Dilanka Rathnasiri

Posted on Nov 3, 2024

Kubernetes Pods

#containers #devops #kubernetes

cover image: Photo by Nilantha Ilangamuwa on Unsplash

In this article, we’ll talk about Kubernetes pods. We'll discuss what they are, how they work, and why they’re essential in the Kubernetes ecosystem.

What are Kubernetes Objects?

Kubernetes Objects are the foundational entities of Kubernetes
We can consider Kubernetes objects as the building blocks of the Kubernetes
Each Kubernetes object has its own specific attributes and responsibilities
There are many types of Kubernetes objects
The following are some of them
- Pods
- Deployments
- Services

What is a Pod?

In Kubernetes, pods are the smallest deployable units of computing
Kubernetes pod is one type of the Kubernetes objects
A pod can contain one container or a group of tightly coupled containers
So, Pods can be considered as abstractions that encapsulate one or more containers
In most use cases, we use pods that contain a single container
Pods are ephemeral and disposable

Creating a Pod with a Manifest File

Here’s an example of a basic pod manifest file:

apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  containers:
  - name: nginx
    image: nginx:latest
    ports:
    - containerPort: 80

Then we can create this pod using kubectl with one of the following commands

kubectl create -f nginx.yaml

kubectl apply -f nginx.yaml

The above manifest file creates a pod with a single nginx container
In many use cases, we don't directly create and manipulate pods
Instead, we’ll use workload resources like Deployments or ReplicaSets to manage pods at scale

We can delete above pod using kubectl with one of the following commands

kubectl delete -f nginx.yaml

kubectl delete pod nginx

Multi-container pods

Containers that are tightly coupled and required to work together can be encapsulated into a single pod
These containers are automatically co-located and co-scheduled in the same physical or virtual machine
So, this allows them to
- communicate and coordinate with each other
- share resources and dependencies

Example Multi-Container Pod Manifest:

apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  containers:
  - name: nginx
    image: nginx:latest
    ports:
    - containerPort: 80
  - name: redis
    image: redis:latest
    ports:
    - containerPort: 6379

The above pod has two containers; nginx container and redis container
There are several types of multi-container pods
The following are 3 types used commonly
- Sidecar Containers
- Ambassador Containers
- Adapter Containers

Sidecar Containers

Sidecar containers are the secondary containers that run along with the main application container within the same Pod
They provide additional services or functionalities such as logging, monitoring, etc.
In most cases, we don't directly manage sidecar containers
Instead, Helm charts manage sidecar containers

Init Container

Init container is a container that runs before the main application containers of the pod
They’re used for tasks required to be completed once before the application container startup
Init containers are also regular containers
But unlike regular containers,
- Init containers always run to completion
- Init containers run only at the pod startup
A pod can have multiple init containers and they execute sequentially
Init containers are specified within the initContainers section of the manifest file
Init containers run sequentially in the order of initContainers section of the manifest file
Only one init container runs at a time
If an init container fails, it'll be restarted until it succeeds
However, we can control the restart behavior by using restartPolicy of the pod
If an init container fails, the whole pod will fail
Example Kubernetes Manifest with Init Containers:

apiVersion: v1
kind: Pod
metadata:
  name: myapp-pod
  labels:
    app.kubernetes.io/name: MyApp
spec:
  containers:
    - name: myapp-container
      image: busybox:1.28
      command: ['sh', '-c', 'echo The app is running! && sleep 3600']
  initContainers:
    - name: init-myservice
      image: busybox:1.28
      command: ['sh', '-c', "until nslookup myservice.$(cat /var/run/secrets/kubernetes.io/serviceaccount/namespace).svc.cluster.local; do echo waiting for myservice; sleep 2; done"]
    - name: init-mydb
      image: busybox:1.28
      command: ['sh', '-c', "until nslookup mydb.$(cat /var/run/secrets/kubernetes.io/serviceaccount/namespace).svc.cluster.local; do echo waiting for mydb; sleep 2; done"]

In the above example init-myservice and init-mydb are init containers
Containers of the above manifest run in the following order,
1. init-myservice init container starts first and runs to completion
2. After successfully completing init-myservice init container, init-mydb init container starts and runs to the completion
3. After successfully completing init-mydb init container, myapp-container container starts

Summary

In this article, we covered the basics of Kubernetes pods. We looked at what pods are, how they can encapsulate one or more containers, and the different types of containers that can run within a pod, including sidecar and init containers.

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