Overview

Within OpenShift Online, Kubernetes manages containerized applications across a set of containers or hosts and provides mechanisms for deployment, maintenance, and application-scaling. The Docker service packages, instantiates, and runs containerized applications. A Kubernetes cluster consists of one or more masters and a set of nodes.

OpenShift Online uses Kubernetes 1.9 and Docker 1.13.

Masters

The master is the host or hosts that contain the master components, including the API server, controller manager server, and etcd. The master manages nodes in its Kubernetes cluster and schedules pods to run on nodes.

Table 1. Master Components
Component Description

API Server

The Kubernetes API server validates and configures the data for pods, services, and replication controllers. It also assigns pods to nodes and synchronizes pod information with service configuration. Can be run as a standalone process.

etcd

etcd stores the persistent master state while other components watch etcd for changes to bring themselves into the desired state. etcd can be optionally configured for high availability, typically deployed with 2n+1 peer services.

Controller Manager Server

The controller manager server watches etcd for changes to replication controller objects and then uses the API to enforce the desired state. Can be run as a standalone process. Several such processes create a cluster with one active leader at a time.

Nodes

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.

Kubelet

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 manifest.

Service Proxy

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)
metadata:
  creationTimestamp: null
  labels: (3)
    kubernetes.io/hostname: node1.example.com
  name: node1.example.com (4)
spec:
  externalID: node1.example.com (5)
status:
  nodeInfo:
    bootID: ""
    containerRuntimeVersion: ""
    kernelVersion: ""
    kubeProxyVersion: ""
    kubeletVersion: ""
    machineID: ""
    osImage: ""
    systemUUID: ""
1 apiVersion defines the API version to use.
2 kind set to Node identifies this as a definition for a node object.
3 metadata.labels lists any labels that have been added to the node.
4 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 command.
5 spec.externalID defines the fully-qualified domain name where the node can be reached. Defaults to the metadata.name value when empty.