A node provides the runtime environments for containers. Each node in a
Kubernetes cluster has the required services to be managed by the
master. Nodes also have the required services to run pods,
including the Docker service, a kubelet, and a service proxy.
OpenShift Online creates nodes from a cloud provider, physical systems, or virtual
systems. Kubernetes interacts with node objects
that are a representation of those nodes. The master uses the information from
node objects to validate nodes with health checks. A node is ignored until it
passes the health checks, and the master continues checking nodes until they are
valid. The Kubernetes documentation has more information on node management.
Each node has a kubelet that updates the node as specified by a container
manifest, which is a YAML file that describes a pod. The kubelet uses a set of
manifests to ensure that its containers are started and that they
continue to run.
A container manifest can be provided to a kubelet by:
A file path on the command line that is checked every 20 seconds.
An HTTP endpoint passed on the command line that is checked every 20 seconds.
The kubelet watching an etcd server, such as /registry/hosts/$(hostname -f), and acting on any changes.
The kubelet listening for HTTP and responding to a simple API to submit a new
Each node also runs a simple network proxy that reflects the services defined in
the API on that node. This allows the node to do simple TCP and UDP stream
forwarding across a set of back ends.
Node Object Definition
The following is an example node object definition in Kubernetes:
apiVersion: v1 (1)
kind: Node (2)
name: node1.example.com (4)
externalID: node1.example.com (5)
apiVersion defines the API version to use.
kind set to
Node identifies this as a definition for a node
metadata.labels lists any
labels that have been added
to the node.
metadata.name is a required value that defines the name of the node
object. This value is shown in the
NAME column when running the
oc get nodes
spec.externalID defines the fully-qualified domain name where the node
can be reached. Defaults to the
metadata.name value when empty.