node.md

Node

What is a node?

Node is a worker machine in Kubernetes, previously known as Minion. Node may be a VM or physical machine, depending on the cluster. Each node has the services necessary to run Pods and be managed from the master systems. The services include docker, kubelet and network proxy. See The Kubernetes Node section in design doc for more details.

Node Status

Node status describes current status of a node. For now, there are the following pieces of information:

Node Addresses

Host IP address is queried from cloudprovider and stored as part of node status. If kubernetes runs without cloudprovider, node's ID will be used. IP address can change, and there are different kind of IPs, e.g. public IP, private IP, dynamic IP, ipv6, etc. It makes more sense to save it as a status rather than spec.

Node Phase

Node Phase is the current lifecycle phase of node, one of Pending, Running and Terminated. Node Phase management is under development, here is a brief overview: In kubernetes, node will be created in Pending phase, until it is discovered and checked in by kubernetes, at which time, kubernetes will mark it as Running. The end of a node's lifecycle is Terminated. A terminated node will not receive any scheduling request, and any running pods will be removed from the node.

Node with Running phase is necessary but not sufficient requirement for scheduling Pods. For a node to be considered a scheduling candidate, it must have appropriate conditions, see below.

Node Condition

Node Condition describes the conditions of Running nodes. (However, it can be present also when node status is different, e.g. Unknown) Current valid condition is Ready. In the future, we plan to add more. Ready means kubelet is healthy and ready to accept pods. Different condition provides different level of understanding for node health. Node condition is represented as a json object. For example, the following conditions mean the node is in sane state:

"conditions": [
  {
    "kind": "Ready",
    "status": "True",
    },
]

Node Capacity

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

Node Info

General information about the node, for instance kernel version, kubernetes version, docker version (if used), OS name. The information is gathered by Kubernetes from the node.

Node Management

Unlike Pods and Services, a Node is not inherently created by Kubernetes: it is either created from cloud providers like GCE, or from your physical or virtual machines. What this means is that when Kubernetes creates a node, it only creates a representation for the node. After creation, Kubernetes will check whether the node is valid or not. For example, if you try to create a node from the following content:

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

Kubernetes will create a Node object internally (the representation), and validate the node by health checking based on the metadata.name field: we assume metadata.name can be resolved. If the node is valid, i.e. all necessary services are running, it is eligible to run a Pod; otherwise, it will be ignored for any cluster activity, until it becomes valid. Note that Kubernetes will keep invalid node unless explicitly deleted by client, and it will keep checking to see if it becomes valid.

Currently, there are two agents that interacts with Kubernetes node interface: Node Controller and Kube Admin.

Node Controller

Node controller is a component in Kubernetes master which manages Node objects. It performs two major functions: cluster-wide node synchronization and single node life-cycle management.

Node controller has a sync loop that creates/deletes Nodes from Kubernetes based on all matching VM instances listed from cloud provider. The sync period can be controlled via flag --node_sync_period. If a new instance gets created, Node Controller creates a representation for it. If an existing instance gets deleted, Node Controller deletes the representation. Note however, Node Controller is unable to provision the node for you, i.e. it won't install any binary; therefore, to join Kubernetes cluster, you as an admin need to make sure proper services are running in the node. In the future, we plan to automatically provision some node services.

Self-Registration of nodes

When kubelet flag --register-node is true (the default), then the kubelet will attempt to register itself with the API server. This is the preferred pattern, used by most distros.

For self-registration, the kubelet is started with the following options:

• --apiservers= tells the kubelet the location of the apiserver.
• --kubeconfig tells kubelet where to find credentials to authenticate itself to the apiserver.
• --cloud_provider= tells the kubelet how to talk to a cloud provider to read metadata about itself.
• --register-node tells the kubelet to create its own node resource.

Currently, any kubelet is authorized to create/modify any node resource, but in practice it only creates/modifies its own. (In the future, we plan to limit authorization to only allow a kubelet to modify its own Node resource.)

Manual Node Administration

A cluster administrator can create and modify Node objects.

If the administrator wishes to create node objects manually, set kubelet flag --register-node=false.

The administrator can modify Node resources (regardless of the setting of --register-node). Modifications include setting labels on the Node, and marking it unschedulable.

Labels on nodes can be used in conjunction with node selectors on pods to control scheduling.

Making a node unscheduleable will prevent new pods from being scheduled to that node, but will not affect any existing pods on the node. This is useful as a preparatory step before a node reboot, etc. For example, to mark a node unschedulable, run this command:

kubectl update nodes 10.1.2.3 --patch='{"apiVersion": "v1", "unschedulable": true}'