CODEWITHSUNDEEP
kubernetesserviceheadless
Cain
Cain

Reputation: 665

What’s the usage scenario of kubernetes headless service without selector?

I know a scenario of kubernetes headless service with selector. But what’s the usage scenario of kubernetes headless service without selector?

Upvotes: 2

Views: 2515

Answers (2)

Malgorzata
Malgorzata

Reputation: 7023

Services without selectors are used if you want to have an external database cluster in production, but in your test environment you use your own databases, to point your Service to a Service in a different Namespace or on another cluster, when you are migrating a workload to Kubernetes. Service without selectors are often used to alias external services into the cluster DNS.

Here ia an example of service without selector:

apiVersion: v1
kind: Service
metadata:
  name: example-service
spec:
  ports:
    - protocol: TCP
      port: 80
      targetPort: 9376

This Service has no selector, the corresponding Endpoint object is not created automatically. You can manually map the Service to the network address and port where it’s running, by adding an Endpoint object manually:

apiVersion: v1
kind: Endpoints
metadata:
  name: example-service
subsets:
  - addresses:
      - ip: 192.0.2.42
    ports:
      - port: 9376

If you have more that one IP address for redundancy, you can repeat them in the addresses array. Once the endpoints are populated, the load balancer will start redirecting traffic from your Kubernetes service to the IP addresses,

Note: The endpoint IPs must not be: loopback (127.0.0.0/8 for IPv4, ::1/128 for IPv6), or link-local (169.254.0.0/16 and 224.0.0.0/24 for IPv4, fe80::/64 for IPv6). Endpoint IP addresses cannot be the cluster IPs of other Kubernetes Services, because kube-proxy doesn’t support virtual IPs as a destination.

You can access a Service without a selector the same as if it had a selector.

Take a look: services-without-selector, example-service-without-selector.

Upvotes: 3

coderanger
coderanger

Reputation: 54211

Aliasing external services into the cluster DNS.

Upvotes: 2

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