In this new entry, continuing with Kubernetes articles, we’ll explore how the node selector in Kubernetes works and how to apply it. This functionality is particularly helpful in environments with different nodes having varying resources.

In production environments, it’s common to have different nodes to cater to various needs. For instance, one node might prefer microservices, while another might prioritize infrastructure services, or we might need to locate pods geographically.This is where the node selector comes into play.

In a previous post we deep about node affinity,

What is Node Selector in Kubernetes?

A node selector, also known as nodeSelector, is a field specified in the Pod specification. It’s defined as a key-value map.

The purpose of nodeSelector, as mentioned, is to determine whether a pod can be executed on a selected node. The node must also have a matching key-value pair. This way, the pod and node match, allowing the pod to run exclusively on the selected node.

Example Application of a Node Selector

In the current environment I’m working in, we have four different nodes, and pods are assigned to nodes based on their functionality. For instance, microservices pods are assigned to one node, while infrastructure applications like Jenkins might be placed on another node with specific resources.

Let’s walk through the steps and requirements to apply a nodeSelector to a pod.

Create a Label on a Node to Apply Node Selector

To assign a pod to a node, the first step is to create the corresponding label on the node. Let’s obtain a list of nodes:

kubectl get nodes

Once we’ve selected the node to which we want to add the label, execute the following command, kubectl label nodes <node-name> <label-key>=<label-value>, for example:

kubectl label nodes ip-190-168-140-57.ec2.internal nodegroup-type=microservices

As always, make sure the node value has a correct value. You can verify using:

kubectl describe nodes

Alternatively, check a specific node:

kubectl describe nodes ip-190-168-140-57.ec2.internal

As seen in the above code, the node is modified, and now it has a node selector to associate a pod with it.

Name:               ip-190-168-140-57.ec2.internal
Roles:              <none>
Labels:             alpha.eksctl.io/cluster-name=eks-dev-cluster
                    alpha.eksctl.io/instance-id=i-0100bdd9c9bd76460
                    alpha.eksctl.io/nodegroup-name=instances-stateless
                    beta.kubernetes.io/arch=amd64
                    beta.kubernetes.io/instance-type=c5.2xlarge
                    beta.kubernetes.io/os=linux
                    failure-domain.beta.kubernetes.io/region=us-east-1
                    failure-domain.beta.kubernetes.io/zone=us-east-1c
                    kubernetes.io/arch=amd64
                    kubernetes.io/hostname=ip-190-168-140-57.ec2.internal
                    kubernetes.io/os=linux
                    node.kubernetes.io/instance-type=c5.2xlarge
                    nodegroup-instances=mixed-instances-stateless
                    nodegroup-type=microservices
                    topology.kubernetes.io/region=us-east-1
                    topology.kubernetes.io/zone=us-east-1c

Creating a NodeSelector on a Pod Associated with a Node

To associate a pod with a node, you need to assign the node’s label to the pod’s nodeSelector.

Here’s an example of creating a pod associated with a node having the “microservices” label. Let’s say we’re deploying a service we’ve built locally:

apiVersion: v1
kind: Pod
metadata:
  namespace: microservice-stack
  name: location
  labels:
    env: dev
spec:
  containers:
  - name: location
    image: location:latest    
  nodeSelector:
    nodegroup-type=microservices

Now, we can verify the localtion of our pod. To do this, use the following command. We’ll add the namespace as we’ve deployed it within a specific namespace:

kubectl get pod location -o wide -n microservice-stack

In our case, we can see that we have deployed the pod on the specific node:

sqlCopy code

NAME                         READY   STATUS    RESTARTS   AGE    IP           NODE                              NOMINATED NODE   READINESS GATES
location                      1/1     Running   0          2m   10.0.2.199   ip-190-168-140-57.ec2.internal   <none>           <none>

Conclusion

In this article on Kubernetes Node Selector, we’ve seen a simple example and use case of when and how to apply nodeSelector in Kubernetes. In upcoming articles, we’ll also explore the usage of nodeAffinity, which operates based on resources.