# Kubernetes - Label, Annotaions
###### tags: `Kubernetes`

## Foreword
> When our Pods is increasing, management will be complicated. For example, how to create a Pod on different Node or how to manage Pods on different level.
>
> Thus, Kubernetes has the element which called “Label”. According to the different property, we can give the Pod different Labels.
>

## Label
### Defination
> Labels are key/value pairs that are attached to objects, such as pods.

### Example
* "release" : "stable", "release" : "canary"
* "environment" : "dev", "environment" : "qa", "environment" : "production"
* "tier" : "frontend", "tier" : "backend", "tier" : "cache"
* "partition" : "customerA", "partition" : "customerB"
> If the objects have Label we can know the objects belong which level immediately.
>
* An object can have many Labels.
* We can use the “selector” to find the Label which is we want.

## Annotation
### Defination
> Annotations is the object which we can use to attach arbitrary non-identifying metadata to objects.
>
### Example
* Fields managed by a declarative configuration layer. Attaching these fields as annotations distinguishes them from default values set by clients or servers, and from auto-generated fields and fields set by auto-sizing or auto-scaling systems.
* Build, release, or image information like timestamps, release IDs, git branch, PR numbers, image hashes, and registry address.
* Pointers to logging, monitoring, analytics, or audit repositories.
* Client library or tool information that can be used for debugging purposes: for example, name, version, and build information.
## Practice
### Step1 Create Pod
> We can use “my-pod.yaml” to create a Pod.

apiVersion: v1
kind: Pod
metadata:
  name: label-demo
  labels:
    environment: production
    app: nginx
spec:
  containers:
  - name: nginx
    image: nginx:1.14.2
    ports:
    - containerPort: 80

> Use `Kubectl applt -f my-pod.yaml` to create a Pod.
### Step2 Get Information
> Than use `kubectl get pods` to get the information. Add the `--show-labels` can get the Lebels of Pod.
>


> We can use `kubectl describe pods label-demo` to get the Annotataions.
>

> If we want to add Labels on Pod, we can use `kubectl label pods pod-demo env=production` to add Labels.
Then we can see the new Labels on this Pod.
>

> We also can add Labels on Nose by `kubectl label node <node_name> <labels>`.
>
### Supplement
> And we can use “nodeSelector: <labels>” to select the node which we want to create this objest.
>
Eample

apiVersion: v1
kind: Pod
metadata:
  name: label-demo
  labels:
    environment: production
    app: nginx
spec:
  containers:
  - name: nginx
    image: nginx:1.14.2
    ports:
    - containerPort: 80
  nodeSelector: 
    kubernetes.io/os=linux

## Summary
> We can use the Labels the mark the object. Also, we can use the nodeSelector to select the node where we want to create.

## Reference
> https://ithelp.ithome.com.tw/articles/10194613
>
> https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/