Kubernetes - Pods Overview

Pods

Pods are the smallest deployable units of computing that you can create and manage in Kubernetes.

What is a Pod?

Note: While Kubernetes supports more container runtimes than just Docker, Docker is the most commonly known runtime, and it helps to describe Pods using some terminology from Docker.

The shared context of a Pod is a set of Linux namespaces, cgroups, and potentially other facets of isolation - the same things that isolate a Docker container. Within a Pod's context, the individual applications may have further sub-isolations applied.

In terms of Docker concepts, a Pod is similar to a group of Docker containers with shared namespaces and shared filesystem volumes

Using Pods

• Pods that run a single container. The "one-container-per-Pod" model is the most common Kubernetes use case; in this case, you can think of a Pod as a wrapper around a single container; Kubernetes manages Pods rather than managing the containers directly.
• Pods that run multiple containers that need to work together. A Pod can encapsulate an application composed of multiple co-located containers that are tightly coupled and need to share resources. These co-located containers form a single cohesive unit of service—for example, one container serving data stored in a shared volume to the public, while a separate sidecar container refreshes or updates those files.
  
  

Pod networking

Each Pod is assigned a unique IP address for each address family. Every container in a Pod shares the network namespace, including the IP address and network ports. Inside a Pod (and only then), the containers that belong to the Pod can communicate with one another using localhost. When containers in a Pod communicate with entities outside the Pod, they must coordinate how they use the shared network resources (such as ports). Within a Pod, containers share an IP address and port space, and can find each other via localhost.

 To Create the pod ( with name nginx-pod by using nginx docker image)  with command

    kubectl run nginx-pod --image nginx

To list the pod with its IP address and NodeName

    kubectl get pods -o wide

To interact with pod's terminal(go inside the pod)

    kubectl exec -it nginx-pod-- bash

To Exit from the pod run "exit" command

To access the application running in the pod ( consider 192.168.135.2 is IP of the pod)

     curl 192.168.135.2

To find all the properties of the pod in yaml format

    kubectl get pods nginx-pod -o yaml

To find all the properties of the pod in json format

    kubectl get pods nginx-pod-o json

To Delete the Pod

    kubectl delete pod nginx-pod

Manifest file in Kubernetes 

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Kubernetes Documentation

My First Pod Manifest file (nginx-pod.yaml)

apiVersion: v1 # apiVersion for the resource

kind: Pod  # type of resource (Pod)

metadata:

  name: myapp # Pod name

  labels:  # user defined key value pair

    name: myapp # name is the key and myapp is value

spec:

  containers: # Containers information for the pod

  - name: myapp # name of the container to access via pod

    image: nginx # docker image for myapp container

    # below code is optional

    resources: # resources define the memory or cpu limits for a container

      limits:  # maximum limit for the container

        memory: "128Mi"  # maximum 128 mb of memory used by the container

        cpu: "500m"  # maximum cpu usage 500mz

    ports: # Ports where the container's app is running

      - containerPort: 80 # nginx runs on port 80

To create Pod by using above file you need to run below command

            kubectl create -f nginx-pod.yaml

To Verify Pod is created with its IP address and other properties

             kubectl get pods -o wide

To Verify the events and other run-time properties of the pod.

          kubectl describe pod myapp

If you make any change in the manifest file then use the  apply(change/create) command

          kubectl apply -f nginx-pod.yaml

To find the logs generated by pod

         kubectl logs myapp

To interact with pod

         kubectl exec -it myapp -- bash

To delete the pod using manifest file

          kubectl delete -f nginx-pod.yaml

To verify pod is deleted or not

        kubectl get pods

MultiContainer Pod

Create 2 containers (nginx, tomcat) within a single pod multicont.yaml

apiVersion: v1

kind: Pod

metadata:

  name: myapp

  labels:  

    name: myapp

spec:

  containers:

  - name: nginx-container

    image: nginx

     # below code is optional

    resources:

      limits:  

        memory: "64Mi"  

        cpu: "100m"  

    ports:

      - containerPort: 80 # nginx runs on port 80

  - name: tomcat-container

    image: tomcat  

    # below code is optional

    resources:

      limits:  

        memory: "64Mi"  

        cpu: "200m"  

    ports:

      - containerPort: 8080 # tomcat runs on port 8080

To create Pod by using above file you need to run below command

            kubectl create -f multicont.yaml

To Verify Pod is created with its IP address and other properties

             kubectl get pods -o wide

To Verify the events and other run-time properties of the pod.

          kubectl describe pod myapp

If you make any change in the manifest file then use the  apply(change/create) command

          kubectl apply -f multicont.yaml

To find the logs generated by pod nginx-container

        kubectl logs myapp -c nginx-container

To find the logs generated by pod nginx-container

        kubectl logs myapp -c tomcat-container

To interact with pod for nginx-container

    kubectl exec -it myapp -c nginx-container -- bash

To interact with pod for tomcat-container

    kubectl exec -it myapp -c tomcat-container -- bash

To access nginx app using multicontainer pod (IP 192.168.135.3)

    curl 192.168.135.3:80

To access tomcat app using multicontainer pod (IP 192.168.135.3)

    curl 192.168.135.3:8080

To delete the pod using manifest file

          kubectl delete -f multicont.yaml

To verify pod is deleted or not

        kubectl get pods

Pod Lifecycle

This page describes the lifecycle of a Pod. Pods follow a defined lifecycle, starting in the Pending phase, moving through Running if at least one of its primary containers starts OK, and then through either the Succeeded or Failed phases depending on whether any container in the Pod terminated in failure.

LAB

To create a pod with docker image nginx

     Kubectl run nginx-pod --image=nginx

1.# nginx-pod.yaml

apiVersion: v1

kind: Pod

metadata:

  name: nginx-pod

  labels:

    app: nginx

    tier: dev

spec:

  containers:

  - name: nginx-container

    image: nginx

 

 

 

2. Create and display Pods

# Create and display PODs

kubectl create -f nginx-pod.yaml

kubectl get pod

kubectl get pod -o wide

kubectl get pod nginx-pod -o yaml

kubectl describe pod nginx-pod

3. Test & Delete

# To get inside the pod

kubectl exec -it nginx-pod -- bash

 

# Create test HTML page

cat <<EOF > /usr/share/nginx/html/test.html

<!DOCTYPE html>

<html>

<head>

<title>Testing..</title>

</head>

<body>

<h1 style="color:rgb(90,70,250);">Hello, DevopsWorld...!</h1>

<h2>Congratulations, you passed :-) </h2>

</body>

</html>

EOF

exit

 

# Expose PODS using NodePort service

kubectl expose pod nginx-pod --type=NodePort --port=80

 

# Display Service and find NodePort

kubectl describe svc nginx-pod

kubectl get svc

# Open Web-browser and access webapge using

http://nodeip:nodeport/test.html

# Delete pod & svc

kubectl delete svc nginx-pod

kubectl delete pod nginx-pod

Pod Assignment

 Next Assign Pods to Nodes