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128 lines
5.3 KiB
Markdown
128 lines
5.3 KiB
Markdown
# Node
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## What is a node?
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`Node` is a worker node in Kubernetes, previously known as `Minion`. Node
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may be a VM or physical machine, depending on the cluster. Each node has
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the services necessary to run [Pods](pods.md) and be managed from the master
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systems. The services include docker, kubelet and network proxy. See
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[The Kubernetes Node](../DESIGN.md#the-kubernetes-node) section in design
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doc for more details.
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## Node Status
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Node status describes current status of a node. For now, there are three
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pieces of information:
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### HostIP
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Host IP address is queried from cloudprovider and stored as part of node
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status. If kubernetes runs without cloudprovider, node's ID will be used.
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IP address can change, and there are different kind of IPs, e.g. public
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IP, private IP, dynamic IP, ipv6, etc. It makes more sense to save it as
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a status rather than spec.
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### Node Phase
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Node Phase is the current lifecycle phase of node, one of `Pending`,
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`Running` and `Terminated`. Node Phase management is under development,
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here is a brief overview: In kubernetes, node will be created in `Pending`
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phase, until it is discovered and checked in by kubernetes, at which time,
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kubernetes will mark it as `Running`. The end of a node's lifecycle is
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`Terminated`. A terminated node will not receive any scheduling request,
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and any running pods will be removed from the node.
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Node with `Running` phase is necessary but not sufficient requirement for
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scheduling Pods. For a node to be considered a scheduling candidate, it
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must have appropriate conditions, see below.
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### Node Condition
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Node Condition describes the conditions of `Running` nodes. Current valid
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condition is `NodeReady`. In the future, we plan to add more.
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`NodeReady` means kubelet is healthy and ready to accept pods. Different
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condition provides different level of understanding for node health.
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Node condition is represented as a json object. For example,
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the following conditions mean the node is in sane state:
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```json
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"conditions": [
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{
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"kind": "Ready",
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"status": "True",
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},
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]
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```
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## Node Management
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Unlike [Pod](pods.md) and [Service](services.md), `Node` is not inherently
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created by Kubernetes: it is either created from cloud providers like GCE,
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or from your physical or virtual machines. What this means is that when
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Kubernetes creates a node, it only creates a representation for the node.
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After creation, Kubernetes will check whether the node is valid or not.
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For example, if you try to create a node from the following content:
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```json
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{
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"id": "10.1.2.3",
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"kind": "Minion",
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"apiVersion": "v1beta1",
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"resources": {
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"capacity": {
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"cpu": 1000,
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"memory": 1073741824
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},
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},
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"labels": {
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"name": "my-first-k8s-node",
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},
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}
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```
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Kubernetes will create a `Node` object internally (the representation), and
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validate the node by health checking based on the `id` field: we assume `id`
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can be resolved. If the node is valid, i.e. all necessary services are running,
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it is eligible to run a `Pod`; otherwise, it will be ignored for any cluster
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activity, until it becomes valid. Note that Kubernetes will keep invalid node
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unless explicitly deleted by client, and it will keep checking to see if it
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becomes valid.
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Currently, there are two agents that interacts with Kubernetes node interface:
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Node Controller and Kube Admin.
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### Node Controller
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Node controller is a component in Kubernetes master which manages `Node`
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objects. It performs two major functions: cluster-wide node synchronization
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and single node life-cycle management.
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Node controller has a sync loop that creates/deletes `Node`s from Kubernetes
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based on all matching VM instances listed from cloud provider. The sync period
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can be controlled via flag "--node_sync_period". If a new instance
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gets created, Node Controller creates a representation for it. If an existing
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instance gets deleted, Node Controller deletes the representation. Note however,
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Node Controller is unable to provision the node for you, i.e. it won't install
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any binary; therefore, to
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join Kubernetes cluster, you as an admin need to make sure proper services are
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running in the node. In the future, we plan to automatically provision some node
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services. In case of no cloud provider, Node Controller simply registers all
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machines from `--machines` flag, any further interactions need to be done manually
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by using `kubectl`. If you are paranoid, leave `--machines` empty and create all
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machines from `kubectl` one by one - the two approaches are equivalent.
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Optionally you can skip cluster-wide node synchronization with
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'--sync_nodes=false' and can use REST api/kubectl cli to add/remove nodes.
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Node life-cycle management in the Node Controller is still under development, it
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is supposed to manage the Node Status Specification defined above.
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### Manual Node Administration
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A Kubernetes administrator typically uses `kubectl` to manage `Node`. Similar
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to Node Controller, `kubectl` command only creates/deletes node representation.
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Note if Kubernetes is running on cloud provider, `kubectl create` a node will
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be refused if Node Controller has already synchronized nodes from cloud provider.
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Admin can choose to make the node unschedulable using `kubectl`. Unscheduling the node
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will not affect any existing pods on the node but it will disable creation of
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any new pods on the node. Node unschedulable example:
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```
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kubectl update nodes 10.1.2.3 --patch='{"apiVersion": "v1beta1", "unschedulable": true}'
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```
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