A pod is a minimum and basic unit that is handled by Kubernetes. The pod consists of one or more containers that are connected to each other and share storage yet run independently. An application-specific pod model contains one or more containers that go on the same machine.

Kubernetes manages things like automatically restarting a pod when it fails. However, it sometimes happens that the pod does not start when you create it for the first time or update it with new configurations. This can happen for a variety of reasons. So,today, I'll briefly look at why this happens!

Header Reasons Your Kubernetes Pod Won’t Start

A pod may not start or fail due to various reasons, some of which are:

Pod Status is Pending
The pending status implies that the pod’s YAML file has been submitted to Kubernetes, and an API object has been created and saved. However, Kubernetes could not create some of the containers in this pod. A scheduling conflict resulted in the situation not working out. Using kubect to describe the pod, you can see the current pod event and determine why it does not have a schedule.

Here are a few possibilities:

• The cluster nodes cannot meet the CPU, memory, GPU, and other resource requirements requested by the pod.
• The service port needs to be opened to the outside by using HostPort, but it is already busy.

The Pod is in Waiting or ContainerCreating State

This can occur due to one of the following issues:

• There is an incorrectly set image address, a foreign image source that cannot be accessed, an incorrectly entered private image key, or a large image that causes the pull to time out.
• Errors of this kind may result from a problem with the CNI network plug-in settings, such as an IP address can’t be assigned or the pod network cannot be configured.

Make sure that the image is packaged correctly and that the container parameter settings are correct. When the container doesn't start, make sure that the image is packaged correctly.

The Kubelet log should show the cause of the error. Usually, it comes from a failed disk (input/output error) that prevents pod sandboxes from being created.

ImagePullBackOff Error

Generally, the ImagePullBackOff error occurs if the private image key configuration is incorrect. In this case, you can use docker pull to verify that the image can be extracted normally.

If the private image key is set incorrectly or not set, then:

• Check the docker registry type secret

  
  Ver Docker-Registry Secret 
  $ kubectl  get secrets my-secret -o yaml | grep 'dockerconfigjson:' | awk '{print $NF}' | base64 -d
  
  

• Set up a secret of type docker-Registry

 First create a docker record type Secret
$ kubectl create secret docker-registry my-secret --docker-server=DOCKER_REGISTRY_SERVER --docker-username=DOCKER_USER --docker-password=DOCKER_PASSWORD --docker-email=DOCKER_EMAIL

CrashLoopBackOff Error

The CrashLoopBackOff error occurs when a pod is running, but one of its containers restarts due to termination. This basically means that the pod was not able to run and crashed, so Kubernetes restarted it. Unfortunately, thereafter the pod crashed again and was restarted again, forming an endless loop.

This can be caused by a deployment error, aliveness probe misconfiguration, or a server-init configuration error.

• By configuring and deploying Kubernetes correctly, you can solve this error quickly.
• Using a blocking command, you can bypass the error and create an entirely separate deployment.
• Any method that involves back-off restarting failed container should help resolve this error as well.

Pod Is in Error State
In most cases, this error status indicates that the pod is having an error during the startup process. Common causes of the error state include:

• dependent ConfigMap, Secret, or PVWaiting does not exist: The requested resource exceeds the limit established by the administrator, such as exceeding LimitRange.

• There is a cluster security policy violation, such as the violation PodSecurityPolicy, or the container does not have the right to operate the resources in the cluster, such as open RBACA.

The Pod Is in a Terminated or Unknown State
When a node fails, Kubernetes does not remove the pods on its own but marks them as terminated or unknown. The following three methods can be used to remove pods from these states:

• Take the node off the cluster: If a VM is removed after the kube-controller-manager has been installed, the corresponding node will be removed automatically.
• A physical machine node must be manually removed (using kubectl remove node) in a cluster installed on physical machines.
• Node activity resumes normal: To determine whether these pods are to be removed or made to continue running, Kubelet will contact Kube-apiserver from time to time to confirm the expected status of these pods. podSpec parameters are set within. This is usually because the podSpec YAML file content is wrong: you can try using validate parameters to rebuild the container.

Conclusion
A Kubernetes pod can fail due to improper configuration or due to a problem in the code. Quite often, it’s the former, and if you know how to detect the error, resolving it becomes much easier.