DevOps Classroom Series – 10/Apr/2021

Labels and Annotations

  • Labels are the metadata that contains identifiable information to the kubernetes objects. These are basically key-value pairs attached to the objects such as pods
  • Each key must be unique for an object
  • Labels would appear in metadata section and the yaml would appear as
metadata:
  labels:
    key1: value1
    key2: value2
  • Constraints for Labels:
    • Label prefix: This is optional and this must be a DNS subdomain. This cannot be longer thatn 253 characters & cannot contain spaces. The label prefix is always followed by a forwar slash Eg: directdevops.blog/. If no prefix is used the label key is assumed to be private to the user. label prefixes specific to k8s core system kubernetes.io/ and k8s.io/
    • Label name: The label name is required and can be upto 63 characters long.
  • Value of the key in k8s is label-prefix/label-name or label-name

Scenario: Lets create a ngnix pod with labels organized by team/project

  • Refer Here for the k8s manifest and apply the yaml file Preview
  • Add labels to k8s object after creation Preview
  • To change the labels
kubectl label --overwrite pod label-demo1 server=nginx-webserver
  • To Remove the label
kubectl label pod label-demo1 server-

Selecting k8s objects using label selectors

  • Lets create two more pod specs Refer Here Preview
  • We can query the objects using label selector
kubectl get pods -l {label-selector}
  • There are two types of label selectors
    • equality based
    • set-based
  • Equality based selectors:
    • In this selector we have three kinds of operators = == !=
    • Lets try to get all the pods with the label environment is equal to developer Preview Preview
  • SET-BASED Selectors
    • In this we have 3 kinds of operators in notin exists
    • Lets try some examples Preview

Annotations

  • Using labels we can add metadata which can be later used to filter/select objects
  • Annotations on the other hand have fewer constraints, however we cannot filter or select objects by annotations
  • Annotations are generally used by tools or users to get subjective information regarding k8s object.
  • Lets look at one annotation example Refer Here for the manifest Preview
  • Like what we have done in labels we can add/modify/delete annotations from kubectl
kubectl annotate pod <podname> <annotate_key>=<annotate_value>
kubectl annotate --overwrite pod <podname> <annotate_key>=<annotate_value>
kubectl annotate pod <podname> <annotate_key>-

Kubernetes Controllers

  • Pod is the workload in the k8s cluster
  • When we deploy our application in production,
    • we might need more than one replica of pod
    • We can balance load across multiple pods so that one pod is not overloaded
  • k8s supports different controllers that you can use for replications and maintaining state. The controllers we have are
    • ReplicaSets
    • Deployments
    • DaemonSets
    • StatefulSets
    • Jobs
  • A controller is an object in k8s that ensures application runs in the desired state for its entire runtime

ReplicaSet

  • A ReplicaSet is a k8s controller that keeps a certain number of Pods running at any given time
  • ReplicaSet acts a supervisor for multiple Pods across different nodes in a k8s cluster. A ReplicaSet will terminate or start new Pods to match the configuration specified in Replicaset Template.
  • Create a sample replica set Refer Here for the yaml and apply Preview Preview Preview Preview
  • lets delete one pod manually and see what happens Preview Preview
  • Now lets try to delete the rs Preview
  • Now lets create a pod with app: httpd label Refer Here for yaml and apply this Preview
  • Now lets apply the replica set which needs 2 replicas Preview
  • replica set didnt create 2 pods rather it created one pod as there was one pod which matched the label selctor mentioned in replica set specification Preview
  • Manually Scaling the replicas
    • change the spec
    • executing the command Preview
  • Note: command to create token and display the join command in k8s custer
kubeadm token create --print-join-command --ttl=0
kubeadm token list
  • Exercise: Try to create replicaset with 3 replicas
    • for running game of life
    • for running spring pet clinic
    • for nop commerce

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