Node

What is a Node?

• Kubernetes runs workloads by placing containers in pods and running them on nodes

• A node can be a virtual or physical machine depending on the cluster

• Each node is managed by the control plane and contains the services necessary to run pods

• The components of a node include kubelet, kube-proxy, and the container runtime

  -> Refer to Cluster!

Managing Nodes

There are two main ways to add a node to the Kubernetes API server

1. Self-registration to the control plane via the node's kubelet

2. A user (or another user) manually adds the node object

After registration via kubelet or node object, the control plane verifies that the created node object is valid

ex)

{
  "kind": "Node",
  "apiVersion": "v1",
  "metadata": {
    "name": "10.240.79.157",
    "labels": {
      "name": "my-first-k8s-node"
    }
  }
}

Flow

• Kubernetes internally creates a node object

• Kubernetes checks whether the kubelet is registered with the API server matching the node's metadata.name field

  • If the node is healthy,

    • It becomes eligible to run pods

  • If it is not healthy,

    • The node is ignored for all cluster activities until it becomes healthy

Kubernetes `retains` invalid `node objects` and `checks whether the node is healthy`

To stop the health check, the user or a controller must explicitly delete the node object

Uniqueness of Node names

• Two nodes cannot have the same name at the same time

  • Kubernetes assumes that resources with the same name are the same object!

• For nodes, it is implicitly assumed that instances using the same name have the same state (ex. network settings, root disk contents) and the same attributes such as node labels

  • If an instance is modified without changing its name, this can lead to inconsistencies!

  • Therefore, when replacing or updating a node, you must first remove the existing node object from the API server and re-add it after the update!

Node Status

A node's status includes the following information

1. Addresses

2. Conditions

3. Capacity and Allocatable

4. Info

Check node status and details using kubectl

kubectl describe node <NODE_NAME>

The information output by the above command includes addresses, conditions, capacity and allocatable, and info as described below.

Addresses

The addresses field varies depending on the cloud provider or bare metal setup

• HostName

  • The hostname as reported by the node's kernel

  • Can be overridden via the -hostname-override parameter

• ExternalIP

  • Typically, the node's IP address is externally routable

  • That is, it is accessible from outside the cluster

• InternalIP

  • Typically, the node's IP address is only routable within the cluster

Conditions

The conditions field describes all Running state nodes

Node Condition	Description
Ready	True if the node is healthy and ready to accept pods, False if the node is unhealthy and not accepting pods, Unknown if the node controller has not received a response from the node within the last node-monitor-grace-period (default 40 seconds)
DiskPressure	True if there is pressure on disk size, meaning disk capacity is low, False otherwise
MemoryPressure	True if there is pressure on node memory, meaning node memory is low, False otherwise
PIDPressure	True if there is pressure on processes, meaning there are many processes on the node, False otherwise
NetworkUnavailable	True if the network for the node is not correctly configured, False otherwise

Capacity and Allocatable

• Represents the available resources on a node

  • Resources include CPU, memory, and the maximum number of pods that can be scheduled on the node

• The capacity block fields represent the total amount of resources on the node

  • The allocatable block represents the amount of node resources available for regular pods

Info

• Describes general information about the node such as kernel version, Kubernetes version (kubelet and kube-proxy versions), container runtime details, and the operating system the node uses

  • This information is collected by kubelet from the node and sent to the Kubernetes API