Manual Scheduling

Welcome to this lesson on manually scheduling pods in Kubernetes. This guide explains how to assign pods to nodes without relying on Kubernetes’ built-in scheduler. Manual scheduling can be useful in niche scenarios where you need tighter control over pod placement. In this article, we review a basic pod manifest, demonstrate how manual scheduling works, and show you how to use binding objects to reassign pods if necessary.

Understanding the Default Scheduler Behavior

Every pod definition includes a field called nodeName, which is left unset by default. The Kubernetes scheduler automatically scans for pods without a nodeName and selects an appropriate node by updating this field and creating a binding object. Consider the basic pod manifest below:

apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
    name: nginx
spec:
  containers:
  - name: nginx
    image: nginx
    ports:
    - containerPort: 8080

Typically, you do not include the nodeName field in your manifest. The scheduler uses this field only after selecting a node for the pod.

Manually Setting the Node Name

To manually assign a pod to a specific node during creation, populate the nodeName field in the manifest. For example, to schedule the pod on a node called “node02”, update your manifest as follows:

apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
    name: nginx
spec:
  nodeName: node02
  containers:
  - name: nginx
    image: nginx
    ports:
    - containerPort: 8080

After creating the pod with this manifest, check its status with:

kubectl get pods

Sample output before the scheduler assigns the IP:

NAME    READY   STATUS    RESTARTS   AGE
nginx   0/1     Pending   0          3s

And after the pod becomes ready:

kubectl get pods
NAME      READY   STATUS    RESTARTS   AGE   IP         NODE
nginx     1/1     Running   0          9s    10.40.0.4  node02

The nodeName must be set during pod creation. Once the pod is running, Kubernetes does not permit modifications to the nodeName field.

Reassigning a Running Pod Using a Binding Object

If a pod is already running and you need to change its node assignment, you cannot modify its nodeName directly. In this scenario, you can create a binding object that mimics the scheduler’s behavior.

Create a binding object that specifies the target node (“node02”):

apiVersion: v1
kind: Binding
metadata:
  name: nginx
target:
  apiVersion: v1
  kind: Node
  name: node02

The original pod definition remains unchanged:

apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
    name: nginx
spec:
  containers:
  - name: nginx
    image: nginx
    ports:
    - containerPort: 8080

Convert the YAML binding to JSON (e.g., save it as binding.json) and send a POST request to the pod’s binding API using curl:

curl --header "Content-Type: application/json" --request POST --data @binding.json http://$SERVER/api/v1/namespaces/default/pods/nginx/binding

This binding instructs Kubernetes to assign the existing pod to the specified node without altering its original manifest.

Quick Reference Table

Method	Use Case	Example Snippet Reference
Direct Assignment	Set `nodeName` during pod creation	See manifest with `nodeName: node02`
Using a Binding Object	Reassign a running pod	See binding object example

Additional Resources

That’s it for this lesson. Experiment with manually scheduling pods to solidify your understanding of Kubernetes node assignments. Happy scheduling!

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Scheduling Section Introduction

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Install "Kubernetes the kubeadm way"

Understanding the Default Scheduler Behavior

Manually Setting the Node Name

Reassigning a Running Pod Using a Binding Object

Quick Reference Table

Additional Resources

Watch Video

Practice Lab

​Understanding the Default Scheduler Behavior

​Manually Setting the Node Name

​Reassigning a Running Pod Using a Binding Object

​Quick Reference Table

​Additional Resources

Watch Video

Practice Lab

Understanding the Default Scheduler Behavior

Manually Setting the Node Name

Reassigning a Running Pod Using a Binding Object

Quick Reference Table

Additional Resources