118 lines
5.5 KiB
Markdown
118 lines
5.5 KiB
Markdown
---
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reviewers:
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- lavalamp
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title: Kubernetes Components
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content_template: templates/concept
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weight: 20
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card:
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name: concepts
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weight: 20
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---
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{{% capture overview %}}
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This document outlines the various binary components needed to
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deliver a functioning Kubernetes cluster.
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{{% /capture %}}
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{{% capture body %}}
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## Master Components
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Master components provide the cluster's control plane. Master components make global decisions about the
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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).
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Master components can be run on any machine in the cluster. However,
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for simplicity, set up scripts typically start all master components on
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the same machine, and do not run user containers on this machine. See
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[Building High-Availability Clusters](/docs/admin/high-availability/) for an example multi-master-VM setup.
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### kube-apiserver
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{{< glossary_definition term_id="kube-apiserver" length="all" >}}
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### etcd
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{{< glossary_definition term_id="etcd" length="all" >}}
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### kube-scheduler
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{{< glossary_definition term_id="kube-scheduler" length="all" >}}
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### kube-controller-manager
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{{< glossary_definition term_id="kube-controller-manager" length="all" >}}
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These controllers include:
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* Node Controller: Responsible for noticing and 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 (that is, joins Services & Pods).
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* Service Account & Token Controllers: Create default accounts and API access tokens for new namespaces.
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### cloud-controller-manager
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[cloud-controller-manager](/docs/tasks/administer-cluster/running-cloud-controller/) runs controllers that interact with the underlying cloud providers. The cloud-controller-manager binary is an alpha feature introduced in Kubernetes release 1.6.
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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.
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cloud-controller-manager allows cloud vendors code and the Kubernetes code 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.
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The following controllers have cloud provider dependencies:
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* Node Controller: For checking the cloud provider to determine if a node has been deleted in the cloud after it stops responding
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* Route Controller: For setting up routes in the underlying cloud infrastructure
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* Service Controller: For creating, updating and deleting cloud provider load balancers
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* Volume Controller: For creating, attaching, and mounting volumes, and interacting with the cloud provider to orchestrate volumes
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## Node Components
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Node components run on every node, maintaining running pods and providing the Kubernetes runtime environment.
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### kubelet
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{{< glossary_definition term_id="kubelet" length="all" >}}
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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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### Container Runtime
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The container runtime is the software that is responsible for running containers.
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Kubernetes supports several runtimes: [Docker](http://www.docker.com), [containerd](https://containerd.io), [cri-o](https://cri-o.io/), [rktlet](https://github.com/kubernetes-incubator/rktlet) and any implementation of the [Kubernetes CRI (Container Runtime Interface)](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-node/container-runtime-interface.md).
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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, ReplicationControllers, and so on. Namespaced addon objects are created in
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the `kube-system` namespace.
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Selected addons are described below, for an extended list of available addons please see [Addons](/docs/concepts/cluster-administration/addons/).
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### DNS
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While the other addons are not strictly required, all Kubernetes clusters should have [cluster DNS](/docs/concepts/services-networking/dns-pod-service/), 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 environment, which serves DNS records for Kubernetes services.
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Containers started by Kubernetes automatically include this DNS server in their DNS searches.
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### Web UI (Dashboard)
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[Dashboard](/docs/tasks/access-application-cluster/web-ui-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.
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### Container Resource Monitoring
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[Container Resource Monitoring](/docs/tasks/debug-application-cluster/resource-usage-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/concepts/cluster-administration/logging/) mechanism is responsible for
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saving container logs to a central log store with search/browsing interface.
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{{% /capture %}}
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