
  "id": "d88cc0e5-c7f2-4e98-8073-ed76151b5218",
  "version": "1.0"
}

The log file that the application writes to the persistent volume should appear in our locally mounted directory:

cat ./logs/random.log
...
16:52:44.734,fe870020-fcab-42bf-b745-db65ec1cc51f,2137264413
...

Now that our hard requirements (ConfigMap, PVC) are fulfilled our the application runs smoothly.

If you are done, it’s a good idea to cleanup:

kubectl delete pod/random-generator
kubectl delete -f https://k8spatterns.io/PredictableDemands/pv-and-pvc.yml

Resource limits

Let’s create now a real Deployment with resource limits

kubectl create -f https://k8spatterns.io/PredictableDemands/deployment.yml

This will use 200M as upper limit for our application. Since we are using Java 11 as JVM for our Java application, the JVM respects this boundary and allocates only a fraction of this as heap. You can easily check this with: (call CTRL-C when done)

kubectl logs -l app=random-generator -f | grep "=== "
i.k.examples.RandomGeneratorApplication  : === Max Heap Memory:  96 MB
i.k.examples.RandomGeneratorApplication  : === Used Heap Memory: 37 MB
i.k.examples.RandomGeneratorApplication  : === Free Memory:      13 MB
i.k.examples.RandomGeneratorApplication  : === Processors:       1

Let’s now try to change our limits and requests with smaller and larger values.

patch=$(cat <<EOT
[
  {
    "op": "replace",
    "path": "/spec/template/spec/containers/0/resources/requests/memory",
    "value": "30Mi"
  },
  {
    "op": "replace",
    "path": "/spec/template/spec/containers/0/resources/limits/memory",
    "value": "30Mi"
  }
]
EOT
)
kubectl patch deploy random-generator --type=json -p $patch

If you check your Pods now with kubectl get pods and kubectl describe, do you see what you expect ? Also don’t forget to check the logs, too !