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Wendy Shaffer
2017-05-14 15:49:45 -07:00
committed by Andrew Chen
parent 74b390a61a
commit a326f8ff17
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docs/concepts/overview/components.md
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@@ -6,72 +6,56 @@ redirect_from:
- "/docs/admin/cluster-components/" - "/docs/admin/cluster-components/"
- "/docs/admin/cluster-components.html" - "/docs/admin/cluster-components.html"
--- ---
{% capture overview %}
This document outlines the various binary components that need to run to This document outlines the various binary components needed to
deliver a functioning Kubernetes cluster. deliver a functioning Kubernetes cluster.
{% endcapture %}
{% capture body %}
## Master Components ## Master Components
Master components are those that provide the cluster's control plane. For Master components provide the cluster's control plane. Master components make global decisions about the
example, master components are responsible for making 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.
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).
In theory, Master components can be run on any node in the cluster. However, Master components can be run on any node in the cluster. However,
for simplicity, current set up scripts typically start all master components on for simplicity, set up scripts typically start all master components on
the same VM, and does not run user containers on this VM. See the same VM, and do not run user containers on this VM. See
[Building High-Availability Clusters](/docs/admin/high-availability) for an example multi-master-VM setup. [Building High-Availability Clusters](/docs/admin/high-availability) for an example multi-master-VM setup.
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.
### kube-apiserver ### kube-apiserver
[kube-apiserver](/docs/admin/kube-apiserver) exposes the Kubernetes API; it is the front-end for the [kube-apiserver](/docs/admin/kube-apiserver) exposes the Kubernetes API. It is the front-end for the
Kubernetes control plane. It is designed to scale horizontally (i.e., one scales Kubernetes control plane. It is designed to scale horizontally -- that is, it scales by deploying more instances. See [Building High-Availability Clusters](/docs/admin/high-availability).
it by running more of them-- [Building High-Availability Clusters](/docs/admin/high-availability)).
### etcd ### etcd
[etcd](/docs/admin/etcd) is used as Kubernetes' backing store. All cluster data is stored here. [etcd](/docs/admin/etcd) is used as Kubernetes' backing store. All cluster data is stored here. Always have a backup plan for etcd's data for your Kubernetes cluster.
Proper administration of a Kubernetes cluster includes a backup plan for etcd's
data.
### kube-controller-manager ### kube-controller-manager
[kube-controller-manager](/docs/admin/kube-controller-manager) is a binary that runs controllers, which are the [kube-controller-manager](/docs/admin/kube-controller-manager) runs controllers, which are the background threads that handle routine tasks in the cluster. Logically, each controller is a separate process, but to reduce complexity, they are all compiled into a single binary and run in a single process.
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.
These controllers include: These controllers include:
* Node Controller: Responsible for noticing & responding when nodes go down. * Node Controller: Responsible for noticing and responding when nodes go down.
* Replication Controller: Responsible for maintaining the correct number of pods for every replication * Replication Controller: Responsible for maintaining the correct number of pods for every replication
controller object in the system. controller object in the system.
* Endpoints Controller: Populates the Endpoints object (i.e., join Services & Pods). * Endpoints Controller: Populates the Endpoints object (that is, joins Services & Pods).
* Service Account & Token Controllers: Create default accounts and API access tokens for new namespaces. * Service Account & Token Controllers: Create default accounts and API access tokens for new namespaces.
* ... and others.
### cloud-controller-manager ### cloud-controller-manager
`cloud-controller-manager` is a binary that 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 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. As such, 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 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 and 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. 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: The following controllers have cloud provider dependencies:
* Node Controller: For checking the cloud provider to determine if a node has been deleted in the cloud after it stops responding * Node Controller: For checking the cloud provider to determine if a node has been deleted in the cloud after it stops responding
* Route Controller: For setting up routes in the underlying cloud infrastructure * Route Controller: For setting up routes in the underlying cloud infrastructure
* Service Controller: For creating, updating and deleting cloud provider load balancers * Service Controller: For creating, updating and deleting cloud provider load balancers
* Volume Controller: For creating, attaching, and mounting volumes, and interacting with the cloud provider * Volume Controller: For creating, attaching, and mounting volumes, and interacting with the cloud provider to orchestrate volumes
to orchestrate volumes
### kube-scheduler ### kube-scheduler
@@ -81,27 +65,23 @@ selects a node for them to run on.
### addons ### addons
Addons are pods and services that implement cluster features. The pods may be managed Addons are pods and services that implement cluster features. The pods may be managed
by Deployments, ReplicationContollers, etc. Namespaced addon objects are created in by Deployments, ReplicationControllers, and so on. Namespaced addon objects are created in
the "kube-system" namespace. the `kube-system` namespace.
Addon manager takes the responsibility for creating and maintaining addon resources. Addon manager creates and maintains addon resources. See [here](http://releases.k8s.io/HEAD/cluster/addons) for more details.
See [here](http://releases.k8s.io/HEAD/cluster/addons) for more details.
#### DNS #### DNS
While the other addons are not strictly required, all Kubernetes 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.
clusters should have [cluster DNS](/docs/concepts/services-networking/dns-pod-service/), as many examples rely on it.
Cluster DNS is a DNS server, in addition to the other DNS server(s) in your Cluster DNS is a DNS server, in addition to the other DNS server(s) in your environment, which serves DNS records for Kubernetes services.
environment, which serves DNS records for Kubernetes services.
Containers started by Kubernetes automatically include this DNS server Containers started by Kubernetes automatically include this DNS server in their DNS searches.
in their DNS searches.
#### User interface #### User interface
The kube-ui provides a read-only overview of the cluster state. Access The kube-ui provides a read-only overview of the cluster state. For more information, see [Using an HTTP Proxy to Access the Kubernetes API](/docs/tasks/access-kubernetes-api/http-proxy-access-api/)
[Using an HTTP Proxy to Access the Kubernetes API](/docs/tasks/access-kubernetes-api/http-proxy-access-api/)
#### Container Resource Monitoring #### Container Resource Monitoring
@@ -115,41 +95,41 @@ saving container logs to a central log store with search/browsing interface.
## Node components ## Node components
Node components run on every node, maintaining running pods and providing them Node components run on every node, maintaining running pods and providing the Kubernetes runtime environment.
the Kubernetes runtime environment.
### kubelet ### kubelet
[kubelet](/docs/admin/kubelet) is the primary node agent. It: [kubelet](/docs/admin/kubelet) is the primary node agent. It watches for pods that have been assigned to its node (either by apiserver or via local configuration file) and:
* Watches for pods that have been assigned to its node (either by apiserver * Mounts the pod's required volumes.
or via local configuration file) and: * Downloads the pod's secrets.
* Mounts the pod's required volumes * Runs the pod's containers via docker (or, experimentally, rkt).
* Downloads the pod's secrets * Periodically executes any requested container liveness probes.
* Runs the pod's containers via docker (or, experimentally, rkt). * Reports the status of the pod back to the rest of the system, by creating a *mirror pod* if necessary.
* Periodically executes any requested container liveness probes. * Reports the status of the node back to the rest of the system.
* 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.
### kube-proxy ### kube-proxy
[kube-proxy](/docs/admin/kube-proxy) enables the Kubernetes service abstraction by maintaining [kube-proxy](/docs/admin/kube-proxy) enables the Kubernetes service abstraction by maintaining
network rules on the host and performing connection forwarding. network rules on the host and performing connection forwarding.
### docker ### docker
`docker` is of course used for actually running containers. docker is used for running containers.
### rkt ### rkt
`rkt` is supported experimentally as an alternative to docker. rkt is supported experimentally for running containers as an alternative to docker.
### supervisord ### supervisord
`supervisord` is a lightweight process babysitting system for keeping kubelet and docker supervisord is a lightweight process monitoring and control system that can be used to keeo kubelet and docker
running. running.
### fluentd ### fluentd
`fluentd` is a daemon which helps provide [cluster-level logging](#cluster-level-logging). fluentd is a daemon which helps provide [cluster-level logging](#cluster-level-logging).
{% endcapture %}
{% include templates/concept.md %}