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Design docs, concept definitions, and references for APIs and CLIs.

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Nodes

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 is managed by the master components. The services on a node include docker, kubelet and network proxy. See The Kubernetes Node section in the architecture 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

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

Node Phase

Node Phase is the current lifecycle phase of node, one of Pending, Running and Terminated.

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. Currently the only node condition is Ready. The Status of this condition can be True, False, or Unknown. True means the Kubelet is healthy and ready to accept pods. False means the Kubelet is not healthy and is not accepting pods. Unknown means the Node Controller, which manages node lifecycle and is responsible for setting the Status of the condition, has not heard from the node recently (currently 40 seconds). 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"
  }
]

If the Status of the Ready condition is Unknown or False for more than five minutes, then all of the Pods on the node are terminated by the Node Controller.

Node Capacity

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

Node Info

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

Node Management

Unlike Pods and Services, a Node is not inherently created by Kubernetes: it is either taken from cloud providers like Google Compute Engine, or from your pool of physical or virtual machines. What this means is that when Kubernetes creates a node, it is really just creating an object that represents the node in its internal state. 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 the object for the invalid node unless it is explicitly deleted by the client, and it will keep checking to see if it becomes valid.

Currently, there are three components that interact with the Kubernetes node interface: Node Controller, Kubelet, and kubectl.

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 deletes Nodes from Kubernetes based on all matching VM instances listed from the cloud provider. The sync period can be controlled via flag --node-sync-period. If a new VM instance gets created, Node Controller creates a representation for it. If an existing instance gets deleted, Node Controller deletes the representation. Note however, that Node Controller is unable to provision the node for you, i.e. it won’t install any binary; therefore, to join a node to a 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.

In general, node controller is responsible for updating the NodeReady condition of node status to ConditionUnknown when a node becomes unreachable (e.g. due to the node being down), and then later evicting all the pods from the node (using graceful termination) if the node continues to be unreachable. (The current timeouts for those are 40s and 5m, respectively.) It also allocates CIDR blocks to the new nodes.

Self-Registration of Nodes

When kubelet flag --register-node is true (the default), 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:

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, e.g. to constrain a Pod to only be eligible to run on a subset of the nodes.

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 patch nodes $NODENAME -p '{"spec": {"unschedulable": true}}'

Note that pods which are created by a daemonSet controller bypass the Kubernetes scheduler, and do not respect the unschedulable attribute on a node. The assumption is that daemons belong on the machine even if it is being drained of applications in preparation for a reboot.

Node capacity

The capacity of the node (number of cpus and amount of memory) is part of the node resource. Normally, nodes register themselves and report their capacity when creating the node resource. If you are doing manual node administration, then you need to set node capacity when adding a node.

The Kubernetes scheduler ensures that there are enough resources for all the pods on a node. It checks that the sum of the limits of containers on the node is no greater than the node capacity. It includes all containers started by kubelet, but not containers started directly by docker, nor processes not in containers.

If you want to explicitly reserve resources for non-Pod processes, you can create a placeholder pod. Use the following template:

apiVersion: v1
kind: Pod
metadata:
  name: resource-reserver
spec:
  containers:
  - name: sleep-forever
    image: gcr.io/google_containers/pause:0.8.0
    resources:
      limits:
        cpu: 100m
        memory: 100Mi

Set the cpu and memory values to the amount of resources you want to reserve. Place the file in the manifest directory (--config=DIR flag of kubelet). Do this on each kubelet where you want to reserve resources.

API Object

Node is a top-level resource in the kubernetes REST API. More details about the API object can be found at: Node API object.

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