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Label selector

Learn to select resources with label selectors.

Label selector

A selector is a query selecting objects by label. Objects like Service, Deployment, ReplicaSet and NetworkPolicy select pods with a selector. In a Deployment there are two forms:

selector:
  matchLabels:            # equality: all must hold
    app: web
  matchExpressions:       # set operations
    - key: ortam
      operator: In
      values: [prod, staging]

A Service's selector supports only plain equality (app: web).

A Deployment selector is immutable

After a Deployment is created, spec.selector can't be changed; otherwise it would break its link with the old pods. If the selector must change, the deployment is deleted and recreated. That is why only permanent labels (app) go in the selector; changing labels like the version are added to the template but not to the selector.

$ kubectl apply -f web.yaml
The Deployment "web" is invalid: spec.selector: Invalid value: ... field is immutable

Practice A mixed selector

Write a Deployment's selector section (starting with selector:): app: web under matchLabels; key: ortam, operator: In, values: [prod, staging] under matchExpressions.

Starter code

terminal
# buraya yaz

Sample solution

terminal
selector:
  matchLabels:
    app: web
  matchExpressions:
    - key: ortam
      operator: In
      values: [prod, staging]

Expected output

deployment.apps/web created

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 When matchLabels and matchExpressions are written together, when is a pod selected?

  • When either holds
  • Only matchLabels is checked
  • It errors
  • When the conditions of both hold Correct answer

Explanation: All conditions combine with AND.

2 Write the set-based condition.

Question 
selector:
  ____:
    - key: ortam
      operator: In
      values: [prod, staging]
Answer 
selector:
  matchExpressions:
    - key: ortam
      operator: In
      values: [prod, staging]

Explanation: matchExpressions takes a key, an operator and a value list.

3 Match.

Term Meaning
matchLabels Equality conditions
matchExpressions Set conditions
Service selector Equality only
spec.selector (Deployment) Immutable

All lessons in Intermediate

1 What is Kubernetes?

  1. What is Kubernetes?
  2. Why Kubernetes?
  3. Cluster architecture
  4. Control plane
  5. Worker node
  6. Summary - What is Kubernetes

2 Minikube

  1. What is Minikube?
  2. Installing Minikube
  3. minikube start
  4. minikube status
  5. minikube dashboard
  6. Summary - Minikube

3 kubectl basics

  1. What is kubectl?
  2. kubectl get
  3. kubectl describe
  4. kubectl apply
  5. kubectl delete
  6. Summary - kubectl

4 Pod

  1. What is a Pod?
  2. Pod YAML
  3. Creating a Pod
  4. Pod status
  5. Multi-container Pod
  6. Summary - Pod

5 Deployment

  1. What is a Deployment?
  2. Deployment YAML
  3. Replica count
  4. Rolling update
  5. Rollback
  6. Summary - Deployment

6 ReplicaSet

  1. What is a ReplicaSet?
  2. ReplicaSet YAML
  3. ReplicaSet selector
  4. Scaling
  5. Self-healing
  6. Summary - ReplicaSet

7 Service

  1. What is a Service?
  2. ClusterIP
  3. NodePort
  4. LoadBalancer
  5. Service selector
  6. Summary - Service

8 ConfigMap

  1. What is a ConfigMap?
  2. ConfigMap YAML
  3. Environment variable
  4. Mounting as a volume
  5. kubectl create configmap
  6. Summary - ConfigMap

9 Secret

  1. What is a Secret?
  2. Secret types
  3. Secret YAML
  4. Mounting a Secret
  5. kubectl create secret
  6. Summary - Secret

10 Namespace

  1. What is a Namespace?
  2. kubectl get namespaces
  3. Creating a Namespace
  4. The -n flag
  5. Default Namespace
  6. Summary - Namespace

11 Labels and selectors

  1. What is a label?
  2. Adding labels
  3. Label selector
  4. kubectl get -l
  5. Equality and set selectors
  6. Summary - Labels

12 Project: Multi-tier app

  1. Project design
  2. Database Deployment
  3. Web Deployment
  4. ConfigMap and Secret
  5. Putting it together - multi-tier

13 Troubleshooting

  1. kubectl logs
  2. kubectl exec
  3. kubectl describe pod
  4. CrashLoopBackOff
  5. Pending Pod
  6. Summary - Troubleshooting

14 Intermediate final

  1. Challenge - Pod YAML
  2. Challenge - Deployment
  3. Challenge - Service
  4. Challenge - ConfigMap
  5. Challenge - Intermediate project