How to cordon a node with running pods in Kubernetes

Introduction

Kubernetes, the powerful container orchestration platform, provides a wide range of features to manage and maintain the health of your cluster. One such feature is the ability to cordon a node, which allows you to gracefully remove a node from the active workload without disrupting the running pods. In this tutorial, we'll explore the process of cordoning a node with running pods in Kubernetes, discuss practical applications, and cover best practices for effective node management.

Skills Graph

Understanding Node Cordoning in Kubernetes

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 nodes (physical or virtual machines) that make up the cluster. Cordoning a node is an important operation in Kubernetes that allows you to temporarily mark a node as unschedulable, preventing new pods from being placed on that node.

What is Node Cordoning?

Node cordoning is the process of marking a Kubernetes node as unschedulable, which means that new pods will not be scheduled on that node. This is useful in various scenarios, such as:

1. Maintenance: When you need to perform maintenance on a node, you can cordon it to ensure that no new pods are scheduled on that node, making the maintenance process safer and more reliable.

2. Draining Nodes: Before you can safely delete or replace a node, you need to ensure that all the pods running on that node are safely migrated to other nodes. Cordoning the node is the first step in the node draining process.

3. Load Balancing: By cordoning a node, you can temporarily remove it from the pool of available nodes, allowing you to better distribute the workload across the remaining nodes in the cluster.

Cordoning a Node with Running Pods

When you cordon a node, any existing pods on that node will continue to run, but no new pods will be scheduled on the node. If you need to remove all the pods from a node, you'll need to perform a node drain operation, which involves cordoning the node and then evicting all the pods running on that node.

To cordon a node in Kubernetes, you can use the kubectl cordon command:

kubectl cordon <node-name>

This command marks the specified node as unschedulable, preventing new pods from being placed on that node.

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

kubectl get nodes

The output will show the node's status as SchedulingDisabled, indicating that the node has been cordoned.

In the diagram above, Node 1 has been cordoned, but the existing pods (Pod 1 and Pod 2) continue to run on the node. New pods cannot be scheduled on Node 1 until it is uncordoned.

Practical Applications and Best Practices

Cordoning nodes in Kubernetes has several practical applications and best practices:

1. Maintenance and Upgrades: When you need to perform maintenance or upgrades on a node, cordoning the node ensures that no new pods are scheduled on that node, making the process safer and more reliable.

2. Node Draining: Cordoning a node is the first step in the node draining process, which involves safely migrating all the pods running on a node to other nodes before the node can be deleted or replaced.

3. Load Balancing: By cordoning a node, you can temporarily remove it from the pool of available nodes, allowing you to better distribute the workload across the remaining nodes in the cluster.

4. Fault Tolerance: Cordoning a node can be used as a temporary measure to isolate a problematic node, preventing it from impacting the overall cluster performance.

5. Canary Deployments: Cordoning a node can be used in the context of canary deployments, where you can temporarily remove a node from the cluster to test new application versions or configurations.

By understanding the concept of node cordoning and its practical applications, Kubernetes administrators can effectively manage their clusters and ensure the reliability and availability of their containerized applications.

Cordoning Nodes with Running Pods

When you cordon a node in Kubernetes, any existing pods on that node will continue to run, but no new pods will be scheduled on the node. This is an important consideration when cordoning a node, as you need to ensure that the existing pods can continue to operate without interruption.

Cordoning a Node with Existing Pods

To cordon a node with existing pods, you can use the kubectl cordon command:

kubectl cordon <node-name>

This command will mark the specified node as unschedulable, preventing new pods from being placed on that node. The existing pods on the node will continue to run without interruption.

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

kubectl get nodes

The output will show the node's status as SchedulingDisabled, indicating that the node has been cordoned.

In the diagram above, Node 1 has been cordoned, but the existing pods (Pod 1 and Pod 2) continue to run on the node. New pods cannot be scheduled on Node 1 until it is uncordoned.

Handling Existing Pods During Cordoning

When you cordon a node, it's important to consider the impact on the existing pods running on that node. Here are some best practices to handle existing pods during the cordoning process:

1. Graceful Termination: If you need to remove the pods from the cordoned node, you should use the kubectl drain command, which will gracefully terminate the pods and migrate them to other available nodes in the cluster.

2. Pod Disruption Budgets: You can use Pod Disruption Budgets (PDBs) to control the number of pods that can be evicted from a node at the same time. This helps ensure that your application remains available during the cordoning process.

3. Workload Considerations: Consider the nature of the workloads running on the node. For stateful applications or services with persistent data, you may need to take additional steps to ensure data consistency and availability during the cordoning process.

4. Monitoring and Logging: Monitor the status of the cordoned node and the pods running on it. Ensure that you have adequate logging and monitoring in place to detect any issues or disruptions during the cordoning process.

By understanding how to cordon nodes with running pods and following best practices, you can effectively manage your Kubernetes cluster and ensure the continued availability of your containerized applications.

Practical Applications and Best Practices

Cordoning nodes in Kubernetes has several practical applications and best practices that can help you effectively manage your cluster and ensure the reliability and availability of your containerized applications.

Practical Applications of Node Cordoning

1. Maintenance and Upgrades: When you need to perform maintenance or upgrades on a node, cordoning the node ensures that no new pods are scheduled on that node, making the process safer and more reliable.

2. Node Draining: Cordoning a node is the first step in the node draining process, which involves safely migrating all the pods running on a node to other nodes before the node can be deleted or replaced.

3. Load Balancing: By cordoning a node, you can temporarily remove it from the pool of available nodes, allowing you to better distribute the workload across the remaining nodes in the cluster.

4. Fault Tolerance: Cordoning a node can be used as a temporary measure to isolate a problematic node, preventing it from impacting the overall cluster performance.

5. Canary Deployments: Cordoning a node can be used in the context of canary deployments, where you can temporarily remove a node from the cluster to test new application versions or configurations.

Best Practices for Cordoning Nodes

1. Graceful Termination: When cordoning a node, use the kubectl drain command to gracefully terminate the pods and migrate them to other available nodes in the cluster.

2. Pod Disruption Budgets: Utilize Pod Disruption Budgets (PDBs) to control the number of pods that can be evicted from a node at the same time, ensuring your application remains available during the cordoning process.

3. Workload Considerations: Consider the nature of the workloads running on the node. For stateful applications or services with persistent data, take additional steps to ensure data consistency and availability during the cordoning process.

4. Monitoring and Logging: Monitor the status of the cordoned node and the pods running on it. Ensure that you have adequate logging and monitoring in place to detect any issues or disruptions during the cordoning process.

5. Communication and Coordination: Communicate with your team and stakeholders about the planned cordoning activities, and coordinate the process to minimize disruptions to your application's availability.

By understanding the practical applications of node cordoning and following these best practices, you can effectively manage your Kubernetes cluster and ensure the continued reliability and availability of your containerized applications.

Summary

In this Kubernetes tutorial, you have learned the importance of cordoning nodes with running pods, the step-by-step process to achieve this, and the practical applications and best practices for effective node management in your Kubernetes environment. By understanding and implementing these techniques, you can ensure the smooth operation and high availability of your Kubernetes cluster.