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.
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 hack/build-go.sh
Create a container image containing the kube-scheduler binary. Here is the
to build the image:
FROM busybox ADD _output/local/go/bin/kube-scheduler /usr/local/bin/kube-scheduler
docker build -t 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
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.
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.
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.
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.
kubectl create -f pod1.yaml
2. Pod spec with
A scheduler is specified by supplying the scheduler name as a value to the annotation
scheduler.alpha.kubernetes.io/name. 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
3. Pod spec with
In this case, we specify that this pod should be scheduled using the scheduler that we
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.
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