Kubernetes Scheduler Extender

Kubernetes Scheduler Extender: An Introduction

Introduction

Kubernetes is a popular open-source container orchestration platform that automates the deployment, scaling, and management of containerized applications. The Kubernetes scheduling component is responsible for assigning pods to nodes in the cluster based on resource availability and user-defined constraints. However, the default scheduler may not always meet the specific requirements of every workload.

To address this limitation, Kubernetes provides an extensibility mechanism called the "Scheduler Extender." This feature allows users to customize the scheduling process by implementing their own logic using a scheduler extender API. In this article, we will explore the Kubernetes Scheduler Extender and provide a code example to illustrate its usage.

Scheduler Extender API

The Scheduler Extender API allows users to customize the scheduling behavior by implementing three primary interfaces:

1. Filter: This interface is responsible for filtering out nodes that are not suitable for a particular pod.
2. Prioritize: This interface assigns a priority score to each node based on a given set of metrics or policies.
3. Bind: This interface is responsible for binding the pod to a specific node.

By implementing these interfaces, users can define their own scheduling logic to meet specific requirements, such as spreading pods across different availability zones or considering custom resource allocations.

Code Example

Let's say we want to build a simple Scheduler Extender that prefers nodes with a specific label. We will implement the Filter and Prioritize interfaces to achieve this.

1. Implementing the Filter Interface

type LabelFilter struct {
    labelKey   string
    labelValue string
}

func (f *LabelFilter) Filter(pc *v1.Pod, nodeList *v1.NodeList) (*v1.NodeList, error) {
    filteredNodes := make([]*v1.Node, 0)
    for _, node := range nodeList.Items {
        if node.Labels[f.labelKey] == f.labelValue {
            filteredNodes = append(filteredNodes, &node)
        }
    }
    return &v1.NodeList{Items: filteredNodes}, nil
}

2. Implementing the Prioritize Interface

type LabelPrioritizer struct {
    labelKey   string
    labelValue string
}

func (p *LabelPrioritizer) Prioritize(pc *v1.Pod, nodeList *v1.NodeList) (*v1.NodeList, error) {
    for _, node := range nodeList.Items {
        if node.Labels[p.labelKey] == p.labelValue {
            // Assign a higher priority score to nodes with the desired label
            node.Score += 10
        }
    }
    return nodeList, nil
}

3. Registering the Scheduler Extender

To register our custom scheduler extender, we need to modify the Kubernetes scheduler configuration. Add the following lines to the scheduler configuration file (e.g., kube-scheduler.yaml):

apiVersion: kubescheduler.config.k8s.io/v1alpha1
kind: KubeSchedulerConfiguration
profiles:
- schedulerName: custom-scheduler
  plugins:
    score:
      enabled:
      - name: LabelPrioritizer
    preFilter:
      enabled:
      - name: LabelFilter

4. Starting the Scheduler

Start the scheduler with the custom configuration file:

$ kube-scheduler --config=kube-scheduler.yaml

Conclusion

In this article, we explored the Kubernetes Scheduler Extender and provided a code example to demonstrate its usage. By implementing the Filter and Prioritize interfaces, users can customize the scheduling behavior to meet their specific requirements. This extensibility feature allows Kubernetes to be more flexible and adaptable to different workload demands.