I'm doing a prototype where one service depends on an availability of other. Scenario:

• Service A is assumed to be already available in a local network. It was either deployed by K8S or manually (or even a managed one provided by AWS etc.).
• Service B depends on environment variable SERVICE_A_IP and won't start without it. It's treated as a black box and can't be modified.

I want to pass Service A IP to Service B through K8S YAML configuration file. Perfect syntax for this occasion:

...
env:
  - name: SERVICE_A_IP
    valueFrom:
      k8sDeployment:
        name: service_a
        key: deploymentIP
...

During the prototyping stage Service A is an another K8S deployment but it might not be so in a production environment. Thus I need to decouple from SERVICE_A_SERVICE_IP that will be available to Service B (given it's deployed after Service A). I'm not into DNS discovery as well as it would require container modification which is far from a perfect solution.

If I would do it manually with kubectl (or with a shell script) it would be like the following:

$ kubectl run service_a --image=service_a:latest --port=8080
$ kubectl expose deployment service_a
$ SERVICE_A_IP="$(kubectl describe service service_a | \
    grep IP: | \
    cut -f2 -d ':' | \
    xargs)"
$ kubectl run service_b --image=service_b:latest --port=8080 \
    --env="SERVICE_A_IP=${SERVICE_A_IP}"

It works. Though I want to do the same using YAML configuration without injecting SERVICE_A_IP into configuration file with shell (basically modifying the file).

Is there any way to do so? Please take the above setting as set in stone.

UPDATE

Not the best way though still:

$ kubectl create -f service_a.yml
deployment "service_a" created
service "service_a" created
$ SERVICE_A_IP="$(kubectl describe service service_a | \
    grep IP: | \
    cut -f2 -d ':' | \
    xargs)"
$ kubectl create configmap service_a_meta \
    --from-literal="SERVICE_A_IP=${SERVICE_A_IP}"

And then in service_b.yml:

...
env:
  - name: SERVICE_A_IP
    valueFrom:
      configMapKeyRef:
        name: service_a_meta
        key: SERVICE_A_IP
...

That will work but still involves some shell and generally feels way too hax.