Service selector

Learn to select Pods with a Service selector.

Service selector

A Service's selector matches pod labels; it has nothing to do with the deployment's name. This allows powerful patterns: if the pods of two versions carry the same label, traffic spreads over both; changing the label switches traffic to one version (blue-green deployment):

# blue: app=web, surum=v1    green: app=web, surum=v2
spec:
  selector:
    app: web
    surum: v2        # traffic only to v2

kubectl patch svc web -p '{"spec":{"selector":{"app":"web","surum":"v1"}}}'   # go back

A service without a selector

If no selector is written, Kubernetes doesn't fill in the endpoints automatically. This is used to reach a database outside the cluster from inside with a stable name; or the ExternalName type gives a DNS alias:

apiVersion: v1
kind: Service
metadata:
  name: dis-db
spec:
  type: ExternalName
  externalName: db.sirket.com     # dis-db -> db.sirket.com (CNAME)

Exercise A version-based selector

Write a Service YAML: name web; a selector with two labels: app: web and surum: v2; port port: 80, targetPort: 8080.

Show example solution

apiVersion: v1
kind: Service
metadata:
  name: web
spec:
  selector:
    app: web
    surum: v2
  ports:
    - port: 80
      targetPort: 8080

Test yourself

If a Service selector has several labels, what is needed for a pod to be selected?
- Carrying none of them
- Carrying all of the labels
- Carrying any one of them
- Carrying the first label
Show answer

Carrying all of the labels — The labels are combined with AND.

Change the service's selector in place.

kubectl ____ svc web -p '{"spec":{"selector":{"app":"web","surum":"v1"}}}'

Show answer

kubectl patch svc web -p '{"spec":{"selector":{"app":"web","surum":"v1"}}}'

Match.

selector

Matches pod labels
multiple labels

Combined with AND
blue-green

Traffic switch by label
ExternalName

DNS alias

Next lesson: Summary - Service

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