Chapter 2. Introduction to containers and orchestration with Kubernetes 33
A Kubernetes cluster consists of one or more masters and a set of nodes:
/SM590000Master
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 (see Table 2-2).
Table 2-2   OpenShift Master components
/SM590000Service Proxy
Each node also runs a simple network proxy that reflects the services that are defined in 
the API on that node. This feature allows the node to perform simple TCP and UDP stream 
forwarding across a set of back ends.
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.
2.5.4  Red Hat OpenShift access and control
OpenShift 3 allows access to the system from the command-line interface (CLI), web console, 
or Eclipse integrated development environment (IDE). For the purpose of this book, we focus 
on the use of the command-line tool and the web console.
Command Line Interface tool
The OpenShift Container Platform includes a CLI that allows running pre-configured 
commands for managing your applications. IT also includes lower-level tools to interact with 
each component of your system. The oc command-line tool, also known as OpenShift CLI, is 
used to interact with the OpenShift and Kubernetes HTTP API(s).
Component Description
api server The Kubernetes API server va lidates and configures the data for 
Pods, services, and replication controllers. It also assigns Pods to 
nodes and synchronizes Pod information with service configuration.
etcd The etcd component st ores the persistent master state while other 
components watch etcd for changes to bring themselves into the 
wanted state. Also, etcd optionally can be 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 
wanted state. Several such processes create a cluster with one 
active leader at a time.
HAProxy Optional, used when configuri ng highly available masters with the 
native method to balance load between API master endpoints. The 
cluster installation process can configure HAProxy for you by using 
the native method. Alternatively, you can use the native method, but 
pre-configure your own load balancer of choice.
Note: It is recommended to have more than three master nodes to provide high availability 
and quorum.