How to communicate between pods in a service? - kubernetes

Question

Suppose I have a service containing two pods. One of the pods is an HTTP server, and the other pod needs to hit a REST endpoint on this pod. Is there a hostname that the second pod can use to address the first pod?

Answer 1

I'm assuming when you say "service" you aren't referring to the Kubernetes lexicon of a Service object, otherwise your two Pods in the Service would be identical, so let's start by teasing out what a "Service" means in Kubernetes land.
You will have to create an additional Kubernetes object called a Service to get your hostname for your HTTP server's Pod. When you create a Service you will define a .spec.selector that points to a set of labels on the HTTP service's Pod. For the sake of example, let's say the label is app: nginx. The name of that Service object will become the internal DNS record that can be queried by the second Pod.
A simplified example:
apiVersion: v1
kind: Pod
metadata:
name: http-service
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx:1.7.9
ports:
- containerPort: 80
---
kind: Service
apiVersion: v1
metadata:
name: my-http-service
spec:
selector:
app: nginx
ports:
- protocol: TCP
port: 80
targetPort: 80
Now your second Pod can make requests to the HTTP service by the Service name, my-http-service.
It's also worth mentioning that Kubernetes best practice dictates that these Pods be managed by controllers such as Deployments or ReplicaSets for all sorts of reasons, including high availability of your applications.

Answer 2

Note that a service is a different concept in Docker then in K8s. The easiest way of getting what you want would be creating the two pods; say pod-1 and pod-2, with a yaml file similar to this one:
apiVersion: v1
kind: Pod
metadata:
name: NAME
labels:
app: LABEL
spec:
containers:
- name: nginx
image: nginx
ports:
- containerPort: 80
Say NAME and LABEL are nginx and nginx-1, so you have now two pods called nginx and nginx-1, with labels app: nginx and app: nginx-1. Actually, as only one of them is going to be exposed, the other label is irrelevant.
Now you expose the pod either with a yaml file or from command line.
Yaml file:
apiVersion: v1
kind: Service
metadata:
name: server
spec:
ports:
- port: 80
targetPort: 80
protocol: TCP
name: http
selector:
app: nginx
Command line:
kubectl expose pod nginx --port 80 --name server
If you now access the second pod (nginx-1) and curl the service directly, you would end up hitting the pod behind it (nginx):
nerus:~/workspace $ kubectl exec -it nginx-1 bash
root#nginx-1:/# curl -I server
HTTP/1.1 200 OK

Answer 3

You can expose your pod kubectl expose deployment --type=name of pod , then you can use the kubectl describe It will show you port number . Then you access you pod at http://localhost:portnumber in last command ....**.....Hope it will help .

Answer 4

Ironically, you answered your own question: a Service is a stable name and IP that abstracts over the individual coming-and-going of the Pods to which it will route traffic, as described very well in the fine manual.
If the-http-pod needs to reach the-rest-pod, then create a Service that matches the labels on the PodSpec that created the-rest-pod, and from that point forward the-http-pod can always use ${serviceName}.${serviceNamespace}.svc.cluster.local to each any Pod that has matching labels