If you are using a released version of Kubernetes, you should refer to the docs that go with that version.
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).Documentation for other releases can be found at releases.k8s.io.
This document outlines the various binary components that need to run to deliver a functioning Kubernetes cluster.
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).
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 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 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).
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.
kube-controller-manager 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.
These controllers include:
- 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.
kube-scheduler watches newly created pods that have no node assigned, and selects a node for them to run on.
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
Addon objects are created in the "kube-system" namespace.
Example addons are:
- DNS provides cluster local DNS.
- kube-ui provides a graphical UI for the cluster.
- fluentd-elasticsearch provides log storage. Also see the gcp version.
- cluster-monitoring provides monitoring for the cluster.
Node components run on every node, maintaining running pods and providing them the Kubernetes runtime environment.
kubelet 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.
kube-proxy enables the Kubernetes service abstraction by maintaining network rules on the host and performing connection forwarding.
docker is of course used for actually running containers.
rkt is supported experimentally as an alternative to docker.
monit is a lightweight process babysitting system for keeping kubelet and docker
running.