Chapter 2. Infrastructure Components

Within OpenShift Container Platform, 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.

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.

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 Container Platform 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 Container Platform instance using the CLI. To define full configuration and security options when launching node servers, use dedicated node configuration files.

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: ""

OpenShift Container Platform can utilize any server implementing the Docker registry API as a source of images, including the Docker Hub, private registries run by third parties, and the integrated OpenShift Container Platform registry.

OpenShift Container Platform provides an integrated container registry called OpenShift Container Registry (OCR) that adds the ability to automatically provision new image repositories on demand. This provides users with a built-in location for their application builds to push the resulting images.

Whenever a new image is pushed to OCR, the registry notifies OpenShift Container Platform about the new image, passing along all the information about it, such as the namespace, name, and image metadata. Different pieces of OpenShift Container Platform react to new images, creating new builds and deployments.

OCR can also be deployed as a stand-alone component that acts solely as a container registry, without the build and deployment integration. See Installing a Stand-alone Deployment of OpenShift Container Registry for details.

OpenShift Container Platform can create containers using images from third party registries, but it is unlikely that these registries offer the same image notification support as the integrated OpenShift Container Platform registry. In this situation OpenShift Container Platform will fetch tags from the remote registry upon imagestream creation. Refreshing the fetched tags is as simple as running oc import-image <stream>. When new images are detected, the previously-described build and deployment reactions occur.

The OpenShift Container Platform web console is a user interface accessible from a web browser. Developers can use the web console to visualize, browse, and manage the contents of projects.

The web console is started as part of the master. All static assets required to run the web console are served from the openshift binary. Administrators can also customize the web console using extensions, which let you run scripts and load custom stylesheets when the web console loads. You can change the look and feel of nearly any aspect of the user interface in this way.

When you access the web console from a browser, it first loads all required static assets. It then makes requests to the OpenShift Container Platform APIs using the values defined from the openshift start option --public-master, or from the related master configuration file parameter masterPublicURL. The web console uses WebSockets to maintain a persistent connection with the API server and receive updated information as soon as it is available.

Figure 2.1. Web Console Request Architecture

The configured host names and IP addresses for the web console are whitelisted to access the API server safely even when the browser would consider the requests to be cross-origin. To access the API server from a web application using a different host name, you must whitelist that host name by specifying the --cors-allowed-origins option on openshift start or from the related master configuration file parameter corsAllowedOrigins.

The corsAllowedOrigins parameter is controlled by the configuration field. No pinning or escaping is done to the value. The following is an example of how you can pin a host name and escape dots:

corsAllowedOrigins:
- (?i)//my\.subdomain\.domain\.com(:|\z)

You can access CLI downloads from the Help icon in the web console:

Cluster administrators can customize these links further.

Review the tested integrations for OpenShift Container Platform.

After logging in, the web console provides developers with an overview for the currently selected project:

Figure 2.2. Web Console Project Overview

For pods based on Java images, the web console also exposes access to a hawt.io-based JVM console for viewing and managing any relevant integration components. A Connect link is displayed in the pod’s details on the Browse Pods page, provided the container has a port named jolokia.

Figure 2.3. Pod with a Link to the JVM Console

After connecting to the JVM console, different pages are displayed depending on which components are relevant to the connected pod.

Figure 2.4. JVM Console

The following pages are available:

StatefulSet was introduced as a Technology Preview feature in OpenShift Container Platform 3.5 and remains in Technology Preview.

A StatefulSet controller provides a unique identity to its pods and determines the order of deployments and scaling. StatefulSet is useful for unique network identifiers, persistent storage, graceful deployment and scaling, and graceful deletion and termination.

Figure 2.5. StatefulSet in OpenShift Container Platform