Kubernetes sidecar example | Create multi-container Pod

Written by - Deepak Prasad

In this tutorial I will explain about Kubernetes Sidecar using an example.



You must have a basic idea on following topics:
Architecture of Kubernetes
Basic understanding of Kubernetes Pods


Overview on Kubernetes Sidecar Container

  • The sidecar pattern is about co-locating another container in a pod in addition to the main application container.
  • The application container is unaware of the sidecar container and just goes about its business.
  • One Pod can host many containers, using the same of different Docker images. Containers in the Pod can communicate using the localhost address
  • The sidecar container will use the same network namespace as of the main container so you don't have to assign any network interfaces or ip addresses to the sidecar container. The containers will talk to each other using different ports.
  • Each container has it's own filesystem, but volumes are defined at the Pod level and can be mounted into multiple containers to share data


Let's take an example of logging service:

Imagine you have a Linux server with a web server application which stores the logs inside /var/log/app.log . Later the log entries from this file are processed to generate alarms to a central logging server. Suppose you had to apply some patch to the logging service so even your application gets impacted as both are running on the same server.
So we add a sidecar container along with the main container, now the application will run in the main container while sidecar container will be used for the logging service. So now any patch to be applied to logging service can be done separately and can be re-deployed without any impacts to the main application container.


Multi-container Architecture

Following image depicts how a sidecar container and main container would be there in a Pod. You can add n number of sidecar containers to a Pod
Kubernetes sidecar example | Create multi-container Pod


Kubernetes sidecar example

Let us quickly deploy a single pod with two containers i.e one sidecar container with main container. Here is my YAML file to create this Pod:

~]# cat sidecar-example.yaml
apiVersion: v1
kind: Pod
  name: sidecar-pod-1
  - name: log
    emptyDir: {}

  - image: busybox
    name: main-container
     - /bin/sh
     - -c
     - >
      while true; do
        echo "$(date) INFO hello from main-container" >> /var/log/myapp.log ;
        sleep 1;
    - name: log
      mountPath: /var/log

  - name: sidecar-container
    image: busybox
     - /bin/sh
     - -c
     - tail -fn+1 /var/log/myapp.log
    - name: log
      mountPath: /var/log


Let us understand this YAML file:

  • For the sake of understanding I have named my containers as main-container and sidecar-container.
  • The main container will be our application which will continuously write something to /var/log/myapp.log
  • The /var/log/ path is mounted on the containers using separate volume. This path is mounted using volumeMounts in both the containers so that the path is shared across both the containers.
  • The sidecar container will read the log file content using tail -fn+1 /var/log/myapp.log

Let us create this Pod:

 ~]# kubectl create -f sidecar-example.yaml
pod/sidecar-pod-1 created

Check the status of the Pod, it shows that there are two containers and both of them are in Running state:
Kubernetes sidecar example | Create multi-container Pod


List all the containers from the Pod

Use the following command to list the available containers inside the Pod:

 ~]# kubectl get pods sidecar-pod-1 -o jsonpath='{.spec.containers[*].name}'
main-container sidecar-container

So we have two containers inside sidecar-pod-1:

  • main-container
  • sidecar-container


Verify communication between main and sidecar container

Now you can check the output of sidecar-container which should show the logs coming from main-container:

 ~]# kubectl logs -f sidecar-pod-1 -c sidecar-container
Wed Jun  9 08:52:11 UTC 2021 INFO hello from main-container
Wed Jun  9 08:52:12 UTC 2021 INFO hello from main-container
Wed Jun  9 08:52:13 UTC 2021 INFO hello from main-container
Wed Jun  9 08:52:14 UTC 2021 INFO hello from main-container
Wed Jun  9 08:52:15 UTC 2021 INFO hello from main-container
Wed Jun  9 08:52:16 UTC 2021 INFO hello from main-container


Verify Network Namespace between main and sidecar container

As we told initially, all the containers inside the Pod will share the same network namespace. So, first check the IP of the POD:

~]# kubectl get pods -o wide
NAME                            READY   STATUS    RESTARTS   AGE     IP             NODE                   NOMINATED NODE   READINESS GATES
nginx-deploy-5cc69bf9c4-ffdwz   1/1     Running   2          6d18h    worker-2.example.com   <none>           <none>
nginx-deploy-5cc69bf9c4-mgkjp   1/1     Running   2          6d18h   worker-1.example.com   <none>           <none>
sidecar-pod-1                   2/2     Running   0          102m   worker-1.example.com   <none>           <none>

So the IP of our sidecar pod is Let us check this on both of our containers:
Kubernetes sidecar example | Create multi-container Pod

Similarly you can check, both the containers will have the same hostname:

[root@controller ~]# kubectl exec -it sidecar-pod-1 -c main-container -- cat /etc/hostname

[root@controller ~]# kubectl exec -it sidecar-pod-1 -c sidecar-container -- cat /etc/hostname



In this tutorial we learned about Kubernetes sidecar and I also shared an example to help you understand

  • How to create sidecar container in Kubernetes
  • How sidecar container communicate internally with main container inside the Pod
  • How to share volume across all the containers inside the same Pod
  • How all the containers inside the same Pod share the same namespace.


Further Readings

Using a sidecar container with the logging agent
Communicate Between Containers in the Same Pod Using a Shared Volume


Related Searches: kubernetes sidecar container example, kubernetes sidecar injection, kubernetes sidecar yaml, kubernetes sidecar logging, kubernetes shared volume between pods

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Deepak Prasad

He is the founder of GoLinuxCloud and brings over a decade of expertise in Linux, Python, Go, Laravel, DevOps, Kubernetes, Git, Shell scripting, OpenShift, AWS, Networking, and Security. With extensive experience, he excels in various domains, from development to DevOps, Networking, and Security, ensuring robust and efficient solutions for diverse projects. You can reach out to him on his LinkedIn profile or join on Facebook page.

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