Pod

A pod is the smallest building block of Kubernetes object model. In a Kubernetes cluster, a pod represents a running process. Inside a pod, you can have one or more containers. Those containers all share a unique network IP, storage, network and any other specification applied to the pod. Another way to think of a pod is that it is an application-specific “logical host” that holds one or more containers that that are tightly coupled. Let us take for example we have an “app-container” and “logging-container” in a pod. “Logging-container’s” only job is to pull logs from the “app-container”. You can see how having them in a pod eliminates a lot of extra setup to get them to talk. They are co-located so everything is local and they share all the same resources. This is the same thing as being executed on the same physical server in a pre-container world.

Pod is also designed as mortal. When pod dies for some reason, such as getting killed by Kubernetes controller when lacking of resources, it won’t recover itself.

Pod Model Types

There are two model types of pod you can create. “one-container-per-pod” and “multi-container-pod.

·         One-container-per-pod. This model is the most popular. The Pod acts as a wrapper for a single container and since Pod is the smallest object Kubernetes knows, it manages the Pods rather than the containers directly.

·         Multi-container-pod. With this model a pod might hold multiple co-located containers that are tightly coupled and need to share resources. These containers work as a single cohesive unit of service. The Pod wraps these containers and storage resources together as a single unit. Some example use cases are sidecars, proxies, logging.

Pod Life Cycle

A pod status tells us where the pod is in its lifecycle. It is meant to give you an idea not for certain, therefore It is good practice to debug if pod does not come up cleanly. There are 5 phases during a pod lifecycle.

·         Pending – Pod has been accepted, but one or more of the Container images has not been created.

·         Running – The Pod has been bound to a node, and all of the Containers have been created. One Container is still running, or is in the process of starting or restarting.

·         Succeeded – All Containers in the Pod have terminated in success and will not be restarted.

·         Failed – All Containers in the Pod have terminated, and at least one Container has terminated in failure. The Container exited with non-zero status.

·         Unknown – For some reason the state of the Pod could not be obtained.

Lab:

Deploy a Pod on your Kubernetes cluster by running below command. This will deploy two containers in a single pod.

kubectl create -f https://raw.githubusercontent.com/mhausenblas/kbe/master/specs/pods/pod.yaml

Check status of pod.

kubectl get pods

In case if you want to apply constraints on Pod level. These constraints will restrict pods from utilizing additional resources like memory or cpu. we will discuss about constraint in further tutorials deeply. 

kubectl create -f https://raw.githubusercontent.com/mhausenblas/kbe/master/specs/pods/constraint-pod.yaml 

To check pod configuration.

kubectl describe pod constraintpod

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