Kubernetes + kustomize: How to use 'var' in configMapGenerator?

According to the fact that Nginx doesn't support variables in the config file, the file should be adjusted before it's applied to the Nginx container.

Kustomize vars allowed only in particular places and ConfigMap.data is not one of them at this moment.

The error tells you that PHP_FPM variable is defined in kustomization.yaml but never used for replacement. ^{Link1 Link2}

Nevertheless, the problem could be solved by adjusting nginx.conf content using the Init Container and then mount it to the Main container.

Example:

kustomization.yaml:

resources:
- cm-init-pod.yaml
- svc.yaml

patches:
- patch.yaml

configMapGenerator:
  - name: mymap
    files:
      - nginx.conf
vars:
  - name: PHP_FPM
    objref:
      kind: Service
      name: app-service
      apiVersion: v1
    fieldref:
      fieldpath: metadata.name

cm-init-pod.yaml:

apiVersion: v1
kind: Pod
metadata:
  name: cm-vol-pod 
  labels:
    app: nginx
spec:
  containers:
    - name: nginx 
      image: nginx

nginx.conf:

blalb-a
bla-bla
backend ##PHP_FPM##
bl-abla
bla-bla

svc.yaml

apiVersion: v1
kind: Service
metadata:
  name: app-service
spec:
  ports:
  - name: http
    port: 80
    protocol: TCP
    targetPort: 80
  selector:
    app: nginx

patch.yaml

apiVersion: v1
kind: Pod
metadata:
  name: cm-vol-pod 
spec:
  containers:
    - name: nginx 
      image: nginx
      command: [ "bash", "-c", "nginx -g "daemon off;""  ]
      volumeMounts:
      - name: ed
# this is not a real nginx.conf so I've put it next to the original one
        mountPath: /etc/nginx/nginx.conf1 
        subPath: nginx.conf
  initContainers:
    - name: init
      image: nginx
      command: [ "/bin/sh", "-c", "cp /mnt/cm/nginx.conf /mnt/ed/nginx.conf && sed -i 's/##PHP_FPM##/$(PHP_FPM)/' /mnt/ed/nginx.conf" ]
      volumeMounts:
      - name: cm
        mountPath: /mnt/cm
      - name: ed
        mountPath: /mnt/ed
  volumes:
    - name: cm
      configMap:
        name: mymap
    - name: ed
      emptyDir: {}

Results:

$ kustomize build | kubectl apply -f -
# or kubectl apply -k ./

configmap/mymap-k2hbfmf776 created
service/app-service created
pod/cm-vol-pod created

$ kubectl exec -it cm-vol-pod -- bash

root@cm-vol-pod:/# cat /etc/nginx/nginx.conf1

blalb-a
bla-bla
backend app-service
bl-abla
bla-bla

Let's clean up everything

$ kustomize build | kubectl delete -f -

# or

$ kubectl delete -k ./

configmap "mymap-k2hbfmf776" deleted
service "app-service" deleted
pod "cm-vol-pod" deleted