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

Exercise 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.

Show example solution

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

Test yourself

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
Show answer

When the conditions of both hold — All conditions combine with AND.

Write the set-based condition.

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

Show answer

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

Match.

matchLabels

Equality conditions
matchExpressions

Set conditions
Service selector

Equality only
spec.selector (Deployment)

Immutable

Next lesson: kubectl get -l

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