Passing values from initContainers to container spec

Question

I have a kubernetes deployment with the below spec that gets installed via helm 3.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: gatekeeper
spec:
  replicas: 1
  template:
    spec:
      containers:
        - name: gatekeeper
          image: my-gatekeeper-image:some-sha
          args:
            - --listen=0.0.0.0:80
            - --client-id=gk-client
            - --discovery-url={{ .Values.discoveryUrl }}

I need to pass the discoveryUrl value as a helm value, which is the public IP address of the nginx-ingress pod that I deploy via a different helm chart. I install the above deployment like below:

helm3 install my-nginx-ingress-chart
INGRESS_IP=$(kubectl get svc -lapp=nginx-ingress -o=jsonpath='{.items[].status.loadBalancer.ingress[].ip}')
helm3 install my-gatekeeper-chart --set discovery_url=${INGRESS_IP}

This works fine, however, Now instead of these two helm3 install, I want to have a single helm3 install, where both the nginx-ingress and the gatekeeper deployment should be created.

I understand that in the initContainer of my-gatekeeper-image we can get the nginx-ingress ip address, but I am not able to understand how to set that as an environment variable or pass to the container spec.

There are some stackoverflow questions that mention that we can create a persistent volume or secret to achieve this, but I am not sure, how that would work if we have to delete them. I do not want to create any extra objects and maintain the lifecycle of them.

Answer 1

It is not possible to do this without mounting a persistent volume. But the creation of persistent volume can be backed by just an in-memory store, instead of a block storage device. That way, we do not have to do any extra lifecycle management. The way to achieve that is:

apiVersion: v1
kind: ConfigMap
metadata:
  name: gatekeeper
data:
  gatekeeper.sh: |-
    #!/usr/bin/env bash
    set -e

    INGRESS_IP=$(kubectl get svc -lapp=nginx-ingress -o=jsonpath='{.items[].status.loadBalancer.ingress[].name}')
    # Do other validations/cleanup
    echo $INGRESS_IP > /opt/gkconf/discovery_url;
    exit 0

---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: gatekeeper
  labels:
    app: gatekeeper
spec:
  replicas: 1
  selector:
    matchLabels:
      app: gatekeeper
  template:
    metadata:
      name: gatekeeper
      labels:
        app: gatekeeper
    spec:
      initContainers:
        - name: gkinit
          command: [ "/opt/gk-init.sh" ]
          image: 'bitnami/kubectl:1.12'
          volumeMounts:
            - mountPath: /opt/gkconf
              name: gkconf
            - mountPath: /opt/gk-init.sh
              name: gatekeeper
              subPath: gatekeeper.sh
              readOnly: false
      containers:
        - name: gatekeeper
          image: my-gatekeeper-image:some-sha
          # ENTRYPOINT of above image should read the
          # file /opt/gkconf/discovery_url and then launch
          # the actual gatekeeper binary
          imagePullPolicy: Always
          ports:
            - containerPort: 80
              protocol: TCP
          volumeMounts:
            - mountPath: /opt/gkconf
              name: gkconf
      volumes:
        - name: gkconf
          emptyDir:
            medium: Memory
        - name: gatekeeper
          configMap:
            name: gatekeeper
            defaultMode: 0555