Pods & Containers in Kubernetes

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• A pod is a group of one or more containers
  
• A container is a package with software and all its dependencies required to run the application bundled together.
  
• Containers are grouped as pods and they are the smallest execution unit in Kubernetes
  
• Kubernetes work with pods.
  
• All the containers in a pod run in a shared context and are always co-located on the same physical or virtual machine in the cluster
  
• All containers in a pod share the same network namespace and therefore they can locate each other and communicate via localhost
  
• A pod can have a single container or multiple containers which will form an application unit. Example a web application and a redis containers can form a multiple container pod
  
• Example of a multi container pod yaml

apiVersion: "v1"
kind: Pod
metadata:
  name: web1
  labels:
    name: web
    app: demo
spec:
  containers:
    - name: redis
      image: redis
      ports:
        - containerPort: 6379
          name: redis
          protocol: TCP
    - name: python
      image: nolaraj/py-red
      env:       
        - name: "REDIS_HOST"
          value: "localhost"
      ports:
        - containerPort: 5000
          name: http
          protocol: TCP 

• Pods are created using the create command . kubectl create -f FILENAME
  
• When a pod is created it is assigned its own unique IP address. If there are multiple containers within the pod, they can communicate between each other simply by using localhost
  
• A pod represents a single instance of a given application and to scale an application we need to run multiple instances of the pod.
  
• In Kubernetes, this is known as replication.
  
• Pods run on nodes in a Kubernetes cluster and they remain on the node until its process is complete, or its deleted or evicted due to lack of resources.
  
• kubectl get pod command is used to inspect the running status of a Pod. A pod can be in one of the following phase
  
  • Pending: Pod has been created and accepted by the cluster, but one or more of its containers are not yet running.
    
  • Running: Pod has been bound to a node, and all of the containers have been created.
    
  • Succeeded: All containers in the Pod have terminated successfully.
    
  • Failed: All containers in the Pod have terminated, and at least one container has terminated in failure.
    
  • Unknown: The state of the Pod cannot be determined.
    
• The phase of a Pod is a high-level summary of where the Pod is in its lifecycle.
  
• When a Pod starts running, it requests an amount of CPU and memory. This helps Kubernetes schedule the Pod onto an appropriate node to run the workload.
  
• A Pod will not be scheduled onto a node that doesn’t have the resources to honor the Pod’s request. A request is the minimum amount of CPU or memory that Kubernetes guarantees to a Pod.
  
• By default, a Pod has no upper bound on the maximum amount of CPU or memory it can use on a node. You can set limits to control the amount of CPU or memory your Pod can use on a node. A limit is the maximum amount of CPU or memory that Kubernetes guarantees to a Pod.
  

Thank You !!