Multi Container Pod — Sidecar & Init Container — Kubernetes

A pod is the smallest deployable unit of Kubernetes. A pod can have either a single containerized application running in them or multiple containerized applications running in them.

The Pods with multiple containerized applications are known as Multi-Container Pods.

There are two main types of containers in a multi-container pod:

1. Init Containers → These run and are complete before the main application containers start. They are typically used for setup tasks.
2. Sidecar Containers → These run alongside the main application container and provide support, such as logging, monitoring, (or) proxying.

Initialization containers (or) Init containers

“Init Containers are special containers that run before the main application containers in a pod start”

They are designed to perform initialization tasks, such as setting up configuration files, initializing databases, (or) any other operation necessary for the proper functioning of the application.

Key Characteristics:

Sequential Execution

Single Responsibility

Transient Nature

Fail and Restart

Sidecar Containers:

“Sidecar Containers are additional containers that run alongside the main application container within the same pod”

They extend or enhance the functionality of the primary container by providing supporting processes, utilities, (or) services.

Key Characteristics:

Parallel Execution

Isolation of Concerns

Enhanced Functionality

Inter-Container Communication

Create Multiple Containers in a Pod

Create a Manifest file. If a person wants to do anything in Kubernetes first required to create a Manifest. Manifest is a file using the YAML(Yet Another Markup Language)

apiVersion: v1
kind: Pod
metadata:
  name: myapp-pod
  labels:
    app.kubernetes.io/name: MyApp
spec:
  containers:
  - name: myapp-container
    image: busybox:1.28
    env:
    - name: FIRSTNAME
      value: "Ibbus"
    command: ['sh', '-c', 'echo The app is running! && sleep 3600']
  initContainers:
  - name: init-myservice
    image: busybox:1.28
    command: ['sh', '-c']
    args: ['until nslookup myservice.default.svc.cluster.local; do echo waiting for myservice; sleep 2; done']
  - name: init-mydb
    image: busybox:1.28
    command: ['sh', '-c']
    args: ['until nslookup mydb.default.svc.cluster.local; do echo waiting for mydb; sleep 2; done']

Apply the manifest to your Kubernetes cluster

kubectl apply -f multi-container.yaml

Check the status of the pod to ensure it’s running

kubectl get pod

Check full pod

kubectl describe pod/myapp-pod

Stucking the Init state because it’s waiting for the service to be provisioned first.

Check pod logs

kubectl logs myapp-pod

Pause the value

kubectl logs myapp-pod -c init-myservice

Create a deployment

kubectl create deploy nginx-deploy --image nginx --port 80
kubectl get deploy
kubectl get pod

Expose the service

kubectl expose deploy nginx-deploy --name myservice --port 80
kubectl get pod

We can see that myapp-pod is initializing.

Lets wait for a few seconds. Now, we can see that as soon as our service gets configured, our myapp-pod container's state got changed to running.

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