Labels, Selectors, and Annotations
Labels
Labels are key-value pairs attached to Kubernetes objects like Pods to organize and categorize resources. Labels are not unique across resources of the same kind.
- Identify resources based on categories like frontend or backend
- Do not need to be unique across resources of the same type
- Multiple Pods can have the same label, such as frontend
Selectors
Label selectors are used to identify and filter Kubernetes objects based on their labels. They define conditions for label presence and values.
- Select objects based on label criteria
- Can filter Pods by labels, like "tier=frontend"
- Support complex matching conditions
Annotations
Annotations are key-value pairs that store non-identifying metadata for Kubernetes objects. They cannot be used to select objects like labels.
- Used for non-identifying information like contact numbers
- Often utilized by client apps (e.g., kubectl) and extensions
- Cannot filter objects based on annotations
Sample Lab
Below is an example manifest called pods-labels.yml that creates a namespace called labels and then create Pods in that namespace. Notice that each resource definitions are separated by a ---.
---
apiVersion: v1
kind: Namespace
metadata:
name: labels
---
apiVersion: v1
kind: Pod
metadata:
name: red-frontend
namespace: labels # declare namespace in metadata
labels: # labels mapping in metadata
color: red
tier: frontend
annotations: # Example annotation
Lab: Kubernetes Pod Design for Application Developers
spec:
containers:
- image: httpd:2.4.38
name: web-server
---
apiVersion: v1
kind: Pod
metadata:
name: green-frontend
namespace: labels
labels:
color: green
tier: frontend
spec:
containers:
- image: httpd:2.4.38
name: web-server
---
apiVersion: v1
kind: Pod
metadata:
name: red-backend
namespace: labels
labels:
color: red
tier: backend
spec:
containers:
- image: postgres:11.2-alpine
name: db
---
apiVersion: v1
kind: Pod
metadata:
name: blue-backend
namespace: labels
labels:
color: blue
tier: backend
spec:
containers:
- image: postgres:11.2-alpine
name: db
---
apiVersion: v1
kind: Pod
metadata:
name: no-color-backend
namespace: labels
labels:
tier: backend
spec:
containers:
- image: postgres:11.2-alpine
name: db
---
apiVersion: v1
kind: Pod
metadata:
name: no-color-frontend
namespace: labels
labels:
tier: backend
spec:
containers:
- image: postgres:11.2-alpine
name: db
To create the resources:
kubectl apply -f pod-labels.yaml
Use the get command to list all Pods in the cluster.
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
blue-backend 1/1 Running 0 25s
green-frontend 1/1 Running 0 25s
no-color-backend 1/1 Running 0 25s
no-color-frontend 1/1 Running 0 25s
red-backend 1/1 Running 0 25s
red-frontend 1/1 Running 0 25s
Add the -L flag to display labels for each Pod.
$ kubectl get pods -L color,tier
NAME READY STATUS RESTARTS AGE COLOR TIER
blue-backend 1/1 Running 0 50s blue backend
green-frontend 1/1 Running 0 50s green frontend
no-color-backend 1/1 Running 0 50s backend
no-color-frontend 1/1 Running 0 50s backend
red-backend 1/1 Running 0 50s red backend
red-frontend 1/1 Running 0 50s red frontend
Use -l to filter Pods by specific labels, such as showing only those with the "color" label.
$ kubectl get pods -L color,tier -l color
NAME READY STATUS RESTARTS AGE COLOR TIER
blue-backend 1/1 Running 0 75s blue backend
green-frontend 1/1 Running 0 75s green frontend
red-backend 1/1 Running 0 75s red backend
red-frontend 1/1 Running 0 75s red frontend
Filter Pods that do not have the "color" label by using !color.
$ kubectl get pods -L color,tier -l '!color'
NAME READY STATUS RESTARTS AGE COLOR TIER
no-color-backend 1/1 Running 0 96s backend
no-color-frontend 1/1 Running 0 96s backend
Use = to filter Pods with a specific label value, such as "red".
$ kubectl get pods -L color,tier -l color=red
NAME READY STATUS RESTARTS AGE COLOR TIER
red-backend 1/1 Running 0 2m36s red backend
red-frontend 1/1 Running 0 2m36s red frontend
Multiple conditions can also be set for filtering:
$ kubectl get pods -L color,tier -l 'color=red,tier!=frontend'
NAME READY STATUS RESTARTS AGE COLOR TIER
red-backend 1/1 Running 0 4m17s red backend
Use the in condition to specify allowed values, and notin to specify disallowed values.
$ kubectl get pods -L color,tier -l 'color in (blue,green)'
NAME READY STATUS RESTARTS AGE COLOR TIER
blue-backend 1/1 Running 0 5m32s blue backend
green-frontend 1/1 Running 0 5m32s green frontend
To view annotations, use the describe command.
$ kubectl describe pod red-frontend | grep Annotations -A 2
Annotations: Lab: Kubernetes Pod Design for Application Developers
cni.projectcalico.org/podIP: 192.168.23.130/32
cni.projectcalico.org/podIPs: 192.168.23.130/32
To remove an annotation, use the annotate command followed by the annotation name and a dash (-).
kubectl annotate pod red-frontend Lab-
The "Lab: Kubernetes Pod Design for Application Developers" should now be removed.
$ kubectl describe pod red-frontend | grep Annotations -A 2
Annotations: cni.projectcalico.org/podIP: 192.168.23.130/32
cni.projectcalico.org/podIPs: 192.168.23.130/32