Pods- MultiContainer Pods

 

Adding a 2nd container to a Pod

In the microservices architecture, each module should live in its own space and communicate with other modules following a set of rules. But, sometimes we need to deviate a little from this principle. Suppose you have an Nginx web server running and we need to analyze its web logs in real-time. The logs we need to parse are obtained from GET requests to the web server. The developers created a log watcher application that will do this job and they built a container for it. In typical conditions, you’d have a pod for Nginx and another for the log watcher. However, we need to eliminate any network latency so that the watcher can analyze logs the moment they are available. A solution for this is to place both containers on the same pod.

Having both containers on the same pod allows them to communicate through the loopback interface (ifconfig lo) as if they were two processes running on the same host. They also share the same storage volume.

Let us see how a pod can host more than one container. Let’s take a look to the multipod.yaml file. It contains the following lines:

apiVersion: v1
kind: Pod
metadata:
  name: webserver
spec:
  containers:
  - name: webserver
    image: nginx:latest
    ports:
    - containerPort: 80
  - name: webwatcher
    image: afakharany/watcher:latest

Run the following command:

$ kubectl apply -f mulipod.yaml

$ kubectl get po -o wide
NAME        READY   STATUS              RESTARTS   AGE   IP       NODE                                                NOMINATED NODE   READINESS GATES
webserver   0/2     ContainerCreating   0          13s   <none>   gke-standard-cluster-1-default-pool-78257330-5hs8   <none>           <none>

$ kubectl get po,svc,deploy
NAME            READY   STATUS    RESTARTS   AGE
pod/webserver   2/2     Running   0          3m6s
NAME                 TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)   AGE
service/kubernetes   ClusterIP   10.12.0.1    <none>        443/TCP   107m

$ kubectl get po -o wide
NAME        READY   STATUS    RESTARTS   AGE     IP         NODE                                                NOMINATED NODE   READINESS GATES
webserver   2/2     Running   0          3m37s   10.8.0.5   gke-standard-cluster-1-default-pool-78257330-5hs8   <none>           <none>

How to verify 2 containers are running inside a Pod?

$ kubectl describe po

Containers:
  webserver:
    Container ID:   docker://0564fcb88f7c329610e7da24cba9de6555c0183814cf517e55d2816c6539b829
    Image:          nginx:latest
    Image ID:       docker-pullable://nginx@sha256:36b77d8bb27ffca25c7f6f53cadd059aca2747d46fb6ef34064e31727325784e
    Port:           80/TCP
    State:          Running
      Started:      Wed, 08 Jan 2020 13:21:57 +0530
    Ready:          True
    Restart Count:  0
    Requests:
      cpu:        100m
    Environment:  <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-xhgmm (ro)
  webwatcher:
    Container ID:   docker://4cebbb220f7f9695f4d6492509e58152ba661f3ab8f4b5d0a7adec6c61bdde26
    Image:          afakharany/watcher:latest
    Image ID:       docker-pullable://afakharany/watcher@sha256:43d1b12bb4ce6e549e85447678a28a8e7b9d4fc398938a6f3e57d2908a9b7d80
    Port:           <none>
    State:          Running
      Started:      Wed, 08 Jan 2020 13:22:26 +0530
    Ready:          True
    Restart Count:  0
    Requests:

Since we have two containers in a pod, we will need to use the -c option with kubectl when we need to address a specific container. For example:

$ kubectl exec -it webserver -c webwatcher -- /bin/bash

root@webserver:/# cat /etc/hosts
# Kubernetes-managed hosts file.
127.0.0.1       localhost
::1     localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
fe00::0 ip6-mcastprefix
fe00::1 ip6-allnodes
fe00::2 ip6-allrouters
10.8.0.5        webserver

Please exit from the shell (/bin/bash) session.

root@webserver:/# exit

Cleaning up

kubectl delete -f multipod.yaml