• Standard method to group things together.
  • For example a set of different specifies and filter them on their criteria (colour, bird, etc).
  • Add properties to each itme.
    • For example, class = mammal
  • In Kubernetes, you will need labels to group objects by their type, application or their functionality.
  • Example of a labels for an App1 pod –> app
  • Example of a label for another pod called Front-end –> function
  • For Selectors, filter by a specific condition, for example app = App1
  • How do you specify labels in Kubernetes? Under the pod-definition.yaml file, specify the following: ``` apiVersion: v1 kind: Pod metadata: name: simple-webapp labels: app: App1 function: Front-en spec: containers:
    • name: simple-webapp image: simple-webapp ports:
      • containerPort: 8080 ```
  • Add the labels in a key-value format.
  • Once the pod is created, to select the pod by a particular label, use the following command: 
    kubectl get pods --selector app=App1
  • Kubernetes uses labels and selectors internally, to connect different objects together.
  • For example, to create a ReplicaSet, this is an example file: 
    apiVersion: apps/v1
kind: Replicaset
metadata:
name: simple-webapp
labels:
  app: App1
  function: Front-end
spec:
replicas: 3
selector:
  matchLabels:
    app: App1
template:
  metadata:
    labels:
      app: App1
      function: Front-end
  spec:
    containers:
    - name: simple-webapp
      image: simple-webapp
  • In the above file, the labels defined under the template section are the labels configured on the pods.
  • The labels under metadata are those under the replicaset itself.
  • We are trying to get the replicaset to discover the pods, so not interested in the labels under metadata.
  • To connect the replicaset to the pod, we configure the selector field under the replicaset specification, to match the labels defined on the pod.
    • These are the labels that are under the template field.
  • On creation, when the labels under matchLabels and under template –> labels match successfully, then the replicaset is created.
  • It works the same for other objects such as a service.
  • When the service is created, such as in the example below: ``` apiVersion: v1 kind: Service metadata: name: my-service spec: selector: app: App1 ports
    • protocol: TCP port: 80 targetPort: 9376 ```
  • When a service is created, it uses the selector in the service definition file to match the labels set on the pods in the replicaset definition file (the same file as above, under matchLabels and metadata –> labels)
  • Annotations - used to record other details, like build version, emails etc. Example using Annotations: 
    apiVersion: apps/v1
kind: Replicaset
metadata:
name: simple-webapp
labels:
  app: App1
  function: Front-end
annotations:
  buildversion: 1.14
spec:
replicas: 3
selector:
  matchLabels:
    app: App1
template:
  metadata:
    labels:
      app: App1
      function: Front-end
  spec:
    containers:
    - name: simple-webapp
      image: simple-webapp

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