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 pkg/scheduler in the Kubernetes source directory for a canonical example.
You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using Minikube, or you can use one of these Kubernetes playgrounds:
To check the version, enter
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
to build the image:
FROM busybox ADD ./_output/dockerized/bin/linux/amd64/kube-scheduler /usr/local/bin/kube-scheduler
docker build -t gcr.io/my-gcp-project/my-kube-scheduler:1.0 . gcloud docker -- push gcr.io/my-gcp-project/my-kube-scheduler:1.0
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
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
spec.schedulerName on pods, to determine whether this scheduler is responsible for scheduling a particular pod.
Note also that we created a dedicated service account
my-scheduler and bind the cluster role
system:kube-scheduler to it so that it can acquire the same privileges as
Please see the kube-scheduler documentation for detailed description of other command line arguments.
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.
To run multiple-scheduler with leader election enabled, you must do the following:
First, update the following fields in your YAML file:
If RBAC is enabled on your cluster, you must update the
system:kube-scheduler cluster role. Add you scheduler name to the resourceNames of the rule applied for endpoints resources, as in the following example:
$ kubectl edit clusterrole system:kube-scheduler - apiVersion: rbac.authorization.k8s.io/v1 kind: ClusterRole metadata: annotations: rbac.authorization.kubernetes.io/autoupdate: "true" labels: kubernetes.io/bootstrapping: rbac-defaults name: system:kube-scheduler rules: - apiGroups: - "" resourceNames: - kube-scheduler - my-scheduler resources: - endpoints verbs: - delete - get - patch - update
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 in that pod spec. Let’s look at three examples.
When no scheduler name is supplied, the pod is automatically scheduled using the default-scheduler.
Save this file as
pod1.yaml and submit it to the Kubernetes cluster.
kubectl create -f pod1.yaml
A scheduler is specified by supplying the scheduler name as a value to
spec.schedulerName. In this case, we supply the name of the
default scheduler which is
Save this file as
pod2.yaml and submit it to the Kubernetes cluster.
kubectl create -f pod2.yaml
In this case, we specify that this pod should be scheduled using the scheduler that we
my-scheduler. Note that the value of
spec.schedulerName 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.
kubectl create -f pod3.yaml
Verify that all three pods are running.
kubectl get pods
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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