How to communicate between pods in a service?

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.