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## 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](/docs/user-guide/pods) and is managed by the master
components. The services on a node include docker, kubelet and network proxy. See
[The Kubernetes Node](https://github.com/kubernetes/kubernetes/blob/{{page.githubbranch}}/docs/design/architecture.md#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.

* HostName: Generally not used

* ExternalIP: Generally the IP address of the node that is externally routable (available from outside the cluster)

* InternalIP: Generally the IP address of the node that is routable only within the cluster


### Node Phase

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

* Pending: New nodes are created in this state. A node stays in this state until it is configured.

* Running: Node has been configured and the Kubernetes components are running

* Terminated: Node has been removed from the cluster. It will not receive any scheduling requests,
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. 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:

```json
"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](/docs/user-guide/pods) and [Services](/docs/user-guide/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:

```json
{
  "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:

  - `--api-servers=` 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,
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:

```shell
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](#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:

```yaml
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](/docs/api-reference/v1/definitions/#_v1_node).
