If you are using a released version of Kubernetes, you should refer to the docs that go with that version.
The latest 1.0.x release of this document can be found [here](http://releases.k8s.io/release-1.0/docs/admin/node.md).Documentation for other releases can be found at releases.k8s.io.
Table of Contents
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 describes current status of a node. For now, there are the following pieces of information:
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 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 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.
Describes the resources available on the node: CPUs, memory and the maximum number of pods that can be scheduled onto the node.
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.
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 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 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.
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.--kubeconfigtells 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-nodetells 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.)
If your cluster runs short on resources you can easily add more machines to it if your cluster is running in Node self-registration mode. If you're using GCE or GKE it's done by resizing Instance Group managing your Nodes. It can be accomplished by modifying number of instances on Compute > Compute Engine > Instance groups > your group > Edit group Google Cloud Console page or using gcloud CLI:
gcloud preview managed-instance-groups --zone compute-zone resize my-cluster-minon-group --new-size 42
Instance Group will take care of putting appropriate image on new machines and start them, while Kubelet will register its Node with API server to make it available for scheduling. If you scale the instance group down, system will randomly choose Nodes to kill.
In other environments you may need to configure the machine yourself and tell the Kubelet on which machine API server is running.
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 replace nodes 10.1.2.3 --patch='{"apiVersion": "v1", "unschedulable": true}'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 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: 100MiSet 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.
Node is a top-level resource in the kubernetes REST API. More details about the API object can be found at: Node API object.