How to Cordon Kubernetes Nodes and Manage Pod Scheduling

Introduction

Kubernetes is a powerful container orchestration platform that allows you to manage the lifecycle of worker nodes, known as "Kubernetes Nodes." This tutorial explores the concept of "Node Cordoning," a crucial mechanism for maintaining and managing Kubernetes nodes. You will learn how to cordon nodes, understand the benefits of node cordoning, and discover strategies for configuring pod scheduling policies to ensure efficient resource utilization and uninterrupted workloads when nodes are cordoned.

Skills Graph

Understanding Kubernetes Node Cordoning

Kubernetes is a powerful container orchestration platform that manages the deployment, scaling, and management of containerized applications. One of the key features of Kubernetes is the ability to manage the lifecycle of worker nodes, known as "Kubernetes Nodes." In this section, we will explore the concept of "Node Cordoning," which is a crucial mechanism for maintaining and managing Kubernetes nodes.

What is Node Cordoning?

Node Cordoning is the process of marking a Kubernetes node as unschedulable, which means that new pods will not be placed on that node. This is typically done for maintenance or decommissioning purposes, such as when a node needs to be upgraded, repaired, or removed from the cluster.

When a node is cordoned, any existing pods on that node will continue to run, but new pods will not be scheduled on that node. This allows you to perform maintenance or decommissioning tasks on the node without disrupting the running workloads.

Why Use Node Cordoning?

There are several reasons why you might want to use Node Cordoning in a Kubernetes cluster:

1. Node Maintenance: When a node requires maintenance, such as software updates or hardware upgrades, you can cordon the node to prevent new pods from being scheduled on it, allowing you to perform the necessary tasks without disrupting the running workloads.

2. Node Decommissioning: When a node needs to be removed from the Kubernetes cluster, you can cordon the node to gracefully drain any running pods and then safely decommission the node.

3. Resource Optimization: By cordoning nodes, you can control the distribution of pods across your Kubernetes cluster, ensuring that resources are used efficiently and that critical workloads are prioritized.

Cordoning a Node

To cordon a node in Kubernetes, you can use the kubectl cordon command. For example, to cordon a node named "node1," you would run the following command:

kubectl cordon node1

This will mark the "node1" node as unschedulable, preventing new pods from being placed on it.

You can verify the node's status by running the kubectl get nodes command:

kubectl get nodes

The output will show the "node1" node with the "SchedulingDisabled" condition.

Uncordoning a Node

Once the necessary maintenance or decommissioning tasks are complete, you can make the node schedulable again by using the kubectl uncordon command. For example, to uncordon the "node1" node, you would run the following command:

kubectl uncordon node1

This will allow new pods to be scheduled on the "node1" node.

By understanding the concept of Node Cordoning and how to use it, you can effectively manage the lifecycle of your Kubernetes nodes and ensure the smooth operation of your containerized applications.

Cordoning Nodes and Managing Pods

When a Kubernetes node is cordoned, it becomes unschedulable, meaning that new pods will not be placed on that node. However, the existing pods on the cordoned node will continue to run. In this section, we will explore how to manage pods on cordoned nodes and the implications of node cordoning.

Handling Pods on Cordoned Nodes

When a node is cordoned, the existing pods on that node will continue to run. However, if those pods need to be rescheduled or scaled, they will not be placed back on the cordoned node. Instead, Kubernetes will attempt to schedule the pods on other available nodes in the cluster.

If a pod needs to be evicted from a cordoned node, Kubernetes will gracefully terminate the pod and reschedule it on another available node. This process is known as "pod eviction." The pod eviction process ensures that the pod's state is preserved, and the application can continue running without interruption.

To view the pods running on a cordoned node, you can use the following command:

kubectl get pods --field-selector spec.nodeName=<cordoned-node-name>

This command will list all the pods that are currently running on the specified cordoned node.

Scheduling Pods on Cordoned Nodes

By default, Kubernetes will not schedule new pods on a cordoned node. However, in some cases, you may want to override this behavior and schedule specific pods on a cordoned node. This can be useful when you need to perform maintenance on a node but still want to keep certain critical workloads running on that node.

To schedule a pod on a cordoned node, you can set the tolerations field in the pod's specification. Tolerations allow a pod to be scheduled on a node that has a matching taint. In the case of a cordoned node, the node has a "node.kubernetes.io/unschedulable" taint, which can be tolerated by the pod.

Here's an example of a pod specification that tolerates the "node.kubernetes.io/unschedulable" taint:

apiVersion: v1
kind: Pod
metadata:
  name: my-critical-pod
spec:
  tolerations:
    - key: "node.kubernetes.io/unschedulable"
      operator: "Exists"
  containers:
    - name: my-container
      image: my-critical-app:v1

By adding the tolerations field to the pod specification, you can ensure that the pod will be scheduled on the cordoned node, even though the node is marked as unschedulable.

Understanding how to manage pods on cordoned nodes is crucial for maintaining and optimizing your Kubernetes cluster during node maintenance or decommissioning.

Configuring Pod Scheduling Policies for Cordoned Nodes

When a Kubernetes node is cordoned, it becomes unschedulable, meaning that new pods will not be placed on that node. However, you can configure pod scheduling policies to control how pods are scheduled on cordoned nodes. In this section, we will explore the use of node taints and pod tolerations to manage pod scheduling on cordoned nodes.

Node Taints and Pod Tolerations

Kubernetes uses the concept of "node taints" and "pod tolerations" to control the scheduling of pods on nodes. A node taint is a key-value pair that marks a node as unavailable for certain pods. When a node is cordoned, it automatically receives the "node.kubernetes.io/unschedulable" taint.

Pods can be configured to "tolerate" specific taints, which allows them to be scheduled on nodes with those taints. By configuring pod tolerations, you can control which pods are allowed to be scheduled on cordoned nodes.

Here's an example of a pod specification that tolerates the "node.kubernetes.io/unschedulable" taint:

apiVersion: v1
kind: Pod
metadata:
  name: my-critical-pod
spec:
  tolerations:
    - key: "node.kubernetes.io/unschedulable"
      operator: "Exists"
  containers:
    - name: my-container
      image: my-critical-app:v1

In this example, the tolerations field in the pod specification includes the "node.kubernetes.io/unschedulable" taint. This allows the pod to be scheduled on a cordoned node.

Configuring Default Tolerations

In addition to configuring tolerations at the pod level, you can also set default tolerations at the cluster level. This can be useful when you have a large number of pods that need to be scheduled on cordoned nodes.

To set default tolerations, you can use the PodTolerationRestriction admission controller. This controller allows you to configure a set of default tolerations that will be applied to all pods in the cluster.

Here's an example of how to configure default tolerations using the PodTolerationRestriction admission controller:

apiVersion: apiserver.config.k8s.io/v1
kind: AdmissionConfiguration
plugins:
  - name: PodTolerationRestriction
    configuration:
      apiVersion: podtolerationrestriction.admission.config.k8s.io/v1
      kind: Configuration
      defaultAdmissionRule:
        matchExpressions:
          - key: node.kubernetes.io/unschedulable
            operator: Exists
            effect: NoSchedule
      pluginConfig:
        - apiVersion: podtolerationrestriction.admission.config.k8s.io/v1
          kind: PodTolerationConfig
          tolerations:
            - key: node.kubernetes.io/unschedulable
              operator: Exists
              effect: NoSchedule

In this example, the PodTolerationRestriction admission controller is configured to set a default toleration for the "node.kubernetes.io/unschedulable" taint. This means that all pods in the cluster will be able to be scheduled on cordoned nodes by default.

By understanding and configuring pod scheduling policies for cordoned nodes, you can ensure that your critical workloads continue to run on the cordoned nodes while also allowing for maintenance and decommissioning tasks to be performed on the cluster.

Summary

In this tutorial, you have learned about the importance of node cordoning in Kubernetes and how to effectively manage pod scheduling when a node is cordoned. By understanding the process of cordoning nodes and configuring appropriate pod scheduling policies, you can ensure that your Kubernetes cluster continues to run smoothly during maintenance or decommissioning tasks, optimizing resource utilization and maintaining the reliability of your containerized applications.