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Node Reference Information

This section contains the following reference topics about nodes:

You can also read node reference details from elsewhere in the Kubernetes documentation, including:

1 - Kubelet Checkpoint API

FEATURE STATE: Kubernetes v1.25 [alpha]

Checkpointing a container is the functionality to create a stateful copy of a running container. Once you have a stateful copy of a container, you could move it to a different computer for debugging or similar purposes.

If you move the checkpointed container data to a computer that's able to restore it, that restored container continues to run at exactly the same point it was checkpointed. You can also inspect the saved data, provided that you have suitable tools for doing so.

Creating a checkpoint of a container might have security implications. Typically a checkpoint contains all memory pages of all processes in the checkpointed container. This means that everything that used to be in memory is now available on the local disk. This includes all private data and possibly keys used for encryption. The underlying CRI implementations (the container runtime on that node) should create the checkpoint archive to be only accessible by the root user. It is still important to remember if the checkpoint archive is transferred to another system all memory pages will be readable by the owner of the checkpoint archive.

Operations

post checkpoint the specified container

Tell the kubelet to checkpoint a specific container from the specified Pod.

Consult the Kubelet authentication/authorization reference for more information about how access to the kubelet checkpoint interface is controlled.

The kubelet will request a checkpoint from the underlying CRI implementation. In the checkpoint request the kubelet will specify the name of the checkpoint archive as checkpoint-<podFullName>-<containerName>-<timestamp>.tar and also request to store the checkpoint archive in the checkpoints directory below its root directory (as defined by --root-dir). This defaults to /var/lib/kubelet/checkpoints.

The checkpoint archive is in tar format, and could be listed using an implementation of tar. The contents of the archive depend on the underlying CRI implementation (the container runtime on that node).

HTTP Request

POST /checkpoint/{namespace}/{pod}/{container}

Parameters

  • namespace (in path): string, required

    Namespace

  • pod (in path): string, required

    Pod

  • container (in path): string, required

    Container

  • timeout (in query): integer

    Timeout in seconds to wait until the checkpoint creation is finished. If zero or no timeout is specified the default CRI timeout value will be used. Checkpoint creation time depends directly on the used memory of the container. The more memory a container uses the more time is required to create the corresponding checkpoint.

Response

200: OK

401: Unauthorized

404: Not Found (if the ContainerCheckpoint feature gate is disabled)

404: Not Found (if the specified namespace, pod or container cannot be found)

500: Internal Server Error (if the CRI implementation encounter an error during checkpointing (see error message for further details))

500: Internal Server Error (if the CRI implementation does not implement the checkpoint CRI API (see error message for further details))

2 - Articles on dockershim Removal and on Using CRI-compatible Runtimes

This is a list of articles and other pages that are either about the Kubernetes' deprecation and removal of dockershim, or about using CRI-compatible container runtimes, in connection with that removal.

Kubernetes project

You can provide feedback via the GitHub issue Dockershim removal feedback & issues. (k/kubernetes/#106917)

External sources

3 - Node Labels Populated By The Kubelet

Kubernetes nodes come pre-populated with a standard set of labels.

You can also set your own labels on nodes, either through the kubelet configuration or using the Kubernetes API.

Preset labels

The preset labels that Kubernetes sets on nodes are:

What's next

4 - Kubelet Configuration Directory Merging

When using the kubelet's --config-dir flag to specify a drop-in directory for configuration, there is some specific behavior on how different types are merged.

Here are some examples of how different data types behave during configuration merging:

Structure Fields

There are two types of structure fields in a YAML structure: singular (or a scalar type) and embedded (structures that contain scalar types). The configuration merging process handles the overriding of singular and embedded struct fields to create a resulting kubelet configuration.

For instance, you may want a baseline kubelet configuration for all nodes, but you may want to customize the address and authorization fields. This can be done as follows:

Main kubelet configuration file contents:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
port: 20250
authorization:
  mode: Webhook
  webhook:
    cacheAuthorizedTTL: "5m"
    cacheUnauthorizedTTL: "30s"
serializeImagePulls: false
address: "192.168.0.1"

Contents of a file in --config-dir directory:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
authorization:
  mode: AlwaysAllow
  webhook:
    cacheAuthorizedTTL: "8m"
    cacheUnauthorizedTTL: "45s"
address: "192.168.0.8"

The resulting configuration will be as follows:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
port: 20250
serializeImagePulls: false
authorization:
  mode: AlwaysAllow
  webhook:
    cacheAuthorizedTTL: "8m"
    cacheUnauthorizedTTL: "45s"
address: "192.168.0.8"

Lists

You can overide the slices/lists values of the kubelet configuration. However, the entire list gets overridden during the merging process. For example, you can override the clusterDNS list as follows:

Main kubelet configuration file contents:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
port: 20250
serializeImagePulls: false
clusterDNS:
  - "192.168.0.9"
  - "192.168.0.8"

Contents of a file in --config-dir directory:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
clusterDNS:
  - "192.168.0.2"
  - "192.168.0.3"
  - "192.168.0.5"

The resulting configuration will be as follows:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
port: 20250
serializeImagePulls: false
clusterDNS:
  - "192.168.0.2"
  - "192.168.0.3"
  - "192.168.0.5"

Maps, including Nested Structures

Individual fields in maps, regardless of their value types (boolean, string, etc.), can be selectively overridden. However, for map[string][]string, the entire list associated with a specific field gets overridden. Let's understand this better with an example, particularly on fields like featureGates and staticPodURLHeader:

Main kubelet configuration file contents:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
port: 20250
serializeImagePulls: false
featureGates:
  AllAlpha: false
  MemoryQoS: true
staticPodURLHeader:
  kubelet-api-support:
  - "Authorization: 234APSDFA"
  - "X-Custom-Header: 123"
  custom-static-pod:
  - "Authorization: 223EWRWER"
  - "X-Custom-Header: 456"

Contents of a file in --config-dir directory:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
featureGates:
  MemoryQoS: false
  KubeletTracing: true
  DynamicResourceAllocation: true
staticPodURLHeader:
  custom-static-pod:
  - "Authorization: 223EWRWER"
  - "X-Custom-Header: 345"

The resulting configuration will be as follows:

apiVersion: kubelet.config.k8s.io/v1beta1
kind: KubeletConfiguration
port: 20250
serializeImagePulls: false
featureGates:
  AllAlpha: false
  MemoryQoS: false
  KubeletTracing: true
  DynamicResourceAllocation: true
staticPodURLHeader:
  kubelet-api-support:
  - "Authorization: 234APSDFA"
  - "X-Custom-Header: 123"
  custom-static-pod:
  - "Authorization: 223EWRWER"
  - "X-Custom-Header: 345"

5 - Kubelet Device Manager API Versions

This page provides details of version compatibility between the Kubernetes device plugin API, and different versions of Kubernetes itself.

Compatibility matrix

v1alpha1v1beta1
Kubernetes 1.21-✓
Kubernetes 1.22-✓
Kubernetes 1.23-✓
Kubernetes 1.24-✓
Kubernetes 1.25-✓
Kubernetes 1.26-✓

Key:

  • ✓ Exactly the same features / API objects in both device plugin API and the Kubernetes version.
  • + The device plugin API has features or API objects that may not be present in the Kubernetes cluster, either because the device plugin API has added additional new API calls, or that the server has removed an old API call. However, everything they have in common (most other APIs) will work. Note that alpha APIs may vanish or change significantly between one minor release and the next.
  • - The Kubernetes cluster has features the device plugin API can't use, either because server has added additional API calls, or that device plugin API has removed an old API call. However, everything they share in common (most APIs) will work.

6 - Node Status

The status of a node in Kubernetes is a critical aspect of managing a Kubernetes cluster. In this article, we'll cover the basics of monitoring and maintaining node status to ensure a healthy and stable cluster.

Node status fields

A Node's status contains the following information:

You can use kubectl to view a Node's status and other details:

kubectl describe node <insert-node-name-here>

Each section of the output is described below.

Addresses

The usage of these fields varies depending on your cloud provider or bare metal configuration.

  • HostName: The hostname as reported by the node's kernel. Can be overridden via the kubelet --hostname-override parameter.
  • ExternalIP: Typically the IP address of the node that is externally routable (available from outside the cluster).
  • InternalIP: Typically the IP address of the node that is routable only within the cluster.

Conditions

The conditions field describes the status of all Running nodes. Examples of conditions include:

Node conditions, and a description of when each condition applies.
Node ConditionDescription
ReadyTrue if the node is healthy and ready to accept pods, False if the node is not healthy and is not accepting pods, and Unknown if the node controller has not heard from the node in the last node-monitor-grace-period (default is 40 seconds)
DiskPressureTrue if pressure exists on the disk size—that is, if the disk capacity is low; otherwise False
MemoryPressureTrue if pressure exists on the node memory—that is, if the node memory is low; otherwise False
PIDPressureTrue if pressure exists on the processes—that is, if there are too many processes on the node; otherwise False
NetworkUnavailableTrue if the network for the node is not correctly configured, otherwise False

In the Kubernetes API, a node's condition is represented as part of the .status of the Node resource. For example, the following JSON structure describes a healthy node:

"conditions": [
  {
    "type": "Ready",
    "status": "True",
    "reason": "KubeletReady",
    "message": "kubelet is posting ready status",
    "lastHeartbeatTime": "2019-06-05T18:38:35Z",
    "lastTransitionTime": "2019-06-05T11:41:27Z"
  }
]

When problems occur on nodes, the Kubernetes control plane automatically creates taints that match the conditions affecting the node. An example of this is when the status of the Ready condition remains Unknown or False for longer than the kube-controller-manager's NodeMonitorGracePeriod, which defaults to 40 seconds. This will cause either an node.kubernetes.io/unreachable taint, for an Unknown status, or a node.kubernetes.io/not-ready taint, for a False status, to be added to the Node.

These taints affect pending pods as the scheduler takes the Node's taints into consideration when assigning a pod to a Node. Existing pods scheduled to the node may be evicted due to the application of NoExecute taints. Pods may also have tolerations that let them schedule to and continue running on a Node even though it has a specific taint.

See Taint Based Evictions and Taint Nodes by Condition for more details.

Capacity and Allocatable

Describes the resources available on the node: CPU, memory, and the maximum number of pods that can be scheduled onto the node.

The fields in the capacity block indicate the total amount of resources that a Node has. The allocatable block indicates the amount of resources on a Node that is available to be consumed by normal Pods.

You may read more about capacity and allocatable resources while learning how to reserve compute resources on a Node.

Info

Describes general information about the node, such as kernel version, Kubernetes version (kubelet and kube-proxy version), container runtime details, and which operating system the node uses. The kubelet gathers this information from the node and publishes it into the Kubernetes API.

Heartbeats

Heartbeats, sent by Kubernetes nodes, help your cluster determine the availability of each node, and to take action when failures are detected.

For nodes there are two forms of heartbeats:

  • updates to the .status of a Node
  • Lease objects within the kube-node-lease namespace. Each Node has an associated Lease object.

Compared to updates to .status of a Node, a Lease is a lightweight resource. Using Leases for heartbeats reduces the performance impact of these updates for large clusters.

The kubelet is responsible for creating and updating the .status of Nodes, and for updating their related Leases.

  • The kubelet updates the node's .status either when there is change in status or if there has been no update for a configured interval. The default interval for .status updates to Nodes is 5 minutes, which is much longer than the 40 second default timeout for unreachable nodes.
  • The kubelet creates and then updates its Lease object every 10 seconds (the default update interval). Lease updates occur independently from updates to the Node's .status. If the Lease update fails, the kubelet retries, using exponential backoff that starts at 200 milliseconds and capped at 7 seconds.