Kubernetes Multi-container Pods
Kubernetes Multi Container Pods
Multi Container Pods
As per Kubernetes documentation, The primary reason that Pods can have multiple containers is to support helper applications that assist a primary application. Typical examples of helper applications are data pullers, data pushers, and proxies. Helper and primary applications often need to communicate with each other. Typically this is done through a shared filesystem, as shown in this example.
We have not yet discussed volumes, but for now, it is sufficient to know that it is a storage component.
pradeep@learnk8s$ cat multi-container-pod.yaml
apiVersion: v1
kind: Pod
metadata:
name: multi-containers
spec:
restartPolicy: Never
volumes:
- name: common-data
emptyDir: {}
containers:
- name: nginx
image: nginx
volumeMounts:
- name: common-data
mountPath: /usr/share/nginx/html
- name: ubuntu
image: ubuntu
volumeMounts:
- name: common-data
mountPath: /ubuntu-data
command: ["/bin/sh"]
args: ["-c", "echo Hello from the Ubuntu container which is visible in the nginx container as they are sharing the same storage common-data > /ubuntu-data/index.html"]
pradeep@learnk8s$ kubectl create -f multi-container-pod.yaml
pod/multi-containers created
pradeep@learnk8s$ kubectl get pods -o wide | grep multi
multi-containers 1/2 NotReady 0 8m3s 10.244.1.34 k8s-m02 <none> <none>
If you notice, though we have defined two containers (nginx and ubuntu) in this Pod definition, there is only a single IP (10.244.1.34 in this case). This is important to understand. All containers in a pod share the same network resources.
Also, READY column shows 1/2 meaning, there are two containers but only one is Running. Let us find out more on this by describing it.
pradeep@learnk8s$ kubectl describe pods multi-containers
Name: multi-containers
Namespace: default
Priority: 0
Node: k8s-m02/192.168.177.30
Start Time: Wed, 16 Feb 2022 07:19:50 +0530
Labels: <none>
Annotations: <none>
Status: Running
IP: 10.244.1.34
IPs:
IP: 10.244.1.34
Containers:
nginx:
Container ID: docker://1cb2560ba88bf7f5a100c74246f10adecf6ce026d9f6031e43947c32ba021d3c
Image: nginx
Image ID: docker-pullable://nginx@sha256:2834dc507516af02784808c5f48b7cbe38b8ed5d0f4837f16e78d00deb7e7767
Port: <none>
Host Port: <none>
State: Running
Started: Wed, 16 Feb 2022 07:20:09 +0530
Ready: True
Restart Count: 0
Environment: <none>
Mounts:
/usr/share/nginx/html from common-data (rw)
/var/run/secrets/kubernetes.io/serviceaccount from kube-api-access-7jtgb (ro)
ubuntu:
Container ID: docker://d896bd674b06da660ee2e179d104af43344628dcf49edeb90630dc33fa63197e
Image: ubuntu
Image ID: docker-pullable://ubuntu@sha256:669e010b58baf5beb2836b253c1fd5768333f0d1dbcb834f7c07a4dc93f474be
Port: <none>
Host Port: <none>
Command:
/bin/sh
Args:
-c
echo Hello from the Ubuntu container which is visible in the nginx container as they are sharing the same storage common-data > /ubuntu-data/index.html
State: Terminated
Reason: Completed
Exit Code: 0
Started: Wed, 16 Feb 2022 07:20:20 +0530
Finished: Wed, 16 Feb 2022 07:20:20 +0530
Ready: False
Restart Count: 0
Environment: <none>
Mounts:
/ubuntu-data from common-data (rw)
/var/run/secrets/kubernetes.io/serviceaccount from kube-api-access-7jtgb (ro)
Conditions:
Type Status
Initialized True
Ready False
ContainersReady False
PodScheduled True
Volumes:
common-data:
Type: EmptyDir (a temporary directory that shares a pod's lifetime)
Medium:
SizeLimit: <unset>
kube-api-access-7jtgb:
Type: Projected (a volume that contains injected data from multiple sources)
TokenExpirationSeconds: 3607
ConfigMapName: kube-root-ca.crt
ConfigMapOptional: <nil>
DownwardAPI: true
QoS Class: BestEffort
Node-Selectors: <none>
Tolerations: node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 3m8s default-scheduler Successfully assigned default/multi-containers to k8s-m02
Normal Pulling 3m6s kubelet Pulling image "nginx"
Normal Pulled 2m49s kubelet Successfully pulled image "nginx" in 17.095268831s
Normal Created 2m49s kubelet Created container nginx
Normal Started 2m49s kubelet Started container nginx
Normal Pulling 2m49s kubelet Pulling image "ubuntu"
Normal Pulled 2m38s kubelet Successfully pulled image "ubuntu" in 10.692871602s
Normal Created 2m38s kubelet Created container ubuntu
Normal Started 2m38s kubelet Started container ubuntu
We can see that the nginx container is Running but the ubuntu container is terminated with Reason as Completed. That means, Ubuntu container has finished its task. The task that we have given to this container is to echo a message and write that to a file. That’s all.
To continue the verification, login to the nginx container. Remember the kubectl exec command?!
pradeep@learnk8s$ kubectl exec -it multi-containers -c nginx -- /bin/bash
root@multi-containers:/# curl localhost
Hello from the Ubuntu container which is visible in the nginx container as they are sharing the same storage common-data
root@multi-containers:/# exit
exit
The one difference that you might have noticed is the use of -c option here. When there are multiple containers in a pod, we need to be specific about which container we want to work with, by specifying its name. In this case , we want to login to the shell (bash) of the nginx container, and henc the -c nginx option. Also, we specify the command (/bin/bash) that we want to execute after the two dashes followed by a space (--).
We logged in to the nginx container, but the data written by the ubuntu container is visible here as both of them are using the common storage volume called common-data.
