Understanding Kubernetes Cordoned: A Comprehensive Guide

In the realm of container orchestration, Kubernetes has emerged as the de facto standard. One of the crucial features within Kubernetes is the ability to manage nodes effectively. The concept of Kubernetes cordoned plays a significant role in node management. Cordoning a node is essentially a way to mark it as unschedulable, which means that no new pods will be assigned to that particular node. This is extremely useful in scenarios such as node maintenance, upgrades, or when a node is experiencing issues. In this blog post, we will delve deep into the core concepts, typical usage examples, common practices, and best practices related to Kubernetes cordoned.

Table of Contents

  1. Core Concepts
  2. Typical Usage Example
  3. Common Practices
  4. Best Practices
  5. Conclusion
  6. References

Core Concepts

What is Cordoning?

Cordoning a node in Kubernetes is a process of setting a specific taint on the node. A taint is a key - value pair with an associated effect that, when applied to a node, can prevent pods from being scheduled on that node. When you cordon a node, a taint of the type node.kubernetes.io/unschedulable:NoSchedule is added to the node. This taint ensures that no new pods will be scheduled on the node, but existing pods on the node will continue to run.

Why Cordon a Node?

There are several reasons to cordon a node:

  • Maintenance: When you need to perform maintenance on a node, such as upgrading the operating system or the Kubernetes components, cordoning the node prevents new pods from being scheduled on it during the maintenance period.
  • Resource Management: If a node is running out of resources or has a hardware issue, cordoning it can help in redistributing the workload to other healthy nodes.
  • Node Retirement: When you want to remove a node from the cluster, cordoning it is the first step in the process.

Typical Usage Example

Let’s assume you have a Kubernetes cluster with multiple nodes, and you want to perform maintenance on one of the nodes named node-1.

Step 1: Check the Current Status of the Node

First, you need to check the current status of the node using the following command:

kubectl get nodes

This command will display a list of all the nodes in the cluster along with their status.

Step 2: Cordon the Node

To cordon the node-1, use the following command:

kubectl cordon node-1

After running this command, the node will be marked as unschedulable. You can verify this by running the kubectl get nodes command again. The STATUS column for node-1 will show SchedulingDisabled.

Step 3: Perform Maintenance

Now you can safely perform the maintenance tasks on node-1 without worrying about new pods being scheduled on it.

Step 4: Uncordon the Node

Once the maintenance is complete, you can make the node schedulable again by using the following command:

kubectl uncordon node-1

After running this command, the node will be available for new pod scheduling.

Common Practices

Drain the Node After Cordoning

Cordoning a node only prevents new pods from being scheduled on it. Existing pods will continue to run. In most cases, you would want to move these pods to other nodes before performing maintenance. You can use the kubectl drain command to achieve this. The drain command cordons the node and then evicts all the pods from it in a graceful manner.

kubectl drain node-1 --ignore-daemonsets

The --ignore-daemonsets flag is used to ignore the DaemonSet pods, as they are supposed to run on all nodes in the cluster.

Monitor the Cluster During Node Maintenance

While performing maintenance on a cordoned node, it is important to monitor the cluster to ensure that the workload is being redistributed properly. You can use tools like Prometheus and Grafana to monitor the cluster’s performance.

Best Practices

Plan Ahead

Before cordoning a node, make sure you have a clear plan for the maintenance or the task you are going to perform. This includes understanding the impact on the cluster and having a backup plan in case something goes wrong.

Test in a Staging Environment

If possible, test the cordoning and maintenance process in a staging environment before performing it in the production environment. This can help you identify and fix any potential issues.

Document the Process

Keep a record of the cordoning and maintenance process, including the commands used, the time taken, and any issues encountered. This documentation can be useful for future reference and for troubleshooting.

Conclusion

Kubernetes cordoning is a powerful feature that allows you to manage nodes effectively in a Kubernetes cluster. By understanding the core concepts, typical usage examples, common practices, and best practices, you can ensure that your cluster remains stable and reliable during node maintenance and other operations. Cordoning a node is just one step in the process of node management, and it should be used in conjunction with other Kubernetes features like draining and taints to achieve the best results.

References