When you deploy Kubernetes, you get a cluster.
A Kubernetes cluster consists of a set of worker machines, called nodes,
that run containerized applications. Every cluster has at least one worker node.
The worker node(s) host the Pods that are
the components of the application workload. The
control plane manages the worker
nodes and the Pods in the cluster. In production environments, the control plane usually
runs across multiple computers and a cluster usually runs multiple nodes, providing
fault-tolerance and high availability.
This document outlines the various components you need to have for
a complete and working Kubernetes cluster.
The components of a Kubernetes cluster
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. See
Creating Highly Available clusters with kubeadm
for an example control plane setup that runs across multiple VMs.
The API server is a component of the Kubernetes
control plane that exposes the Kubernetes API.
The API server is the front end for the Kubernetes control plane.
The main implementation of a Kubernetes API server is kube-apiserver.
kube-apiserver is designed to scale horizontally—that is, it scales by deploying more instances.
You can run several instances of kube-apiserver and balance traffic between those instances.
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 those data.
You can find in-depth information about etcd in the official documentation.
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.
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:
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 components run on every node, maintaining running pods and providing the Kubernetes runtime environment.
An agent that runs on each node in the cluster. It makes sure that containers are running in a Pod.
The kubelet takes a set of PodSpecs that are provided through various mechanisms and ensures that the containers described in those PodSpecs are running and healthy. The kubelet doesn't manage containers which were not created by Kubernetes.
kube-proxy is a network proxy that runs on each
node in your cluster,
implementing part of the Kubernetes
maintains network rules on nodes. These network rules allow network
communication to your Pods from network sessions inside or outside of
kube-proxy uses the operating system packet filtering layer if there is one
and it's available. Otherwise, kube-proxy forwards the traffic itself.
The container runtime is the software that is responsible for running containers.
Kubernetes supports container runtimes such as
and any other implementation of the Kubernetes CRI (Container Runtime
Addons use Kubernetes resources (DaemonSet,
to implement cluster features. Because these are providing cluster-level features, namespaced resources
for addons belong within the kube-system namespace.
Selected addons are described below; for an extended list of available addons, please
While the other addons are not strictly required, all Kubernetes clusters should have cluster DNS, as many examples rely on it.
Cluster DNS is a DNS server, in addition to the other DNS server(s) in your environment, which serves DNS records for Kubernetes services.
Containers started by Kubernetes automatically include this DNS server in their DNS searches.
Dashboard is a general purpose, web-based UI for Kubernetes clusters. It allows users to manage and troubleshoot applications running in the cluster, as well as the cluster itself.
Container Resource Monitoring records generic time-series metrics
about containers in a central database, and provides a UI for browsing that data.
A cluster-level logging mechanism is responsible for
saving container logs to a central log store with search/browsing interface.
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