137 lines
5.1 KiB
Markdown
137 lines
5.1 KiB
Markdown
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---
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assignees:
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- lavalamp
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title: Kubernetes Components
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---
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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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In theory, Master components can 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](/docs/admin/high-availability) 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](/docs/admin/kube-apiserver) 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](/docs/admin/high-availability)).
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### etcd
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[etcd](/docs/admin/etcd) 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](/docs/admin/kube-controller-manager) 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: Responsible for noticing & responding when nodes go down.
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* Replication Controller: 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: Populates the Endpoints object (i.e., join Services & Pods).
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* Service Account & Token Controllers: 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](/docs/admin/kube-scheduler) 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. The pods may be managed
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by Deployments, ReplicationContollers, etc. Namespaced addon objects are created in
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the "kube-system" namespace.
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Addon manager takes the responsibility for creating and maintaining addon resources.
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See [here](http://releases.k8s.io/HEAD/cluster/addons) for more details.
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#### DNS
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While the other addons are not strictly required, all Kubernetes
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clusters should have [cluster DNS](/docs/admin/dns/), as many examples rely on it.
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Cluster DNS is a DNS server, in addition to the other DNS server(s) in your
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environment, which serves DNS records for Kubernetes services.
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Containers started by Kubernetes automatically include this DNS server
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in their DNS searches.
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#### User interface
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The kube-ui provides a read-only overview of the cluster state. Access
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[the UI using kubectl proxy](/docs/user-guide/connecting-to-applications-proxy/#connecting-to-the-kube-ui-service-from-your-local-workstation)
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#### Container Resource Monitoring
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[Container Resource Monitoring](/docs/user-guide/monitoring) records generic time-series metrics
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about containers in a central database, and provides a UI for browsing that data.
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#### Cluster-level Logging
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A [Cluster-level logging](/docs/user-guide/logging/overview) mechanism is responsible for
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saving container logs to a central log store with search/browsing interface.
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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](/docs/admin/kubelet) is the primary node agent. It:
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* Watches for pods that have been assigned to its node (either by apiserver
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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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* Runs 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](/docs/admin/kube-proxy) 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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### supervisord
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`supervisord` is a lightweight process babysitting system for keeping kubelet and docker
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running.
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### fluentd
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`fluentd` is a daemon which helps provide [cluster-level logging](#cluster-level-logging).
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