Kubernetes Infrastructure - Infrastructure Components | Architecture

Overview

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

OpenShift Origin uses Kubernetes 1.7 and Docker 1.12.

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.
HAProxy	Optional, used when configuring highly-available masters with the `native` method to balance load between API master endpoints. The advanced installation method can configure HAProxy for you with the `native` method. Alternatively, you can use the `native` method but pre-configure your own load balancer of choice.

High Availability Masters

While in a single master configuration, the availability of running applications remains if the master or any of its services fail. However, failure of master services reduces the ability of the system to respond to application failures or creation of new applications. You can optionally configure your masters for high availability (HA) to ensure that the cluster has no single point of failure.

To mitigate concerns about availability of the master, two activities are recommended:

A runbook entry should be created for reconstructing the master. A runbook entry is a necessary backstop for any highly-available service. Additional solutions merely control the frequency that the runbook must be consulted. For example, a cold standby of the master host can adequately fulfill SLAs that require no more than minutes of downtime for creation of new applications or recovery of failed application components.
Use a high availability solution to configure your masters and ensure that the cluster has no single point of failure. The advanced installation method provides specific examples using the native HA method and configuring HAProxy. You can also take the concepts and apply them towards your existing HA solutions using the native method instead of HAProxy.

Moving from a single master cluster to multiple masters after installation is not supported.

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 Docker service, a kubelet, and a service proxy.

OpenShift Origin 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.

Administrators can manage nodes in an OpenShift Origin instance using the CLI. To define full configuration and security options when launching node servers, use dedicated node configuration files.

See the cluster limits section for the recommended maximum number of nodes.

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.

The REST API Reference has more details on these definitions.