Sidecar (how containers in a pod communicate)

Reference Link

1. Introduction

• Sidecar containers extend and enhance the “main” container

• They take existing containers and make them better

2. Creating 2 containers in a pod

• a debian container

• a nginx web-server container

3. twocontainers.yaml

apiVersion: v1
kind: Pod
metadata:
  name: two-containers
spec:

  restartPolicy: Never

  volumes:
  - name: shared-data
    emptyDir: {}

  containers:

  - name: nginx-container
    image: nginx
    volumeMounts:
    - name: shared-data
      mountPath: /usr/share/nginx/html

  - name: debian-container
    image: debian
    volumeMounts:
    - name: shared-data
      mountPath: /pod-data
    command: ["/bin/sh"]
    args: ["-c", "echo Hello from the debian container > /pod-data/index.html"]

• these 2 containers have a shared volume called shared-data

4. Creating the pod with 2 containers

kubectl create -f https://k8s.io/docs/tasks/access-application-cluster/two-container-pod.yaml

hadoop@k8s-00:~$ kubectl create -f https://k8s.io/docs/tasks/access-application-cluster/two-container-pod.yaml
pod "two-containers" created

• view information about the pod and the exisiting 2 containers

    image: debian:latest
    imageID: docker-pullable://debian@sha256:de3eac83cd481c04c5d6c7344cd7327625a1d8b2540e82a8231b5675cef0ae5f
    lastState: {}
    name: debian-container
    ready: false
    restartCount: 0
    state:
      terminated:
        containerID: docker://117897e0812ce815efce5530da34c65b0db3ebe1bc14c452c81f4f4a26d4b422
        exitCode: 0
        finishedAt: 2018-06-18T06:15:54Z
        reason: Completed
        startedAt: 2018-06-18T06:15:54Z
  - containerID: docker://249f487e6f6ed95a8e411ef890c89e10bd1cb8b40abec40ef7225dd9d7c9e56d
    image: nginx:latest
    imageID: docker-pullable://nginx@sha256:3e2ffcf0edca2a4e9b24ca442d227baea7b7f0e33ad654ef1eb806fbd9bedcf0
    lastState: {}
    name: nginx-container
    ready: true
    restartCount: 0
    state:
      running:
        startedAt: 2018-06-18T06:15:42Z

• the above log tells us that debian container has stopped

name: debian-container
    ready: false
    restartCount: 0
    state:
      terminated:

• while the nginx container is still running

image: nginx:latest
    imageID: docker-pullable://nginx@sha256:3e2ffcf0edca2a4e9b24ca442d227baea7b7f0e33ad654ef1eb806fbd9bedcf0
    lastState: {}
    name: nginx-container
    ready: true

• let's get to the shell of the nginx container

kubectl exec -it two-containers -c nginx-container -- /bin/bash

• install curl and ps inside the nginx container

apt-get update
apt-get install curl procsps

• let's check if the nginx is running (verification)

root@two-containers:/# ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.1  32616  5256 ?        Ss   06:15   0:00 nginx: master process nginx -g daemon off;
nginx        6  0.0  0.0  33068  2356 ?        S    06:15   0:00 nginx: worker process

we can see that nginx is running

• the debian container is responsible for creating index.html and placing it in the root directory of nginx container

command: ["/bin/sh"]
    args: ["-c", "echo Hello from the debian container > /pod-data/index.html"]

• let's verify the above index.html by curl command

root@two-containers:/# curl localhost
Hello from the debian container

• therefore, even if the debian container was killed, it was able to create a index.html file and place it in the root directory of nginx container for it to access it.