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.. This work is licensed under a Creative Commons Attribution 4.0 International Licence.
.. http://creativecommons.org/licenses/by/4.0
.. (c) by Yifei Xue (HUAWEI) and Justin Chi (HUAWEI)
K8s introduction
================
Kubernetes Architecture
-----------------------
Currently Compass can deploy kubernetes as NFVI in 3+2 mode by default.
**The following figure shows a typical architecture of Kubernetes.**
.. figure:: images/architecture.png
:alt: K8s architecture
:figclass: align-center
Fig 3. K8s architecture
Kube-apiserver
~~~~~~~~~~~~~~
Kube-apiserver exposes the Kubernetes API. It is the front-end for the Kubernetes control plane.
It is designed to scale horizontally, that is, it scales by deploying more instances.
Etcd
~~~~
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.
Kube-controller-manager
~~~~~~~~~~~~~~~~~~~~~~~
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.
These controllers include:
- Node Controller: Responsible for noticing and 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 (that is, joins Services & Pods).
- Service Account & Token Controllers: Create default accounts and API access tokens for
new namespaces.
kube-scheduler
~~~~~~~~~~~~~~
Kube-scheduler watches newly created pods that have no node assigned, and selects a node for them
to run on.
Kubelet
~~~~~~~
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:
- Mounts the pod's required volumes.
- Downloads the pod's secrets.
- Runs 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
~~~~~~~~~~
Kube-proxy enables the Kubernetes service abstraction by maintaining network rules on the host and
performing connection forwarding.
Docker
~~~~~~
Docker is used for running containers.
POD
~~~
A pod is a collection of containers and its storage inside a node of a Kubernetes cluster. It is
possible to create a pod with multiple containers inside it. For example, keeping a database container
and data container in the same pod.
Understand Kubernetes Networking in Compass configuration
---------------------------------------------------------
**The following figure shows the Kubernetes Networking in Compass configuration.**
.. figure:: images/k8s.png
:alt: Kubernetes Networking in Compass
:figclass: align-center
Fig 4. Kubernetes Networking in Compass
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