Learning Kubernetes

Component

master

Control panel of the entire cluster.

kube-apiserver

Expose API for control panel.

etcd

Key-Value store for all cluster data.

kube-scheduler

Watch newly created pods and assign node to run.

kube-controller-manager

Each controller is logically separated into different process but they are actually combined into single process from single binary.

1. Node controller: Detect node status
2. Replication Controller: Maintain correct number of pods based on replication
3. Endpoint Controller: Provide endpoint object: Services, Pods
4. Account & Token Controller: create default account & access token for new namespace

cloud-controller-manager

Interact with underlying cloud providers.

node

Maintain running pods and provide Kubernetes runtime environment.

kubelet

Run on each node in cluster.
Make sure containers are running in a pod. It takes a set of PodSpecs, then ensure the containers are aligned with it.
It doesnot bmanage containers which were not created by Kubernetes.

kube-proxy

It is a network proxy that runs on each node.
Maintain network rules on nodes, allow network communication to your Pods IN/OUT of your cluster.
It uses either operating system packet filtering layer or forward the traffic itself.

Container Runtime

Kubernetes support several containers:

1. Docker
2. Containerd
3. cri-o
4. rktlet

Kubernetes Objects

Pod

The basic building block and deployable object in Kubernetes object model.
Pod encapsulates an application container, storage, unique network IP, configuration that define the way container runs.
Pod represents a unit of deployment.

Pod can be run in both single and multiple container.

Service

An abstract way to expose application as network service. Application runs on top of a set of pods.
With it, application can use the native discovery mechanism, with their own IP and single DNS name, also with load balance across a set of pods.

Volumn

Disk files in container are not persistant, which means files will lose once kubelet restarts container. Container also might want to share file with others.
Volume solves these requirements.

Namespace

Term used for grouping multiple virtual cluster based on same physical cluster.

ReplicaSet

Maintain a set of replica pods running at any give time. Often used to guarantee the availability of identical pods.

Deployment

Provides declarative configuration for Pod and ReplicaSet, so that you don’t have to manually update the number. Simply set the desired state and deployment do control over change for you.

StatefulSet

Manage the deployment and scaling of pods. Guarantee the order and uniqueness of these pods, with declarative configuration.
This suites will for:

1. stable unique network identifier
2. stable persistent storage
3. ordered graceful deployment and scaling
4. ordered, automated rolling updates

DaemonSet

Ensure nodes run a copy of a Pod, so as to guarantee pod exists in configured nodes.

Job

It creates some pods, finishs their commands and guarantees they successfully terminate.
This is useful when you want to run some on-fly tasks.