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| <h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2> | ||
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| If you are using a released version of Kubernetes, you should | ||
| refer to the docs that go with that version. | ||
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| <strong> | ||
| The latest 1.0.x release of this document can be found | ||
| [here](http://releases.k8s.io/release-1.0/docs/admin/cluster-components.md). | ||
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| Documentation for other releases can be found at | ||
| [releases.k8s.io](http://releases.k8s.io). | ||
| </strong> | ||
| -- | ||
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| <!-- END STRIP_FOR_RELEASE --> | ||
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| <!-- END MUNGE: UNVERSIONED_WARNING --> | ||
| # Kubernetes Cluster Admin Guide: Cluster Components | ||
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| This document outlines the various binary components that need to run to | ||
| deliver a functioning Kubernetes cluster. | ||
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| ## Master Components | ||
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| Master components are those that provide the cluster's control plane. For | ||
| example, master components are responsible for making global decisions about the | ||
| cluster (e.g., scheduling), and detecting and responding to cluster events | ||
| (e.g., starting up a new pod when a replication controller's 'replicas' field is | ||
| unsatisfied). | ||
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| Master components could in theory be run on any node in the cluster. However, | ||
| for simplicity, current set up scripts typically start all master components on | ||
| the same VM, and does not run user containers on this VM. See | ||
| [high-availability.md](high-availability.md) for an example multi-master-VM setup. | ||
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| Even in the future, when Kubernetes is fully self-hosting, it will probably be | ||
| wise to only allow master components to schedule on a subset of nodes, to limit | ||
| co-running with user-run pods, reducing the possible scope of a | ||
| node-compromising security exploit. | ||
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| ### kube-apiserver | ||
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| [kube-apiserver](kube-apiserver.md) exposes the Kubernetes API; it is the front-end for the | ||
| Kubernetes control plane. It is designed to scale horizontally (i.e., one scales | ||
| it by running more of them-- [high-availability.md](high-availability.md)). | ||
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| ### etcd | ||
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| ```etcd``` is used as Kubernetes' backing store. All cluster data is stored here. | ||
| Proper administration of a Kubernetes cluster includes a backup plan for etcd's | ||
| data. | ||
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| ### kube-controller-manager | ||
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| [kube-controller-manager](kube-controller-manager.md) is a binary that runs controllers, which are the | ||
| background threads that handle routine tasks in the cluster. Logically, each | ||
| controller is a separate process, but to reduce the number of moving pieces in | ||
| the system, they are all compiled into a single binary and run in a single | ||
| process. | ||
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| These controllers include: | ||
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| * Node Controller | ||
| * Responsible for noticing & responding when nodes go down. | ||
| * Replication Controller | ||
| * Responsible for maintaining the correct number of pods for every replication | ||
| controller object in the system. | ||
| * Endpoints Controller | ||
| * Populates the Endpoints object (i.e., join Services & Pods). | ||
| * Service Account & Token Controllers | ||
| * Create default accounts and API access tokens for new namespaces. | ||
| * ... and others. | ||
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| ### kube-scheduler | ||
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| [kube-scheduler](kube-scheduler.md) watches newly created pods that have no node assigned, and | ||
| selects a node for them to run on. | ||
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| ### addons | ||
| Addons are pods and services that implement cluster features. They don't run on | ||
| the master VM, but currently the default setup scripts that make the API calls | ||
| to create these pods and services does run on the master VM. See: | ||
| [kube-master-addons](../../cluster/saltbase/salt/kube-master-addons/kube-master-addons.sh) | ||
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| Addon objects are created in the "kube-system" namespace. | ||
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| Example addons are: | ||
| * [DNS](../../cluster/addons/dns/) provides cluster local DNS. | ||
| * [kube-ui](../../cluster/addons/kube-ui/) provides a graphical UI for the | ||
| cluster. | ||
| * [fluentd-elasticsearch](../../cluster/addons/fluentd-elasticsearch) provides | ||
| log storage. Also see the [gcp version](../../cluster/addons/fluentd-gcp). | ||
| * [cluster-monitoring](../../cluster/addons/cluster-monitoring) provides | ||
| monitoring for the cluster. | ||
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| ## Node components | ||
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| Node components run on every node, maintaining running pods and providing them | ||
| the Kubernetes runtime environment. | ||
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| ### kubelet | ||
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| [kubelet](kubelet.md) is the primary node agent. It: | ||
| * Watches for pods that have been assigned to its node (either by apiserver or | ||
| or via local configuration file) and: | ||
| * Mounts the pod's required volumes | ||
| * Downloads the pod's secrets | ||
| * Run the pod's containers via docker (or, experimentally, rkt). | ||
| * Periodically executes any requested container liveness probes. | ||
| * Reports the status of the pod back to the rest of the system, by creating a | ||
| "mirror pod" if necessary. | ||
| * Reports the status of the node back to the rest of the system. | ||
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| ### kube-proxy | ||
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| [kube-proxy](kube-proxy.md) enables the Kubernetes service abstraction by maintaining | ||
| network rules on the host and performing connection forwarding. | ||
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| ### docker | ||
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| ```docker``` is of course used for actually running containers. | ||
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| ### rkt | ||
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| ```rkt``` is supported experimentally as an alternative to docker. | ||
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| ### monit | ||
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| ```monit``` is a lightweight process babysitting system for keeping kubelet and docker | ||
| running. | ||
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| <!-- BEGIN MUNGE: GENERATED_ANALYTICS --> | ||
| []() | ||
| <!-- END MUNGE: GENERATED_ANALYTICS --> |
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