Kubernetes Infrastructure - Infrastructure Components | Architecture

Overview
Masters
- Control Plane Static Pods
- High Availability Masters
Nodes

Overview

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

You can optionally configure your masters for high availability (HA) to ensure that the cluster has no single point of failure.

OpenShift Dedicated 3 uses Kubernetes 1.11 and Docker 1.13.1.

Masters

The master is the host or hosts that contain the control plane components, including the API server, controller manager server, and etcd. The master manages nodes in its Kubernetes cluster and schedules pods to run on those 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.
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. Several such processes create a cluster with one active leader at a time.
HAProxy	Optional, used when configuring highly-available masters with the `native` method to balance load between API master endpoints.

Control Plane Static Pods

The core control plane components, the API server and the controller manager components, run as static pods operated by the kubelet.

For masters that have etcd co-located on the same host, etcd is also moved to static pods. RPM-based etcd is still supported on etcd hosts that are not also masters.

In addition, the node components openshift-sdn and openvswitch are now run using a DaemonSet instead of a systemd service.

Figure 1. Control plane host architecture changes

High Availability Masters

When using the native HA method with HAProxy, master components have the following availability:

Table 2. Availability Matrix with HAProxy
Role	Style	Notes
etcd	Active-active	Fully redundant deployment with load balancing.
API Server	Active-active	Managed by HAProxy.
Controller Manager Server	Active-passive	One instance is elected as a cluster leader at a time.
HAProxy	Active-passive	Balances load between API master endpoints.

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 container runtime, a kubelet, and a service proxy.

OpenShift Dedicated 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 statuses and 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.

Node Bootstrapping

A node’s configuration is bootstrapped from the master, which means nodes pull their pre-defined configuration and client and server certificates from the master. This allows faster node start-up by reducing the differences between nodes, as well as centralizing more configuration and letting the cluster converge on the desired state. Certificate rotation and centralized certificate management are enabled by default.

Figure 2. Node bootstrapping workflow overview

When node services are started, the node checks if the /etc/origin/node/node.kubeconfig file and other node configuration files exist before joining the cluster. If they do not, the node pulls the configuration from the master, then joins the cluster.

ConfigMaps are used to store the node configuration in the cluster, which populates the configuration file on the node host at /etc/origin/node/node-config.yaml.