Big Data

Kubernetes: The Five Errors That Cause ImagePullBackOff

Although Kubernetes is a useful containerized application builder, used by developers from around the globe, it can be a real pain to troubleshoot. Instead of directing a developer to the problem they’re facing, Kubernetes requires levels of troubleshooting to be undergone, moving through the long process of collecting information before a fix is revealed.

With this laborious process in mind, one of the most commonly searched aspects within the Kubernetes community is how to fix certain errors. In this article, we’ll be exploring ImagePullBackOff, touching on exactly how developers within Kubernetes can debug this error if it comes up.

With nearly four million users worldwide, Kubernetes is a building infrastructure that’s used around the globe, meaning people are running into an ImagePullBackOff error all the time. With this in mind, we’ll break down the process of debugging this error as clearly as possible. By the end of this article, you’ll know exactly what the error is, why it’s occurring, and how to fix it.

Let’s get right into it.

Explaining ImagePullBackOff Error in Kubernetes

ImagePullBackOff error relates to the pulling or retrieval of images within a Kubernetes ecosystem. Instead of being able to actively pull an image and begin its processes, the ImagePullBackOff error will arrive when a pod wasn’t able to find a container image.

Breaking this down further, the ‘BackOff’ element of this error means that Kubernetes is still actively trying to pull the image. This will be an error that is routinely encountered, with ‘BackOff’ intervals extending as Kubernetes repeatedly attempts to remedy the error and find the image that the pods needed to be located.

There is commonly a precursor to ImagePullBackOff, known as ErrImagePull, which is the error message received when the kubelet attempts to pull the image but cannot.

Although this error may seem complicated, it’s actually a relatively simple one to fix as we already know that the error lies within an image. Due to that, finding, updating, supplying, or changing this image will solve the issue most of the time.

The Five Errors That Cause ImagePullBackOff

Within Kubernetes, when a developer is returned an error, it can be difficult to know where to begin remediation due to the complexity of their own ecosystem. With this specific error, that’s not the case; developers know the issue relates to an image within their system, meaning they know that the error is being caused by something related to images.

By focusing on the image pool that pods are trying to pull from, you’ll most likely be able to identify one of the following five errors:

  • Tag error – A tag error is where you create an image within your ecosystem but forget to then give it a tag. If your pod then wants to call upon that specific image, it won’t be able to do so as there is no tag that actively identifies the image.
  • Image error – Quite simply, if you’ve entered a specific tagline for the pod to receive when it runs and that tagline leads to nothing, then you’ll receive the ImagePullBackOff error. This is due to the fact that the pod will be calling on something non-existent. Be sure that you’ve actually got an image with the correct tagline that you’re linking to.
  • Human accident – If you’ve made a small mistake when typing out the tagline, either on the image side or on the pod side, then there will be a mismatch between the two systems. This discrepancy will cause an ImagePullBackOff error as the pod continually searches for a tag that doesn’t exist, or an image that isn’t tagged correctly. Always check for typos.
  • Registry overload – Within Kubernetes, it’s incredibly common to use an image repository site, directly downloading and connecting images from that into your system. If you don’t have enough space on the repository, or you’ve run out of image downloads for the plan you’ve chosen, then it will block any attempts that pods make to retrieve images from it. Be sure to revise your repository plan if you’re using one and make sure you have remaining image connectors.
  • Registry Sign-in – Especially when working from more than one computer, developers often forget to sign in for their own repositories. If you haven’t signed in to your repository account on the computer or Kubernetes ecosystem that you’re using, then you won’t be able to access any resources attributed to your account.

Thankfully, due to the specificity of this error, with all the causes pertaining to images, you’ll have absolutely no problem when trying to locate the root cause of the error.

How to Debug ImagePullBackOff In Kubernetes

Now that you know the root causes of each of the errors and how to fix them, let’s cover exactly how you can find out which of the errors is the one that’s impacting your system.

Within Kubernetes, the Describe Pod command is incredibly useful for finding out more information about errors within a pod. By typing ‘kubectl describe pod <pod-id>’, you’ll then get more information about the error. For example, you may receive the following message:

Back-off pulling image unreachableserve/ngix:ID5

This then suggests that the server that you’re attempting to access is unreachable, causing your error to fall into the final two that we suggested above. Alternatively, depending on the error that you get, the information that you receive will alter. Depending on what additional information you get back after running the Describe Pod command, you’ll know the final step to fix your issue.

Final Thoughts

As a widely used system that’s the basis for containerized applications around the globe, Kubernetes has a whole system of different commands and actions you can use to help you debug any errors that you come across.

This is certainly the case for the ImagePullBackOff error, which, after only one command, you’ll be able to find the core reason behind it. From there, work backwards, and you’ll be able to quickly fix the error.

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