Kubernetes Components

Node components run on every node, maintaining running Pods and providing the Kubernetes runtime environment. Here are the node components:

• kubelet The kubelet component interacts with the kube-apiserver, which runs on the control plane node, to pass API calls back and forth between each of the nodes. These API calls are used to run containers, manage any resources necessary, and works with the container engine to manage them on the local node.
• kube-proxy The kube-proxy component is a network proxy that runs on each node in your cluster, implementing part of the Kubernetes Service concept. The kube-proxy maintains network rules on nodes. These network rules allow network communication to your Pods from network sessions inside or outside of your cluster. The kube-proxy uses the operating system packet filtering layer if there is one and it's available. Otherwise, kube-proxy forwards the traffic itself.
• container runtime The container runtime is the software that is responsible for running containers. Kubernetes supports several container runtimes: Docker, containerd, CRI-O, and any implementation of the Kubernetes CRI (Container Runtime Interface).

The control plane's components make global decisions about the cluster (for example, scheduling), as well as detecting and responding to cluster events (for example, starting up a new Pod when a deployment's replicas field is unsatisfied). Control plane components can be run on any machine in the cluster. However, for simplicity, set up scripts typically start all control plane components on the same machine, and do not run user containers on this machine

• kube-apiserver The API server is the component of the Kubernetes control plane that exposes the Kubernetes API. The API server is the front end of the Kubernetes control plane and is designed to scale horizontally. You can run several instances of the kube-apiserver and balance traffic between those instances.
• etcd The etcd component is the consistent and highly-available key value store used as Kubernetes' backing store for all cluster data. If your Kubernetes cluster uses etcd as its backing store, make sure you have a back up plan for your data. More inforamtion on backing up etcd can be found here: backing up etcd.
• kube-scheduler The kube-scheduler is the control plane component that watches for newly created Pods with no assigned node, and selects a node for them to run on. Factors taken into account for scheduling decisions include: individual and collective resource requirements, hardware/software/policy constraints, affinity and anti-affinity specifications, data locality, inter-workload interference, and deadlines.
• kube-controller-manager The kube-controller-manager is the control plane component that runs controller processes. Logically, each controller is a separate process, but to reduce complexity, they are all compiled into a single binary and run in a single process. Some types of these controllers are:

  Node controller

  Responsible for noticing and responding when node go down.

  Job controller

  Wathes for job objects that represent one-off tasks, then creates Pods to run those tasks to completion.

  Endpoints controller

  Populates the endpoints object (that is, joins Services and Pods).

  Service Account & Token controllers

  Create default accounts and API access tokens for new namespaces.

• cloud-controller-manager The cloud-controller-manager is the control plane component that embeds cloud-specific control logic. The cloud controller manager lets you link your cluster into your cloud provider's API, and separates out the components that interact with that cloud platform form components that interact with that cloud platform from components that only interact with your cluster. The cloud-controller-manager only runs controllers that are specific to your cloud provider. If you are running Kubernetes on your own premises, or in a learning environment inside your own PC, the cluster does not have a cloud-controller-manager. As with the kube-controller-manager, the cloud-controller-manager combines several logically independent control loops into a single binary that you run as a single process. You can scale horizontally (run more than one copy) to improve performance or to help tolerate failures. The following controllers can have cloud provider dependencies:

  Node controller

  For checking the cloud provider to determine if a node has been deleted in the cloud after it stops responding.

  Route controller

  For setting up routes in the underlying cloud infrastructure.

  Service controller

  For creating, updating, and deleting cloud provider load balancers.