Setting environment variables based on services in Kubernetes - kubernetes

Question

I am running a simple app based on an api and web interface in Kubernetes. However, I can't seem to get the api to talk to the web interface. In my local environment, I just define a variable API_URL in the web interface with eg. localhost:5001 and the web interface correctly connects to the api. As api and web are running in different pods I need to make them talk to each other via services in Kubernetes. So far, this is what I am doing, but without any luck.
I set-up a deployment for the API
apiVersion: apps/v1
kind: Deployment
metadata:
name: api-deployment
spec:
replicas: 1
selector:
matchLabels:
component: api
template:
metadata:
labels:
component: api
spec:
containers:
- name: api
image: gcr.io/myproject-22edwx23/api:latest
ports:
- containerPort: 5001
I attach a service to it:
apiVersion: v1
kind: Service
metadata:
name: api-cluster-ip-service
spec:
type: NodePort
selector:
component: api
ports:
- port: 5001
targetPort: 5001
and then create a web deployment that should connect to this api.
apiVersion: apps/v1
kind: Deployment
metadata:
name: web-deployment
spec:
replicas: 1
selector:
matchLabels:
component: web
template:
metadata:
labels:
component: web
spec:
containers:
- name: web
image: gcr.io/myproject-22edwx23/web:latest
ports:
- containerPort: 5000
env:
- name: API_URL
value: http://api-cluster-ip-service:5001
afterwards, I add a service for the web interface + ingress etc but that seems irrelevant for the issues. I am wondering if the setting of API_URL correctly picks up the host of the api via http://api-cluster-ip-service:5001?
Or can I not rely on Kubernetes getting the appropriate dns for the api and should the web app call the api via the public internet.

Answer 1

If you want to check API_URL variable value, simply run
kubectl exec -it web-deployment-pod env | grep API_URL
The kube-dns service listens for service and endpoint events from the Kubernetes API and updates its DNS records as needed. These events are triggered when you create, update or delete Kubernetes services and their associated pods.
kubelet sets each new pod's search option in /etc/resolv.conf
Still, if you want to http from one pod to another via cluster service it is recommended to refer service's ClusterIP as follows
api-cluster-ip-service.default.svc.cluster.local
You should have service IP assigned to env variable within your web pod, so there's no need to re-invent it:
sukhoversha#sukhoversha:~/GCP$ kk exec -it web-deployment-675f8fcf69-xmqt8 env | grep -i service
API_CLUSTER_IP_SERVICE_PORT=tcp://10.31.253.149:5001
API_CLUSTER_IP_SERVICE_PORT_5001_TCP=tcp://10.31.253.149:5001
API_CLUSTER_IP_SERVICE_PORT_5001_TCP_PORT=5001
API_CLUSTER_IP_SERVICE_PORT_5001_TCP_ADDR=10.31.253.149
To read more about DNS for Services. A service defines environment variables naming the host and port.

Answer 2

If you want to use environment variables you can do the following:
Example in Python:
import os
API_URL = os.environ['API_CLUSTER_IP_SERVICE_SERVICE_HOST'] + ":" + os.environ['API_CLUSTER_IP_SERVICE_SERVICE_PORT']
Notice that the environment variable is based on your service name. If you want to check all environment variables available in a Pod:
kubectl get pods #get {pod name}
kubectl exec -it {pod_name} printenv
P.S. Be careful that a Pod gets its environment variables during its creation and it will not be able to get it from services created after it.