concept external service in category kubernetes

This is an excerpt from Manning's book Kubernetes in Action. Login to get full access to this book.

Up to now, we’ve talked about services backed by one or more pods running inside the cluster. But cases exist when you’d like to expose external services through the Kubernetes services feature. Instead of having the service redirect connections to pods in the cluster, you want it to redirect to external IP(s) and port(s).

This allows you to take advantage of both service load balancing and service discovery. Client pods running in the cluster can connect to the external service like they connect to internal services.

5.2.3. Creating an alias for an external service

Instead of exposing an external service by manually configuring the service’s Endpoints, a simpler method allows you to refer to an external service by its fully qualified domain name (FQDN).

To create a service that serves as an alias for an external service, you create a Service resource with the type field set to ExternalName. For example, let’s imagine there’s a public API available at api.somecompany.com. You can define a service that points to it as shown in the following listing.

Listing 5.10. An ExternalName-type service: external-service-externalname.yaml

apiVersion: v1
kind: Service
metadata:
  name: external-service
spec:
  type: ExternalName                         #1
  externalName: someapi.somecompany.com      #2
  ports:
  - port: 80

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After the service is created, pods can connect to the external service through the external-service.default.svc.cluster.local domain name (or even external-service) instead of using the service’s actual FQDN. This hides the actual service name and its location from pods consuming the service, allowing you to modify the service definition and point it to a different service any time later, by only changing the externalName attribute or by changing the type back to ClusterIP and creating an Endpoints object for the service—either manually or by specifying a label selector on the service and having it created automatically.

ExternalName services are implemented solely at the DNS level—a simple CNAME DNS record is created for the service. Therefore, clients connecting to the service will connect to the external service directly, bypassing the service proxy completely. For this reason, these types of services don’t even get a cluster IP.