How to get kubernetes service external ip dynamically inside manifests file? - kubernetes

Question

We are creating a deployment in which the command needs the IP of the pre-existing service pointing to a statefulset. Below is the manifest file for the deployment. Currently, we are manually entering the service external IP inside this deployment manifest. Now we would like it to auto-populate during runtime. Is there a way to achieve this dynamically using environment variables or another way?
apiVersion: apps/v1
kind: Deployment
metadata:
name: app-api
namespace: app-api
spec:
selector:
matchLabels:
app: app-api
replicas: 1
template:
metadata:
labels:
app: app-api
spec:
containers:
- name: app-api
image: asia-south2-docker.pkg.dev/rnd20/app-api/api:09
command: ["java","-jar","-Dallow.only.apigateway.request=false","-Dserver.port=8084","-Ddedupe.searcher.url=http://10.10.0.6:80","-Dspring.cloud.zookeeper.connect-string=10.10.0.6:2181","-Dlogging$.file.path=/usr/src/app/logs/springboot","/usr/src/app/app_api/dedupe-engine-components.jar",">","/usr/src/app/out.log"]
livenessProbe:
httpGet:
path: /health
port: 8084
httpHeaders:
- name: Custom-Header
value: ""
initialDelaySeconds: 60
periodSeconds: 60
ports:
- containerPort: 4016
resources:
limits:
cpu: 1
memory: "2Gi"
requests:
cpu: 1
memory: "2Gi"
NOTE: The IP in question here is the Internal load balancer IP, i.e. the external IP for the service and the service is in a different namespace. Below is the manifest for the same
apiVersion: v1
kind: Service
metadata:
name: app
namespace: app
annotations:
cloud.google.com/load-balancer-type: "Internal"
labels:
app: app
spec:
selector:
app: app
type: LoadBalancer
ports:
- name: container
port: 80
targetPort: 8080
protocol: TCP

Answer 1

You could use the following command instead:
command:
- /bin/bash
- -c
- |-
set -exuo pipefail
ip=$(dig +search +short servicename.namespacename)
exec java -jar -Dallow.only.apigateway.request=false -Dserver.port=8084 -Ddedupe.searcher.url=http://$ip:80 -Dspring.cloud.zookeeper.connect-string=$ip:2181 -Dlogging$.file.path=/usr/src/app/logs/springboot /usr/src/app/app_api/dedupe-engine-components.jar > /usr/src/app/out.log
It first resolves the ip address using dig (if you don't have dig in your image - you need to substitute it with something else you have), then execs your original java command.
As of today I'm not aware of any "native" kubernetes way to provide IP meta information directly to the pod.

Answer 2

If you are sure they exist before, and you deploy into the same namespace, you can read them from environment variables. It's documented here: https://kubernetes.io/docs/concepts/services-networking/service/#environment-variables.
When a Pod is run on a Node, the kubelet adds a set of environment variables for each active Service. It adds {SVCNAME}_SERVICE_HOST and {SVCNAME}_SERVICE_PORT variables, where the Service name is upper-cased and dashes are converted to underscores. It also supports variables (see makeLinkVariables) that are compatible with Docker Engine's "legacy container links" feature.
For example, the Service redis-master which exposes TCP port 6379 and has been allocated cluster IP address 10.0.0.11, produces the following environment variables:
REDIS_MASTER_SERVICE_HOST=10.0.0.11
REDIS_MASTER_SERVICE_PORT=6379
REDIS_MASTER_PORT=tcp://10.0.0.11:6379
REDIS_MASTER_PORT_6379_TCP=tcp://10.0.0.11:6379
REDIS_MASTER_PORT_6379_TCP_PROTO=tcp
REDIS_MASTER_PORT_6379_TCP_PORT=6379
REDIS_MASTER_PORT_6379_TCP_ADDR=10.0.0.11
Note, those wont update after the container is started.