Skip to main content

Kubernetes: 8. Labels & Selectors

Labels

  • Labels are a way of grouping the objects
  • While Kubernetes understands the objects it create, it is easier to identify the objects by using custom labels
  • With labels you group the objects by types (Pods, Services, ReplicaSet etc) or by Applications
  • For a pod, labels are defined under the metadata section
Selectors
  • Selectors are used to filter the objects using labels defined on them
  • Using kubectl and selector pods can be listed by filtering on the labels attached to them
  • If a Selector has multiple labels, they are understood as logical AND, which means pods must match all labels.
pod-definition.yaml
apiVersion: v1
kind: Pod
metadata:
    name: myapp-pod
    labels:
        app: myapp
        location: IN

spec:
    containers:
    - name: nginx-container
      image: nginx

kubectl get pods --selector app=myapp 
-> Get pods filtered by labels

kubectl get pods --selector app=myapp,tier=frontend 
-> Get pods filtered by labels app=myapp and tier=frontend

Replica Set
  • Kubernetes also uses labels to build objects.
  • For example take a case of replica set, 
    • Labels under metadata are replicaSet labels
    • labels under template.metadata are pod labels
    • labels under selector.matchLabels are selector labels
  • Kubernetes uses the selector.matchLabels to identify which pods have to be created as defined in spec.template by matching it with spec.template.metadata.labels

replica-set-definition.yaml
apiVersion: apps/v1
kind: ReplicaSet
metadata:
    name: myapp-rs
    labels:
        app: myapp
        type: front-end

spec:
    template:
        metadata:
            name: myapp-pod
            labels:
                app: myapp
                type: front-end
            spec:
                containers:
                - name: nginx-container
                  image: nginx


    replicas: 3
    selector:
        matchLabels:
            type: front-end

Service
  • For a Service object Kubernetes matches the labels defined under spec.selector to metadata.labels as defined under pod-definition file
apiVersion: v1
kind: Service
metadata:
    name: myapp-rs
    labels:
        app: my-svc-app

spec:
    selector:
        app: myapp
    ports:
    - protocol: TCP
      port: 8080
      targetPort: 9376

Annotations
  • Annotations though sound similar to labels are to be used to store the custom information specific to object
  • With labels, you can use them to group objects or filter them with selectors
  • Annotations are custom information about the object
pod-definition.yaml
apiVersion: v1
kind: Pod
metadata:
    name: myapp-pod
    labels:
        app: myapp
        location: IN
    annotations:
        buildVersion: 1.34
        contact: am@memories.com

spec:
    containers:
    - name: nginx-container
      image: nginx
Labels:

Comments

Post a Comment

Popular posts from this blog

Kubernetes: 6. Imperative Commands

Imperative commands are useful in quickly creating the resources on Kubernetes kubectl --dry-run -> When created with this option, Kubernetes just validates the definition and does not actually create the resource -o=yaml -> Gets the output in YAML format kubectl run nginx-pod --image=nginx:alpine  -> Create a pod with name nginx-pod and image nginx:alpine kubectl run httpd --image=httpd:alpine  -> By default, run implies run-a-pod kubectl run redis --image=redis:alpine --labels=tier=db  -> Create a pod with name redis and image redis:alpine and labels set to tier=db kubectl run custom-nginx --image=nginx --port=8080  -> Create a pod with name custom-nginx and image nginx to run on port 8080 kubectl expose pod redis --port=6379 --name=redis-service  -> Create a service with name redis-service to expose pod named redis on service-port 6379 kubectl create deployment webapp --image=kodekloud/webapp-color --replicas=3  -> Create a depl...
Post a Comment
Read more

Kubernetes: 21. Secrets

Passwords In the webapps we store the properties file for storing and retrieving the data required by application But we never store the application passwords, truststore, keystore passwords etc here We might store them in an encrypted format, but storing them as plain text is not the correct way In Kubernetes we store these sensitive information in Secrets https://medium.com/avmconsulting-blog/secrets-management-in-kubernetes-378cbf8171d0 Secrets Secrets are used to store the sensitive information They are similar to ConfigMaps, except that they are stored in hashed or encoded format Note that they are only encoded (using base64) but are not encrypted So secrets are a safe option to store sensitive information but infact they are not the safest option As such secret objects should be not checked into source code tools, its best to store them encrypted at REST in ETCD Again as in ConfigMaps, we have to create the secrets object first and then inject them into the pods There are 2 ways ...
Post a Comment
Read more

Kubernetes: 19. Configure Application

Configuring application consists of Configuring commands and arguments on applications Configuring environment variables Configuring secrets Docker Commands docker run ubuntu  -> Runs ubuntu container and exit, container CMD is set to [bash], so the container quitely exits docker run ubuntu echo "Hello World" -> Runs ubuntu container, prints "Hello World" exits quitely. To update the default settings, create your own image from the base image lets call this ubuntu-sleeper image FROM ubuntu CMD sleep 5 CMD can also be mentioned in the JSON format like CMD ["sleep", "5"] Note that with JSON format the first element should always be the command to execute,  for eg, it CANNOT be ["sleep 5"] Run build the new ubuntu-sleeper image and run the new image docker build -t ubuntu-sleeper .  -> Build the image docker run ubuntu-sleeper -> Run the new image So the new image will launch ubuntu container, sleep for 5 seconds and quitely ex...
Post a Comment
Read more