I'm migrating an application to Docker/Kubernetes. This application has 20+ well-known ports it needs to be accessed on. It needs to be accessed from outside the kubernetes cluster. For this, the application writes its public accessible IP to a database so the outside service knows how to access it. The IP is taken from the downward API (status.hostIP).
One solution is defining the well-known ports as (static) nodePorts in the service, but I don't want this, because it will limit the usability of the node: if another service has started and incidentally taken one of the known ports the application will not be able to start. Also, because Kubernetes opens the ports on all nodes in the cluster, I can only run 1 instance of the application per cluster.
Now I want to make the application aware of the port mappings done by the NodePort-service. How can this be done? As I don't see a hard link between the Service and the Statefulset object in Kubernetes.
Here is my (simplified) Kubernetes config:
apiVersion: v1
kind: Service
metadata:
name: my-app-svc
labels:
app: my-app
spec:
ports:
- port: 6000
targetPort: 6000
protocol: TCP
name: debug-port
- port: 6789
targetPort: 6789
protocol: TCP
name: traffic-port-1
selector:
app: my-app
type: NodePort
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
name: my-app-sf
spec:
serviceName: my-app-svc
replicas: 1
selector:
matchLabels:
app: my-app
template:
metadata:
labels:
app: my-app
spec:
containers:
- name: my-app
image: my-repo/myapp/my-app:latest
imagePullPolicy: Always
env:
- name: K8S_ServiceAccountName
valueFrom:
fieldRef:
fieldPath: spec.serviceAccountName
- name: K8S_ServerIP
valueFrom:
fieldRef:
fieldPath: status.hostIP
- name: serverName
valueFrom:
fieldRef:
fieldPath: metadata.name
ports:
- name: debug
containerPort: 6000
- name: traffic1
containerPort: 6789This can be done with an initContainer.
You can define an initContainer to get the nodeport and save into a directory that shared with the container, then container can get the nodeport from that directory later, a simple demo like this:
apiVersion: v1
kind: Pod
metadata:
name: my-app
spec:
containers:
- name: my-app
image: busybox
command: ["sh", "-c", "cat /data/port; while true; do sleep 3600; done"]
volumeMounts:
- name: config-data
mountPath: /data
initContainers:
- name: config-data
image: tutum/curl
command: ["sh", "-c", "TOKEN=`cat /var/run/secrets/kubernetes.io/serviceaccount/token`; curl -kD - -H \"Authorization: Bearer $TOKEN\" https://kubernetes.default:443/api/v1/namespaces/test/services/app 2>/dev/null | grep nodePort | awk '{print $2}' > /data/port"]
volumeMounts:
- name: config-data
mountPath: /data
volumes:
- name: config-data
emptyDir: {}