Scheduling an Application using Labels and LabelConstraints

Goal: Explore the labels mechanisms and schedule an application based on labels and label constraints

Introduction to Scheduling mechanisms

Note

After its initial scheduling, an application location is re-evaluated every 60 seconds by the Scheduler - the so-called “rescheduling”. In the following scenarios, we observe both the initial scheduling of an application, and a migration triggered by the rescheduling. To correctly observe this mechanism, it is recommended to check the Scheduler logs, the Application status, and the resources running on the Kubernetes clusters directly, by running the following commands in separate terminals:

  • Watch the echo-demo Kubernetes Application status, and more precisely its current location:
watch "rok kube app get echo-demo -o json | jq .status.running_on"
  • Watch Scheduler logs:
docker logs -f krake_krake-ctrl-scheduler_1
  • Observe k8s resources on both Minikube clusters:
watch kubectl --kubeconfig clusters/config/minikube-cluster-1 get all

watch kubectl --kubeconfig clusters/config/minikube-cluster-2 get all

Preparation

  • Register the two clusters with a location Label.

Note

Each label always have a key and a value. We follow the same specifications as Kubernetes.

rok kube cluster register -k clusters/config/minikube-cluster-1 -l location=DE
rok kube cluster register -k clusters/config/minikube-cluster-2 -l location=SK

Spawn the demo application

  • Create an application with a location LabelConstraints, and observe where it is deployed.
rok kube app create -f git/krake/rak/functionals/echo-demo.yaml echo-demo -L location=DE
rok kube app get echo-demo -o json | jq .status.running_on

Observe a migration

  • Update an application’s LabelConstraints and observe the migration to the second Kubernetes cluster.
rok kube app update echo-demo -L location=SK
rok kube app get echo-demo -o json | jq .status.running_on  # The Application is now running on "minikube-cluster-2"

Cleanup

  • Delete the echo-demo Kubernetes Application and both Krake Kubernetes Clusters
rok kube app delete echo-demo
rok kube cluster delete minikube-cluster-1
rok kube cluster delete minikube-cluster-2