简介
Kubernetes 跟 Docker 等很多项目最大的不同，就在于它不推荐你使用命令行的方式直接运行容器（虽然 kubectl run 支持)，而是采用yaml/json 文件的方式。最直接的好处是，你会有一个文件能记录下 Kubernetes到底“run”了什么。使用文件的优点归纳起来
- Convenience，比如
kubectl create -f https://k8s.io/examples/application/deployment.yaml --record命令行可这样玩不了 - Maintenance， 比如使用git 管理
- Flexibility，也就是说表达能力更强
简化 Kubernetes Yaml 文件创建由于Yaml文件格式比较复杂，即使是老司机有时也不免会犯错或需要查询文档，因此可以dry-run 一下，kubectl run myapp --image=nginx --dry-run -o yaml 会输出模拟运行 nginx 镜像的yaml 文件内容，copy-paste 即可。或者你可以 kubectl get deployment my-nginx -o yaml 查看一个已有 kubernetes object 的配置，依葫芦画瓢。
了解kubernetes yaml 主要从两个维度：
- yaml 文件的普遍特征
- Kubernetes Object 的共同特征
yaml 的一些知识
Introduction to YAML: Creating a Kubernetes deployment
- YAML, which stands for Yet Another Markup Language，yaml 是一个标记语言
- YAML is a superset of JSON， yaml 是json 的超集
-
there are only two types of structures you need to know about in YAML:
- Lists
- Maps
yaml Maps
apiVersion: v1
kind: Pod
metadata:
name: rss-site
labels:
app: web
- Maps let you associate name-value pairs
- 只要“平行/级”，就是同一个层级的key-value。有了缩进，就表示一个map value。层级之间缩进空格数任意，哪怕一个空格也可以，但不要使用tab。 For example, name and labels are at the same indentation level, so the processor knows they’re both part of the same map; it knows that app is a value for labels because it’s indented further.
yaml list
args
- sleep
- "1000"
- message
- "Bring back Firefly!"
you can have virtually any number of items in a list, which is defined as items that start with a dash (-) indented from the parent.
Describing a Kubernetes Object
Understanding Kubernetes Objects
Kubernetes Object
Kubernetes Objects are persistent entities in the Kubernetes system. A Kubernetes object is a “record of intent”–once you create the object, the Kubernetes system will constantly work to ensure that object exists.
- What containerized applications are running (and on which nodes)
- The resources available to those applications
- The policies around how those applications behave, such as restart policies, upgrades, and fault-tolerance
Every Kubernetes object includes two nested object fields that govern the object’s configuration: the object spec and the object status.
- The spec, which you must provide, describes your desired state
- The status describes the actual state of the object, and is supplied and updated by the Kubernetes system. pod 状态可以使用
kubectl get pod pod_name -o yaml来查看，或者kubectl describe pod pod_name。
At any given time, the Kubernetes Control Plane actively manages an object’s actual state to match the desired state you supplied. 基于这种机制 不管是kubectl create -f 还是 kubectl replace -f 都可以是 kubectl apply -f，这或许也是kubernetes 声明式api 的一个体现吧。
yaml 配置共同点
- apiVersion - Which version of the Kubernetes API you’re using to create this object
- kind - What kind of object you want to create
- metadata - Data that helps uniquely identify the object, including a name string, UID, and optional namespace
- spec - The precise format of the object spec is different for every Kubernetes object, and contains nested fields specific to that object. 每一个 Kubernetes object 就得参见 Kubernetes API Reference了
metadata 与 spec 分别代表了 共性与个性，数据表设计也可以参照这个思路
PodPreset
开发人员习惯的写的，是最简单的pod
apiVersion: v1
kind: Pod
metadata:
name: website
labels:
app: website
role: frontend
spec:
containers:
- name:website
image: nginx
ports:
- containerPort:80
但对运维来说，在实际环境中还需添加大量的配置，此时，运维可以事先定义一个PodPreset.yaml，并创建一个PodPresetkubectl create -f preset.yaml。 之后开发创建的pod（有一个规则匹配） 都会自动加上 preset.yaml 指定的配置。
kubectl
在命令行中，所有 key-value 格式的参数，都使用“=”而非“:”表示。
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