Kubernetes to find Pod IP from another Pod - kubernetes

Question

I have the following pods hello-abc and hello-def.
And I want to send data from hello-abc to hello-def.
How would pod hello-abc know the IP address of hello-def?
And I want to do this programmatically.
What's the easiest way for hello-abc to find where hello-def?
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
name: hello-abc-deployment
spec:
replicas: 1
template:
metadata:
labels:
app: hello-abc
spec:
containers:
- name: hello-abc
image: hello-abc:v0.0.1
imagePullPolicy: Always
args: ["/hello-abc"]
ports:
- containerPort: 5000
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
name: hello-def-deployment
spec:
replicas: 1
template:
metadata:
labels:
app: hello-def
spec:
containers:
- name: hello-def
image: hello-def:v0.0.1
imagePullPolicy: Always
args: ["/hello-def"]
ports:
- containerPort: 5001
---
apiVersion: v1
kind: Service
metadata:
name: hello-abc-service
spec:
ports:
- port: 80
targetPort: 5000
protocol: TCP
selector:
app: hello-abc
type: NodePort
---
apiVersion: v1
kind: Service
metadata:
name: hello-def-service
spec:
ports:
- port: 80
targetPort: 5001
protocol: TCP
selector:
app: hello-def
type: NodePort

Answer 1

Preface
Since you have defined a service that routes to each deployment, if you have deployed both services and deployments into the same namespace, you can in many modern kubernetes clusters take advantage of kube-dns and simply refer to the service by name.
Unfortunately if kube-dns is not configured in your cluster (although it is unlikely) you cannot refer to it by name.
You can read more about DNS records for services here
In addition Kubernetes features "Service Discovery" Which exposes the ports and ips of your services into any container which is deployed into the same namespace.
Solution
This means, to reach hello-def you can do so like this
curl http://hello-def-service:${HELLO_DEF_SERVICE_PORT}
based on Service Discovery https://kubernetes.io/docs/concepts/services-networking/service/#environment-variables
Caveat: Its very possible that if the Service port changes, only pods that are created after the change in the same namespace will receive the new environment variables.
External Access
In addition, you can also reach this your service externally since you are using the NodePort feature, as long as your NodePort range is accessible from outside.
This would require you to access your service by node-ip:nodePort
You can find out the NodePort which was randomly assigned to your service with kubectl describe svc/hello-def-service
Ingress
To reach your service from outside you should implement an ingress service such as nginx-ingress
https://github.com/helm/charts/tree/master/stable/nginx-ingress
https://github.com/kubernetes/ingress-nginx
Sidecar
If your 2 services are tightly coupled, you can include both in the same pod using the Kubernetes Sidecar feature. In this case, both containers in the pod would share the same virtual network adapter and accessible via localhost:$port
https://kubernetes.io/docs/concepts/workloads/pods/pod/#uses-of-pods
Service Discovery
When a Pod is run on a Node, the kubelet adds a set of environment
variables for each active Service. It supports both Docker links
compatible variables (see makeLinkVariables) and simpler
{SVCNAME}_SERVICE_HOST and {SVCNAME}_SERVICE_PORT variables, where the
Service name is upper-cased and dashes are converted to underscores.
Read more about service discovery here:
https://kubernetes.io/docs/concepts/services-networking/service/#environment-variables

Answer 2

You should be able to reach hello-def-service from pods in hello-abc via DNS as specified here: https://kubernetes.io/docs/concepts/services-networking/dns-pod-service/#services
However, kube-dns or CoreDNS has to be configured/installed in your k8s cluster before DNS records can be utilized in your cluster.
Specifically, you should be reach hello-def-service via the DNS record http://hello-def-service for the service running in the same namespace as hello-abc-service
And you should be able to reach hello-def-service running in another namespace ohter_namespace via the DNS record hello-def-service.other_namespace.svc.cluster.local.
If, for some reason, you do not have DNS add-ons installed in your cluster, you still can find the virtual IP of the hello-def-service via environment variables in hello-abc pods. As is documented here.