Kubernetes pods and volumes

One of my friends asked me a question that was around how a single volume can be mounted across 2 different containers in separate paths for the same pod. Like under the hood, what's exactly happening? How are they seeing the same files even though they are mounted in different paths across 2 different containers?

At that time, I was not working actively on k8s, but the question seemed very valid. I thought maybe I could write a blog for him and also learn a bit.

To understand this, I found a simple pod definition example on the following link and tweaked it a bit, and added an explanation below with diagrams. 

The below pod template has one volume of type emptyDir defined called shared-data and 2 containers (nginx and debian) defined which mount the same volume on 2 different paths. I tweaked the start-up command a bit but basically the 2 containers share the same volume, but mount the volume on 2 different paths /usr/share/nginx/html and /pod-data

apiVersion: v1

kind: Pod

metadata:

  name: two-containers

spec:

  restartPolicy: Never

  volumes:

  - name: shared-data

    emptyDir: {}

  containers:

  - name: nginx-container

    image: nginx

    volumeMounts:

    - name: shared-data

      mountPath: /usr/share/nginx/html

  - name: debian-container

    image: debian

    volumeMounts:

    - name: shared-data

      mountPath: /pod-data

    command: ["/bin/sh"]

    args: ["-c", "sleep infinity"]

The volume shared-data is of type emptyDir means that the lifespan of the volume is attached to the lifespan of the pod (so basically you lose data when pod restarts) and the volume will be created/stored on the node where the pod is scheduled to run. 

I could access a simple k8s cluster online (for free) on https://killercoda.com/

There you have one control node and one worker node (node01)

I will try and create the pod and this is how the overall layout looks like (might not be super accurate but gives an idea in general)

So, since the volume is of type emptyDir (I guess you are aware there are many other types of volumes like pvc, config map, nfc, hostPath etc and k8s maintains that abstraction) it is basically a shared folder on the node01 file system.

Below screenshot shows the pod created on node, node01

To prove that the file is stored directly on node, I will create a dummy file under the debian container under the path /pod-data called test123.txt

Now, I will ssh into the k8s worker node, node01 where the pod is running and try and find the file test123.txt in its file system

As you can see there is a folder called share-data available on the worker node host and this is the only file/folder that exists.

What this means is that this is a physical folder representing the volume shared between the 2 containers nginx and debian who mount this volume under different respective paths.

This makes sense since the containers could have different folder structures internally but fundamentally they see the same set of files under their respective mounted folders.

As you can see below, the same file test123.txt is now also available in the nginx container but the relative path is different as specified in the spec. 

If I do the reverse operation that is create a folder in the nginx container with a file under it I would see the same effects. 

I should see this file and folder created on the node

Also, I should see this is available on the debian container under /pod-data

Now something crazy. I will try to create the file on the worker node node01 directly lets say called ghost.txt and check if this reflects on the 2 containers. 

As you can see below, it works

We have used a simple example with a volume of type emptyDir but fundamentally, it should be the same for other volumes as well. Feel free to add comments here and we can discuss.

Start of my ML journey

The hype around ML, LLM, Neural Network, etc is crazy now. It always feels to me like a train I have missed but would really want to get on. But the point to start is always something that has eluded me. I have been in the "IT" industry now for close to 20 years but I still feel I can learn new things. I come from the generation of "Core Java backend" development era or the "monolith" era or whatever you wanna term the 2000s computer era. Things like cloud, container, devops, microservices, k8s, big data, etc have always been ahead of me and its always been sort of catchup. And now ML/AI/LLM are captivating everyones imagination and I feel like I am again playing he catch up game.

I am the "40 year old dev" who still believes he can learn and still has to learn a lot. But as age progresses, lot of aspects of life change and sometimes one can regret with statements like "I wish I can go back in time". Responsibilities increase. The society norms like "ohh you should be owning a house by now", etc start mounting pressure. "You should focus on family" or "You should not be coding now". It would have been easier as a bachelor to devote more time on learning but as I stated before, everything has been an after thought in my life. 

Then I came across this line that if you love something, you will always have enough time. I guess I need to follow that mantra and not lose hope.

So I have narrowed down few resources that I need to devote time to. I will try making notes everyday as I embark on this journey.

 

A new start

 Reviving this blog after 2008... Will post technical stuff that am learning... It should be fun...

French Open `07 remains Nadal's open

French Open according to me and most of the tennis fans is equivalent to Wimbeldon when it comes to the excitement level. I being a big Federer fan wanted him to beat the hell out of his opponents especially Nadal and was hoping for a Nadal Federer final showdown and I wasnt let down. But the way Nadal played throughout the tournament was amazing. Federer also played well but against Nadal, with the ball bouncing above his head and the deadly Nadal top-spin, he was left clueless. Much before the tournament started, I had seen a voting in which 99% had voted for a Federer-Nadal French open final and this is what exactly happened. Hewitt in a press conference was asked about what makes Nadal and Federer way above rest of the players and his reply was that they have so much confidence in their skills that they go for winners even when they are down and under pressure. Nadal now has the record of winning the French Open three times in a row. The only person who achieved this feat before him is the great Bjorn Borg.

Federer is a magician with the tennis ball but not on clay courts. He doesnt look as comforatble as Nadal while moving around the clay court. And on break points, Nadal is mentally tougher than Federer. I feel sad for myself when I think that Nadal is younger than me and he has achieved so much. He is only 20!!!!!! He looks so matured as a player and is physically so strong. He never looks like he is ever going to get tired let it be the tie break of 5th set.

Can Nadal beat Federer in Wimbeldon? Well, Nadal struggled in Artois open in the quarters on the grass courts. But he is a warrior and he is a player who plays based on the grandness of the occassion.

Cant wait to see Wimbeldon.