Node Server Components


In Kubernetes, servers that perform work by running containers are known as nodes. Node servers have a few requirements that are necessary for communicating with master components, configuring the container networking, and running the actual workloads assigned to them.

A Container Runtime

The first component that each node must have is a container runtime. Typically, this requirement is satisfied by installing and running Docker, but alternatives like rkt and runc are also available.
The container runtime is responsible for starting and managing containers, applications encapsulated in a relatively isolated but lightweight operating environment. Each unit of work on the cluster is, at its basic level, implemented as one or more containers that must be deployed. The container runtime on each node is the component that finally runs the containers defined in the workloads submitted to the cluster.

kubelet

The main contact point for each node with the cluster group is a small service called kubelet. This service is responsible for relaying information to and from the control plane services, as well as interacting with the etcd store to read configuration details or write new values.
The kubelet service communicates with the master components to authenticate to the cluster and receive commands and work. Work is received in the form of a manifest which defines the workload and the operating parameters. The kubelet process then assumes responsibility for maintaining the state of the work on the node server. It controls the container runtime to launch or destroy containers as needed.

kube-proxy

To manage individual host subnetting and make services available to other components, a small proxy service called kube-proxy is run on each node server. This process forwards requests to the correct containers, can do primitive load balancing, and is generally responsible for making sure the networking environment is predictable and accessible, but isolated where appropriate.