<ins> --- reviewers: - davidopp - madhusudancs title: Configure Multiple Schedulers content_type: task weight: 20 --- <!-- overview --> Kubernetes ships with a default scheduler that is described [here](/docs/admin/kube-scheduler/). 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](https://github.com/kubernetes/kubernetes/tree/{{< param "githubbranch" >}}/pkg/scheduler) in the Kubernetes source directory for a canonical example. ## {{% heading "prerequisites" %}} {{< include "task-tutorial-prereqs.md" >}} {{< version-check >}} <!-- steps --> ## 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](https://github.com/kubernetes/kubernetes) and build the source. ```shell 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: ```docker FROM busybox ADD ./_output/local/bin/linux/amd64/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)](https://cloud.google.com/container-registry/). For more details, please read the GCR [documentation](https://cloud.google.com/container-registry/docs/). ```shell 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 ``` ## 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](/docs/concepts/workloads/controllers/deployment/) for this example. A [Deployment](/docs/concepts/workloads/controllers/deployment/) manages a [Replica Set](/docs/concepts/workloads/controllers/replicaset/) which in turn manages the pods, thereby making the scheduler resilient to failures. Here is the deployment config. Save it as `my-scheduler.yaml`: {{< codenew file="admin/sched/my-scheduler.yaml" >}} 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 `kube-scheduler`. Please see the [kube-scheduler documentation](/docs/admin/kube-scheduler/) for detailed description of other command line arguments. ## 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: ```shell kubectl create -f my-scheduler.yaml ``` Verify that the scheduler pod is running: ```shell 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. ### Enable leader election To run multiple-scheduler with leader election enabled, you must do the following: First, update the following fields in your YAML file: * `--leader-elect=true` * `--lock-object-namespace=<lock-object-namespace>` * `--lock-object-name=<lock-object-name>` {{< note >}} The control plane creates the lock objects for you, but the namespace must already exist. You can use the `kube-system` namespace. {{< /note >}} If RBAC is enabled on your cluster, you must update the `system:kube-scheduler` cluster role. Add your scheduler name to the resourceNames of the rule applied for `endpoints` and `leases` resources, as in the following example: ``` kubectl edit clusterrole system:kube-scheduler ``` ```yaml 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: - coordination.k8s.io resources: - leases verbs: - create - apiGroups: - coordination.k8s.io resourceNames: - kube-scheduler - my-scheduler resources: - leases verbs: - get - update - apiGroups: - "" resourceNames: - kube-scheduler - my-scheduler resources: - endpoints verbs: - delete - get - patch - update ``` ## 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 in that pod spec. Let's look at three examples. - Pod spec without any scheduler name {{< codenew file="admin/sched/pod1.yaml" >}} 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. ```shell kubectl create -f pod1.yaml ``` - Pod spec with `default-scheduler` {{< codenew file="admin/sched/pod2.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 `default-scheduler`. Save this file as `pod2.yaml` and submit it to the Kubernetes cluster. ```shell kubectl create -f pod2.yaml ``` - Pod spec with `my-scheduler` {{< codenew file="admin/sched/pod3.yaml" >}} In this case, we specify that this pod should be scheduled using the scheduler that we deployed - `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. ```shell kubectl create -f pod3.yaml ``` Verify that all three pods are running. ```shell kubectl get pods ``` <!-- discussion --> ### 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. ```shell kubectl get events ``` </ins>