How Pods Communicate: Shared Volumes and IPC in Kubernetes

Kubernetes Networking Models

Kubernetes provides a container‑oriented solution where Pods act as virtual environments that can host one or more containers. Managing container communication inside Pods and forwarding container ports are key aspects of Kubernetes networking, which offers four models:

Container‑to‑container communication

Pod‑to‑Pod communication

Pod‑to‑Service communication

External‑to‑internal communication

Communication via Shared Volumes

When multiple containers run in a single Pod, they can share data through a shared volume . The most common approach is to use an emptyDir volume, which lives as long as the Pod exists. Below is a minimal manifest that defines a Pod with an emptyDir volume named html and two containers that mount it:

apiVersion: v1
kind: Pod
metadata:
  name: mc1
spec:
  volumes:
  - name: html
    emptyDir: {}
  containers:
  - name: 1st
    image: nginx
    volumeMounts:
    - name: html
      mountPath: /usr/share/nginx/html
  - name: 2nd
    image: debian
    volumeMounts:
    - name: html
      mountPath: /html
    command: ["/bin/sh", "-c"]
    args:
    - while true; do
        date >> /html/index.html;
        sleep 1;
      done

The first container runs Nginx and serves the files from the shared directory, while the second container appends the current date to index.html every second. You can verify the content with:

$ kubectl exec mc1 -c 1st -- /bin/cat /usr/share/nginx/html/index.html
$ kubectl exec mc1 -c 2nd -- /bin/cat /html/index.html

Communication via IPC

Containers in the same Pod share the same IPC namespace, allowing them to use standard inter‑process communication mechanisms such as SystemV semaphores or POSIX shared memory. The following manifest defines a Pod with a producer and a consumer container that communicate through a Linux message queue:

apiVersion: v1
kind: Pod
metadata:
  name: mc2
spec:
  containers:
  - name: producer
    image: allingeek/ch6_ipc
    command: ["./ipc", "-producer"]
  - name: consumer
    image: allingeek/ch6_ipc
    command: ["./ipc", "-consumer"]
  restartPolicy: Never

After creating the Pod, monitor its status with:

$ kubectl get pods -w

And view the logs to confirm that the consumer receives all messages produced by the producer:

$ kubectl logs mc2 -c producer
$ kubectl logs mc2 -c consumer

Conclusion

Multi‑container Pods enable side‑car patterns such as data extractors, pushers, or proxies. Shared volumes provide a simple way to exchange files, but their data is lost when the Pod is deleted. IPC offers an alternative for in‑memory communication. Understanding these mechanisms prepares you to explore other Kubernetes networking models like Pod‑to‑Pod or Pod‑to‑Service communication.