kubernetes 1.12 从零开始（初）: 课程介绍与官方文档汇总

Master的组件：
    kube-apiserver、etcd、kube-scheduler、kube-controller-manager、cloud-controller-manager、

Node的组件：
    kubelet、kube-proxy、Container Runtime

可选插件：
    DNS、Web UI、Container Resource Monitoring、Cluster-level Logging

Object的定义是`声明式`的，spec中记录的是期待达成状态，status中记录的是当前状态。

nodes and persistentVolumes, are not in any namespace.

松耦合设计

We don’t want to pollute labels with non-identifying, 
especially large and/or structured, data. Non-identifying information should be recorded using annotations.

Equality-based requirement:  Three kinds of operators are admitted =,==,!=
Set-based requirement:       Three kinds of operators are supported: in,notin and exists (only the key identifier)

Similarly the comma separator acts as an AND operator:  partition,environment notin (qa)

Set-based requirements can be mixed with equality-based requirements:  partition in (customerA, customerB),environment!=qa

kubectl get pods -l 'environment in (production, qa)'

Service and ReplicationController:  equality-based requirement selectors are supported

Newer resources, such as Job, Deployment, Replica Set, and Daemon Set, support set-based requirements as well.

Annotations are not used to identify and select objects. 
The metadata in an annotation can be small or large, structured or unstructured, and can include characters not permitted by labels.

Supported operators: =, ==, and !=
field selectors can be chained together as a comma-separated list

use field selectors across multiple resource types:
    kubectl get statefulsets,services --field-selector metadata.namespace!=default

这只是建议使用的labels

apiVersion: apps/v1
kind: StatefulSet
metadata:
  labels:
    app.kubernetes.io/name: mysql
    app.kubernetes.io/instance: wordpress-abcxzy
    app.kubernetes.io/version: "5.7.21"
    app.kubernetes.io/component: database
    app.kubernetes.io/part-of: wordpress
    app.kubernetes.io/managed-by: helm

Guaranteed:  request == limit，            Pod分配的资源是固定受限的
Burstable:   request <  limit(包括未设置)  Pod分配的资源在request~limit之间变动
BestEffort:  request和limit均未设置        Pod的资源不受限也没有最低保证     

扩展资源是CPU、Memory、磁盘等通用资源之外的、Kubernetes感知不到的资源，例如Node上的特殊硬件。

Projected Volume可以将secret、configMap、serviceAccountToken、downwardAPI几种不同类型的Source挂载到同一个目录中。

Security Context定义了Pod和Container的操作权限和访问控制。

Service Account被挂载到Pod中，Pod中的进程可以用Service Account来通过其它服务的认证。

将私有镜像仓库的账号和密码保存成Kubernetes中的secret，然后在Pod的声明中指定要使用的secret。

liveness probes用来判定Container是否需要被重启
readiness probes 用来判定Container是否可以开始接收请求

Init Container在Pod中的业务容器启动之前*完成运行*，可以用来为业务容器准备运行环境。

postStart：容器启动后执行的命令，与容器中的entrypoint异步执行
preStop：  容器停止前执行的命令，容器需要等到preStop指定的命令运行结束后才能终止，除非等待时间超过了grace period

容器的环境变量的值可以从ConfigMap中读取，ConfigMap也可以被以volumn的形式挂载到容器中

Kubernetes v1.11 alpha特性。

一个名为Kompose的工具。

Retain   :  用户删除PVC时，动态分配的PV保留，标记为released，不能被其它的PVC使用 
Delete   :  用户删除PVC时，动态分配的PV一同删除
Recycle  :  (deprecated)用户删除PVC时，对应PV中的数据被删除，PV可以再次用于其它PVC。

Starting from Kubernetes 1.8, resource usage metrics, such as container CPU and memory usage, are available in Kubernetes through the Metrics API.
These metrics can be either accessed directly by user, for example by using kubectl top command, or used by a controller in the cluster, e.g. Horizontal Pod Autoscaler, to make decisions.