Concepts

Detailed explanations of Kubernetes system concepts and abstractions.

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Kubernetes Components

This document outlines the various binary components needed to deliver a functioning Kubernetes cluster.

Master Components

Master components provide the cluster’s control plane. Master components make global decisions about the cluster (for example, scheduling), and detecting and responding to cluster events (starting up a new pod when a replication controller’s ‘replicas’ field is unsatisfied).

Master components can be run on any machine in the cluster. However, for simplicity, set up scripts typically start all master components on the same machine, and do not run user containers on this machine. See Building High-Availability Clusters for an example multi-master-VM setup.

kube-apiserver

Component on the master that exposes the Kubernetes API. It is the front-end for the Kubernetes control plane.

It is designed to scale horizontally – that is, it scales by deploying more instances. See Building High-Availability Clusters.

etcd

Consistent and highly-available key value store used as Kubernetes’ backing store for all cluster data.

Always have a backup plan for etcd’s data for your Kubernetes cluster. For in-depth information on etcd, see etcd documentation.

kube-scheduler

Component on the master that watches newly created pods that have no node assigned, 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

Component on the master that runs controllersA control loop that watches the shared state of the cluster through the apiserver and makes changes attempting to move the current state towards the desired state..

Logically, each controllerA control loop that watches the shared state of the cluster through the apiserver and makes changes attempting to move the current state towards the desired state. is a separate process, but to reduce complexity, they are all compiled into a single binary and run in a single process.

These controllers include:

cloud-controller-manager

cloud-controller-manager runs controllers that interact with the underlying cloud providers. The cloud-controller-manager binary is an alpha feature introduced in Kubernetes release 1.6.

cloud-controller-manager runs cloud-provider-specific controller loops only. You must disable these controller loops in the kube-controller-manager. You can disable the controller loops by setting the --cloud-provider flag to external when starting the kube-controller-manager.

cloud-controller-manager allows cloud vendors code and the Kubernetes core to evolve independent of each other. In prior releases, the core Kubernetes code was dependent upon cloud-provider-specific code for functionality. In future releases, code specific to cloud vendors should be maintained by the cloud vendor themselves, and linked to cloud-controller-manager while running Kubernetes.

The following controllers have cloud provider dependencies:

Node Components

Node components run on every node, maintaining running pods and providing the Kubernetes runtime environment.

kubelet

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

kube-proxy enables the Kubernetes service abstraction by maintaining network rules on the host and performing connection forwarding.

Container Runtime

The container runtime is the software that is responsible for running containers. Kubernetes supports several runtimes: Docker, rkt, runc and any OCI runtime-spec implementation.

Addons

Addons are pods and services that implement cluster features. The pods may be managed by Deployments, ReplicationControllers, and so on. Namespaced addon objects are created in the kube-system namespace.

Selected addons are described below, for an extended list of available addons please see Addons.

DNS

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.

Web UI (Dashboard)

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

Container Resource Monitoring records generic time-series metrics about containers in a central database, and provides a UI for browsing that data.

Cluster-level Logging

A Cluster-level logging mechanism is responsible for saving container logs to a central log store with search/browsing interface.

Analytics

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