When I began my journey of learning Kubernetes, I always thought why Kubernetes has made the pod its smallest entity, why not the container. But when I started diving deep in it I realized, there is a big rationale behind it and now I thank Kubernetes for making the Pod as an only object, not containers.
After being inspired by the working of a Pod, I would like to share my experience and knowledge with you guys.
What exactly Pod means?
The literal meaning of pod means the peel of pea which holds the beans and following the same analogy in Kubernetes pod means a logical object which holds a container or more than one container.
The bookish definition could be – a pod represents a request to execute one or more containers on the same node.
The question that needs to be raised why pod?So let me clear this, pods are considered the fundamental building blocks of Kubernetes, because all the Kubernetes workloads, like Deployments, ReplicaSets or Jobs are eventually expressed in terms of pods.
Pods are the one and only objects in Kubernetes that results in the execution of containers which means No Pod No Containers !!!
Now after the context setting over pod I would like to answer my beloved question:- Why Pod over container??
My answer is why not 🙂
Let’s take an example, suppose you have an application which generates two types of logs one is access log and other logs are error log. Now you have to add log shipper agent, In case of the container, you will install the log shipper in the container image. Now you got another request to add application monitoring in the application. So again you have to recreate the container image with APM agent in it.
Don’t you think this is quite an untidy way to do it? Of course, it is, why I have to add these things in my application image, it makes my image quite bulky and difficult to manage.
What if I tell you that Kubernetes has its own way of dealing situations like this.
Yup the solution is a sidecar. Just like in real life if I have a two sitter bike and I want to take 3 persons on a ride, So I will add a sidecar in my bike to take 2 persons together on the ride.
In a similar fashion, I can do the same thing with Kubernetes as well. To solve the above problem I will just create 3 containers (application, log-shipper and APM agent) in the same pod. Now the question is how they will access the data between them and how the networking magic will happen.
The answer is quite simple containers within the pod can share Pod IP address and can listen on localhost. For volume, we can share volumes also across the containers in a pod.
The architecture would be something like this:-
Now another interesting query arises that when to use sidecar and when not.
Just as shown in the above image we should not keep application and database as a sidecar in the same pod. The reason behind it is Kubernetes does not scale a container it scales a pod. So when autoscaling will happen it scales the application as well as database which could not be required.
Instead of that, we should keep log-shippers, health-check containers and monitoring agent as a sidecar because anyhow application will scale these agents also needs to be scaled with the application.
Now I am assuming you are also madly in love with the pods.
For diving deep in the pod stay tuned for the next part of this blog Why I love pods in Kubernetes? Part – 2. In my next part, I will discuss the different phases and lifecycle of the pod and how pod makes our life really smooth.
Thanks for reading, I’d really appreciate any and all feedback, please leave your comment below if you guys have any feedback.
Cheers till the next time.