Coders Classroom: Kubernetes for Developers #8: Kubernetes Object Name, Labels, Selectors and Namespace

Object Name

Every Kubernetes object has a Name that is unique for that type of resource within the same namespace. It means, we can only have one Pod name called “mynginx” within the same namespace. however, we can use same name for another Kubernetes object like Deployment, Service, etc.

So, we can set same name i.e., “mynginx” to Pod, Deployment, Service, etc.

Kubernetes Object Name, Labels, Selectors and Namespace

The following characteristics need to be followed while setting name to Kubernetes object.

Name should not exceed 253 characters
It contains only lowercase alphanumeric characters (a to z), hyphen (-), period(.)
It should start with an alphanumeric character
It should end with an alphanumeric character

apiVersion: v1
kind: Pod
metadata:
  name: mynginx
spec:

Labels

Label is a key-value pair which is attached to pods, deployments, etc.

It acts an identifier on K8 object (Ex: Pod or Deployment). So, the other Kubernetes objects (Ex: Service, DaemonSet) can communicate by matching same label names.

It can be attached to Kubernetes objects at creation time or directly on live objects by using Imperative way

Same label key/value can be assigned to multiple Kubernetes objects

Each object in the label can have set of key/value and each key must be unique for a given object

Label key should not exceed 63 characters and allowed characters are alphanumeric, dash(-), underscore(_), dot(.)

Label value should not exceed 63 characters and allowed characters are alphanumeric, dash(-), underscore(_), dot(.)

metadata:
  name: pod-label-demo
  labels:
    environment: production
    app: nginx

Example: Creating labels on Pod

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

There are two labels i.e. app:nginx, environment:production are assigned to Pod

Example: Creating labels on Deployment

apiVersion: v1
kind: Deployment
metadata:
  name: deployment-label-demo
  labels:
    environment: production
    app: nginx
spec:

There are two labels i.e. app:nginx, environment:production are assigned to Deployment

Use following kubectl commands to find each k8 object and respective labels

// show all labels on each Pod
> kubectl get pods --show-labels

// show all labels on each Deployment
> kubectl get deployments --show-labels

// show all Pods where label app:nginx
> Kubectl get pods -l app=nginx

// add label to running Pod
// syntax
kubectl label Pod <podname> <key>=<value>

// example
> kubectl label Pod pod-label-demo tier=frontend

// delete Pods using label
> kubectl delete pod -l app=nginx

Selector

It is used for grouping Kubernetes objects and perform actions accordingly.

a) It is used by Kubernetes Deployment object to talk to all Pods with particular label key/value

b) It is used by Kubernetes Service object to expose all Pods with particular label key/value

Example: Adding labels on Pod template and Selector tag for identifying all Pod labels in the Deployment

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-nginx-deployment 
spec:
  replicas: 2
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
        - name: nginx
          image: nginx:1.7.9
          ports:
            - containerPort: 80

Here, we have added selector tag with matchLabels(app:nginx) used by deployment for identifying all the Pods where label equal to app:nginx and apply Deployment rules

Example: Create Service and communicate with Pods using labels

apiVersion: v1
kind: Service
metadata:
  name: my-nginx-service
spec:
  type: LoadBalancer
  ports:
    - port: 80
  selector:
    app: nginx

Here, we have added selector tag used by Service for identifying all the Pods where label name equal to app:nginx and apply Service rules

Note: matchLabels are not supported by Service. It only supported by Deployment, Replica Set, Daemon Set and Job.

Namespace

Kubernetes supports multiple virtual clusters by using Namespaces. It helps when multiple teams using same cluster and create separate Roles, Binding and Environments for each team.

Kubernetes resource name should be unique within a namespace, but not across namespaces
Namespaces cannot be nested
Each Kubernetes resource can only be in one namespace
Don’t create namespace with prefix “kube-“. Because, it is reserved for K8 system
By default, each resource created under “default” namespace

// create namespace using Imperative way
> kubectl create namespace my-namespace

# create namespace using Declarative way
apiVersion: v1
kind: Namespace
metadata:
  name: my-namespace

> kubectl create -f my-namespace.yaml

# create Pod under my-namespace
apiVersion: v1
kind: Pod
metadata:
  name: my-nginx-pod
  namespace: my-namespace

// get all namespaces
> kubectl get namespace

// get all Pods with specific namespace
> kubectl get pods –namespace=my-namespace

// delete namespace
> kubectl delete namespace my-namespace

// setting namespace preference for kubectl subsequent commands in the current session
> kubectl config set-context --current --namespace=my-name

Kubernetes for Developers Journey.

Happy Coding :)

Coders Classroom

Thursday, February 4, 2021

Kubernetes for Developers #8: Kubernetes Object Name, Labels, Selectors and Namespace

Labels

Selector

Namespace

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