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153 lines
5.6 KiB
Markdown
153 lines
5.6 KiB
Markdown
<!-- BEGIN MUNGE: UNVERSIONED_WARNING -->
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<!-- BEGIN STRIP_FOR_RELEASE -->
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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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
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refer to the docs that go with that version.
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<strong>
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The latest 1.0.x release of this document can be found
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[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
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[releases.k8s.io](http://releases.k8s.io).
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</strong>
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--
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<!-- END STRIP_FOR_RELEASE -->
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<!-- END MUNGE: UNVERSIONED_WARNING -->
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# Kubernetes Cluster Admin Guide: Cluster Components
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This document outlines the various binary components that need to run to
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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
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example, master components are responsible for making global decisions about the
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cluster (e.g., scheduling), and detecting and responding to cluster events
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(e.g., starting up a new pod when a replication controller's 'replicas' field is
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unsatisfied).
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Master components could in theory be run on any node in the cluster. However,
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for simplicity, current set up scripts typically start all master components on
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the same VM, and does not run user containers on this VM. See
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[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
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wise to only allow master components to schedule on a subset of nodes, to limit
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co-running with user-run pods, reducing the possible scope of a
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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
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Kubernetes control plane. It is designed to scale horizontally (i.e., one scales
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it by running more of them-- [high-availability.md](high-availability.md)).
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### etcd
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[etcd](etcd.md) is used as Kubernetes' backing store. All cluster data is stored here.
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Proper administration of a Kubernetes cluster includes a backup plan for etcd's
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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
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background threads that handle routine tasks in the cluster. Logically, each
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controller is a separate process, but to reduce the number of moving pieces in
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the system, they are all compiled into a single binary and run in a single
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process.
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These controllers include:
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* Node Controller
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* Responsible for noticing & responding when nodes go down.
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* Replication Controller
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* Responsible for maintaining the correct number of pods for every replication
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controller object in the system.
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* Endpoints Controller
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* Populates the Endpoints object (i.e., join Services & Pods).
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* Service Account & Token Controllers
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* Create default accounts and API access tokens for new namespaces.
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* ... 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
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selects a node for them to run on.
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### addons
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Addons are pods and services that implement cluster features. They don't run on
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the master VM, but currently the default setup scripts that make the API calls
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to create these pods and services does run on the master VM. See:
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[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:
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* [DNS](../../cluster/addons/dns/) provides cluster local DNS.
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* [kube-ui](../../cluster/addons/kube-ui/) provides a graphical UI for the
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cluster.
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* [fluentd-elasticsearch](../../cluster/addons/fluentd-elasticsearch/) provides
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log storage. Also see the [gcp version](../../cluster/addons/fluentd-gcp/).
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* [cluster-monitoring](../../cluster/addons/cluster-monitoring/) provides
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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
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the Kubernetes runtime environment.
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### kubelet
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[kubelet](kubelet.md) is the primary node agent. It:
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* Watches for pods that have been assigned to its node (either by apiserver or
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or via local configuration file) and:
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* Mounts the pod's required volumes
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* Downloads the pod's secrets
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* Run the pod's containers via docker (or, experimentally, rkt).
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* Periodically executes any requested container liveness probes.
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* Reports the status of the pod back to the rest of the system, by creating a
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"mirror pod" if necessary.
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* 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
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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
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running.
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<!-- BEGIN MUNGE: GENERATED_ANALYTICS -->
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[]()
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<!-- END MUNGE: GENERATED_ANALYTICS -->
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