Pods- Multicontainer pod -Use Case

 

Example of Multi-Container Pod

Let’s talk about communication between containers in a Pod. Having multiple containers in a single Pod makes it relatively straightforward for them to communicate with each other. They can do this using several different methods.

Use Cases for Multi-Container Pods

The primary purpose of a multi-container Pod is to support co-located, co-managed helper processes for a primary application. There are some general patterns for using helper processes in Pods:

Sidecar containers help the main container. Some examples include log or data change watchers, monitoring adapters, and so on. A log watcher, for example, can be built once by a different team and reused across different applications. Another example of a sidecar container is a file or data loader that generates data for the main container.

Proxies, bridges, and adapters connect the main container with the external world. For example, Apache HTTP server or nginx can serve static files. It can also act as a reverse proxy to a web application in the main container to log and limit HTTP requests. Another example is a helper container that re-routes requests from the main container to the external world. This makes it possible for the main container to connect to the localhost to access, for example, an external database, but without any service discovery.

Shared volumes in a Kubernetes Pod

In Kubernetes, you can use a shared Kubernetes Volume as a simple and efficient way to share data between containers in a Pod. For most cases, it is sufficient to use a directory on the host that is shared with all containers within a Pod.

Kubernetes Volumes enables data to survive container restarts, but these volumes have the same lifetime as the Pod. That means that the volume (and the data it holds) exists exactly as long as that Pod exists. If that Pod is deleted for any reason, even if an identical replacement is created, the shared Volume is also destroyed and created anew.

A standard use case for a multi-container Pod with a shared Volume is when one container writes logs or other files to the shared directory, and the other container reads from the shared directory. For example, we can create a Pod like so (pods03.yaml):

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

In this file (pods03.yaml) a volume named html has been defined. Its type is emptyDir, which means that the volume is first created when a Pod is assigned to a node, and exists as long as that Pod is running on that node. As the name says, it is initially empty. The 1st container runs nginx server and has the shared volume mounted to the directory /usr/share/nginx/html. The 2nd container uses the Debian image and has the shared volume mounted to the directory /html. Every second, the 2nd container adds the current date and time into the index.html file, which is located in the shared volume. When the user makes an HTTP request to the Pod, the Nginx server reads this file and transfers it back to the user in response to the request.

kubectl apply -f pods03.yaml

[Captains-Bay]🚩 >  kubectl get po,svc
NAME      READY     STATUS    RESTARTS   AGE
po/mc1    2/2       Running   0          11s

NAME             TYPE        CLUSTER-IP    EXTERNAL-IP   PORT(S)   AGE
svc/kubernetes   ClusterIP   10.15.240.1   <none>        443/TCP   1h
[Captains-Bay]🚩 >  kubectl describe po mc1
Name:         mc1
Namespace:    default
Node:         gke-k8s-lab1-default-pool-fd9ef5ad-pc18/10.140.0.16
Start Time:   Wed, 08 Jan 2020 14:29:08 +0530
Labels:       <none>
Annotations:  kubectl.kubernetes.io/last-applied-configuration={"apiVersion":"v1","kind":"Pod","metadata":{"annotations":{},"name":"mc1","namespace":"default"},"spec":{"containers":[{"image":"nginx","name":"1st","v...
              kubernetes.io/limit-ranger=LimitRanger plugin set: cpu request for container 1st; cpu request for container 2nd
Status:       Running
IP:           10.12.2.6
Containers:
  1st:
    Container ID:   docker://b08eb646f90f981cd36c605bf8fead3ca62178c7863598fd4558cb026ed067dd
    Image:          nginx
    Image ID:       docker-pullable://nginx@sha256:36b77d8bb27ffca25c7f6f53cadd059aca2747d46fb6ef34064e31727325784e
    Port:           <none>
    State:          Running
      Started:      Wed, 08 Jan 2020 14:29:09 +0530
    Ready:          True
    Restart Count:  0
    Requests:
      cpu:        100m
    Environment:  <none>
    Mounts:
      /usr/share/nginx/html from html (rw)
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-xhgmm (ro)
  2nd:
    Container ID:  docker://63180b4128d477810d6062342f4b8e499de684ffd69ad245c29118e1661eafcb
    Image:         debian
    Image ID:      docker-pullable://debian@sha256:c99ed5d068d4f7ff36c7a6f31810defebecca3a92267fefbe0e0cf2d9639115a
    Port:          <none>
    Command:
      /bin/sh
      -c
    Args:
      while true; do date >> /html/index.html; sleep 1; done
    State:          Running
      Started:      Wed, 08 Jan 2020 14:29:14 +0530
    Ready:          True
    Restart Count:  0
    Requests:
      cpu:        100m
    Environment:  <none>
    Mounts:
      /html from html (rw)
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-xhgmm (ro)
Conditions:
  Type              Status
  Initialized       True 
  Ready             True 
  ContainersReady   True 
  PodScheduled      True 
Volumes:
  html:
    Type:    EmptyDir (a temporary directory that shares a pod's lifetime)
    Medium:  
  default-token-xhgmm:
    Type:        Secret (a volume populated by a Secret)
    SecretName:  default-token-xhgmm
    Optional:    false
QoS Class:       Burstable
Node-Selectors:  <none>
Tolerations:     node.kubernetes.io/not-ready:NoExecute for 300s
                 node.kubernetes.io/unreachable:NoExecute for 300s
Events:
  Type    Reason     Age   From                                              Message
  ----    ------     ----  ----                                              -------
  Normal  Scheduled  18s   default-scheduler                                 Successfully assigned default/mc1 to gke-k8s-lab1-default-pool-fd9ef5ad-pc18
  Normal  Pulling    17s   kubelet, gke-k8s-lab1-default-pool-fd9ef5ad-pc18  pulling image "nginx"
  Normal  Pulled     17s   kubelet, gke-k8s-lab1-default-pool-fd9ef5ad-pc18  Successfully pulled image "nginx"
  Normal  Created    17s   kubelet, gke-k8s-lab1-default-pool-fd9ef5ad-pc18  Created container
  Normal  Started    17s   kubelet, gke-k8s-lab1-default-pool-fd9ef5ad-pc18  Started container
  Normal  Pulling    17s   kubelet, gke-k8s-lab1-default-pool-fd9ef5ad-pc18  pulling image "debian"
  Normal  Pulled     13s   kubelet, gke-k8s-lab1-default-pool-fd9ef5ad-pc18  Successfully pulled image "debian"
  Normal  Created    12s   kubelet, gke-k8s-lab1-default-pool-fd9ef5ad-pc18  Created container
  Normal  Started    12s   kubelet, gke-k8s-lab1-default-pool-fd9ef5ad-pc18  Started container

$ kubectl exec mc1 -c 1st -- /bin/cat /usr/share/nginx/html/index.html
...
Wed Jan  8 08:59:14 UTC 2020
Wed Jan  8 08:59:15 UTC 2020
Wed Jan  8 08:59:16 UTC 2020
 
$ kubectl exec mc1 -c 2nd -- /bin/cat /html/index.html
...
Wed Jan  8 08:59:14 UTC 2020
Wed Jan  8 08:59:15 UTC 2020
Wed Jan  8 08:59:16 UTC 2020

Cleaning Up

kubectl delete -f pods03.yaml