Achieving Zero-Downtime Load Migration in Kubernetes GKE with Autoscaling
Tags: kubernetes, advance, operations
Published: 2023-08-30 14:03:24

## Introduction

The magic of autoscaling ensures that your application scales seamlessly with demand. But what happens when you need to migrate your workloads from one node pool to another without causing disruptions? In this article, we'll dive into the process of migrating loads between GKE node pools while autoscaling is enabled, all without interrupting your services.

## TL;DR (Too Long; Didn't Read)

Migrating loads between GKE node pools while keeping autoscaling operational might sound complex, but it's an essential operation in scenarios like resource optimization, maintenance, or refining your scaling strategy. To execute this successfully, you'll:

1. Create a new node pool: Prepare a destination for your workloads.
2. Cordoning nodes: Pause new pod scheduling on nodes in the source node pool.
3. Disable autoscaling: Temporarily halt automatic scaling for controlled migration.
4. Draining or rolling restarts: Shift running workloads off the source nodes.
5. Monitor and validate: Keep an eye on cluster health and application performance.

## Why Do We Need This?

There are a few key scenarios that highlight the importance of seamless load migration:

1. **Optimal Resource Utilization:** New node pools might offer better resources or updated OS versions, making migration crucial for performance optimization.
2. **Maintenance and Upgrades:** During system updates or maintenance tasks, smooth load migration ensures continuous availability.
3. **Scaling Flexibility:** As your application scales, distributing the load across multiple node pools can help maintain optimal performance without overwhelming individual nodes.

## Step-by-Step Guide

**1. Create a the New Node Pool**

In the Google Cloud Console:

- Navigate to your GKE cluster.
- Under "Cluster," select "Node Pools."
- Click "Create a Node Pool" to set up a destination for your workloads.

Of course, those are steps for doing it manually but if you use IaC with Terraform, just let you know that you have to create the nodepool  as the first step.


**2. Cordoning Nodes**

Cordoning nodes prevents new pods from being scheduled on nodes in the source node pool. This step prepares the pool for migration while keeping existing workloads running.

```bash
kubectl cordon NODE_NAME
```

**3. Disable Autoscaling**

By disabling autoscaling, you gain control over the migration process. This ensures that the source node pool won't unexpectedly scale during migration.

- In the Google Cloud Console, navigate to your GKE cluster.
- Under "Cluster," select "Node Pools."
- Pick the source node pool and click "Edit."
- Turn off autoscaling and save the changes.

**4. Draining Nodes or Rolling Restarts**

For a smooth transition, you can either drain nodes or perform rolling restarts on deployments.

- **Draining Nodes:** Evict pods gracefully from nodes you're migrating using:

  ```bash
  kubectl drain NODE_NAME --ignore-daemonsets --delete-emptydir-data
  ```

Although draining the node is the most easiest and fast way to evict the pods from one node to other node, this can be impossible if PodDisruptionBudgets are set. If you find any problem related with this, follow with Rolling Restart procedure.

- **Rolling Restarts:** If managed by Deployments, this approach gradually moves pods to other nodes with minimal service disruption. Identify the deployments allocated on the nodes, and rollout restart them.

  ```bash
  kubectl rollout restart DEPLOYMENT_NAME
  ```


Kubernetes' scheduler automatically places evicted pods onto new nodes in the destination node pool, ensuring minimal downtime.

## 6. Monitor and Validate

Keep a close watch on your GKE cluster to ensure the migration's success. Check the status of your workloads and their performance in the new node pool.

## Conclusion

Seamlessly migrating loads between GKE node pools with autoscaling enabled might seem intricate, but with careful planning and execution, you can achieve it without service interruptions. GKE empowers you to manage these transitions efficiently as your application evolves, maintaining high availability and performance. Embrace the flexibility and tools GKE offers, and confidently manage your infrastructure's growth and change.