Service selector
- Docker & Kubernetes course
- Intermediate
- Lesson 41/82
- 4 min
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 backA 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)Practice 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.
Starter code
# buraya yazSample solution
apiVersion: v1
kind: Service
metadata:
name: web
spec:
selector:
app: web
surum: v2
ports:
- port: 80
targetPort: 8080Expected output
service/web created
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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 Correct answer
- Carrying any one of them
- Carrying the first label
Explanation: The labels are combined with AND.
2 Change the service's selector in place.
kubectl ____ svc web -p '{"spec":{"selector":{"app":"web","surum":"v1"}}}'kubectl patch svc web -p '{"spec":{"selector":{"app":"web","surum":"v1"}}}'Explanation: patch changes part of an object.
3 Match.
| Term | Meaning |
|---|---|
selector |
Matches pod labels |
multiple labels |
Combined with AND |
blue-green |
Traffic switch by label |
ExternalName |
DNS alias |