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Configuring Multiple Schedulers

Kubernetes ships with a default scheduler that is described here. If the default scheduler does not suit your needs you can implement your own scheduler. Not just that, you can even run multiple schedulers simultaneously alongside the default scheduler and instruct Kubernetes what scheduler to use for each of your pods. Let’s learn how to run multiple schedulers in Kubernetes with an example.

A detailed description of how to implement a scheduler is outside the scope of this document. Please refer to the kube-scheduler implementation in plugin/pkg/scheduler in the Kubernetes source directory for a canonical example.

1. Package the scheduler

Package your scheduler binary into a container image. For the purposes of this example, let’s just use the default scheduler (kube-scheduler) as our second scheduler as well. Clone the Kubernetes source code from Github and build the source.

git clone https://github.com/kubernetes/kubernetes.git
cd kubernetes
make

Create a container image containing the kube-scheduler binary. Here is the Dockerfile to build the image:

FROM busybox
ADD _output/local/go/bin/kube-scheduler /usr/local/bin/kube-scheduler

Save the file as Dockerfile, build the image and push it to a registry. This example pushes the image to Google Container Registry (GCR). For more details, please read the GCR documentation.

docker build -t my-kube-scheduler:1.0 .
gcloud docker push gcr.io/my-gcp-project/my-kube-scheduler:1.0

2. Define a Kubernetes Deployment for the scheduler

Now that we have our scheduler in a container image, we can just create a pod config for it and run it in our Kubernetes cluster. But instead of creating a pod directly in the cluster, let’s use a Deployment for this example. A Deployment manages a Replica Set which in turn manages the pods, thereby making the scheduler resilient to failures. Here is the deployment config. Save it as my-scheduler.yaml:

multiple-schedulers/my-scheduler.yaml
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  labels:
    component: scheduler
    tier: control-plane
  name: my-scheduler
  namespace: kube-system
spec:
  replicas: 1
  template:
    metadata:
      labels: 
        component: scheduler
        tier: control-plane
        version: second
    spec:
      containers:
      - command: [/usr/local/bin/kube-scheduler, --address=0.0.0.0,
                  --scheduler-name=my-scheduler]
        image: gcr.io/my-gcp-project/my-kube-scheduler:1.0
        livenessProbe:
          httpGet:
            path: /healthz
            port: 10251
          initialDelaySeconds: 15
        name: kube-second-scheduler
        readinessProbe:
          httpGet:
            path: /healthz
            port: 10251
        resources:
          requests:
            cpu: '0.1'
        securityContext:
          privileged: false
        volumeMounts: []
      hostNetwork: false
      hostPID: false
      volumes: []

An important thing to note here is that the name of the scheduler specified as an argument to the scheduler command in the container spec should be unique. This is the name that is matched against the value of the optional scheduler.alpha.kubernetes.io/name annotation on pods, to determine whether this scheduler is responsible for scheduling a particular pod.

Please see the kube-scheduler documentation for detailed description of other command line arguments.

3. Run the second scheduler in the cluster

In order to run your scheduler in a Kubernetes cluster, just create the deployment specified in the config above in a Kubernetes cluster:

kubectl create -f my-scheduler.yaml

Verify that the scheduler pod is running:

$ kubectl get pods --namespace=kube-system
NAME                                           READY     STATUS    RESTARTS   AGE
....
my-scheduler-lnf4s-4744f                       1/1       Running   0          2m
...

You should see a “Running” my-scheduler pod, in addition to the default kube-scheduler pod in this list.

4. Specify schedulers for pods

Now that our second scheduler is running, let’s create some pods, and direct them to be scheduled by either the default scheduler or the one we just deployed. In order to schedule a given pod using a specific scheduler, we specify the name of the scheduler as an annotation in that pod spec. Let’s look at three examples.

  1. Pod spec without any scheduler annotation
multiple-schedulers/pod1.yaml
apiVersion: v1
kind: Pod
metadata:
  name: no-annotation
  labels:
    name: multischeduler-example
spec:
  containers:
  - name: pod-with-no-annotation-container
    image: gcr.io/google_containers/pause:2.0

When no scheduler annotation is supplied, the pod is automatically scheduled using the default-scheduler.

Save this file as pod1.yaml and submit it to the Kubernetes cluster.

shell kubectl create -f pod1.yaml 2. Pod spec with default-scheduler annotation

multiple-schedulers/pod2.yaml
apiVersion: v1
kind: Pod
metadata:
  name: annotation-default-scheduler
  annotations:
    scheduler.alpha.kubernetes.io/name: default-scheduler
  labels:
    name: multischeduler-example
spec:
  containers:
  - name: pod-with-default-annotation-container
    image: gcr.io/google_containers/pause:2.0

A scheduler is specified by supplying the scheduler name as a value to the annotation with key scheduler.alpha.kubernetes.io/name. In this case, we supply the name of the default scheduler which is default-scheduler.

Save this file as pod2.yaml and submit it to the Kubernetes cluster.

shell kubectl create -f pod2.yaml 3. Pod spec with my-scheduler annotation

multiple-schedulers/pod3.yaml
apiVersion: v1
kind: Pod
metadata:
  name: annotation-second-scheduler
  annotations:
    scheduler.alpha.kubernetes.io/name: my-scheduler
  labels:
    name: multischeduler-example
spec:
  containers:
  - name: pod-with-second-annotation-container
    image: gcr.io/google_containers/pause:2.0

In this case, we specify that this pod should be scheduled using the scheduler that we deployed - my-scheduler. Note that the value of the annotation with key scheduler.alpha.kubernetes.io/name should match the name supplied to the scheduler command as an argument in the deployment config for the scheduler.

Save this file as pod3.yaml and submit it to the Kubernetes cluster.

shell kubectl create -f pod3.yaml

Verify that all three pods are running.

shell kubectl get pods

Verifying that the pods were scheduled using the desired schedulers

In order to make it easier to work through these examples, we did not verify that the pods were actually scheduled using the desired schedulers. We can verify that by changing the order of pod and deployment config submissions above. If we submit all the pod configs to a Kubernetes cluster before submitting the scheduler deployment config, we see that the pod annotation-second-scheduler remains in “Pending” state forever while the other two pods get scheduled. Once we submit the scheduler deployment config and our new scheduler starts running, the annotation-second-scheduler pod gets scheduled as well.

Alternatively, one could just look at the “Scheduled” entries in the event logs to verify that the pods were scheduled by the desired schedulers.

kubectl get events

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