What is a Kubernetes Container?
A Kubernetes container is a lightweight, portable, and extensible virtual machine having its memory, space, CPU, filesystem, etc. It is considered lightweight due to its capability of sharing the operating system among applications having relaxed isolation properties. Moreover, it is portable across the cloud and has different operating system distributions. No matter what environment the Kubernetes cluster is running in, it will always depict the same behavior for all environments because the dependencies included in it standardize its performance.
Before the evolution of containers, a separate virtual machine was used for each application because any changes in the shared dependencies on one virtual machine can cause strange results. This causes a loss of memory resources, CPU wastage, and scarcity of other resources. And then came the containers, which virtualized the host OS and isolated the dependencies for each application in the same environment. The container engine in the container allows the applications to use the same OS isolated from other applications running on the host virtual machine.
What is a Container Image?
A container image is the representation of the dependencies included in the container in the form of binary data. It is an executable and ready-to-run software package that is capable of running standalone. It contains all the dependencies including application libraries, system libraries, code, essential default settings, etc. required to run an application on any Kubernetes environment or operating system. Each node in the container uses the container image to run applications and pods on it.
In the Kubernetes cluster, the kubectl agent is responsible for running container images on each node. It pulls the image on each node present in the cluster. It is also responsible to report everything that happens back up to the central Kubernetes API. If the container image doesn’t exist already on the cluster node, then kubectl instructs the container to pull the image at the run time.
What is the ImagePullBackOff Error?
There are some situations when Kubernetes can experience issues pulling the container image from the registry of the container. If these issues result in an error, then the pods enter the ImagePullBackOff state. When a new deployment is created or an existing deployment is updated in the Kubernetes cluster, the container image needs to be pulled. Kubectl pulls the image on each worker node in the cluster that matches the scheduling request. So, when kubectl fails to pull the image, it faces the ImagePullBackOff error.
In other words, the ‘ImagePull’ section of the ImagePullBackOff error refers to the inability of Kubernetes to pull the image of the container from a public or private container registry. The ‘BackOff’ section refers to the continuously increasing backoff delay pulling the image. The backoff delay keeps on increasing with each attempt until the limit of backoff reaches 5 minutes. The main or obvious reason for the ImagePullBackOff error is that Kubernetes fails to pull the container image at runtime. However, there may be many causes for this issue, including the following:
- The image path is incorrect.
- Kubeclt fails to authenticate with the container registry.
- A network failure.
- Container registry rate limits.
- Incorrect container registry name
- Authentication failure due to the image being private.
- Incorrect image name and tag.
- The image does not exist.
- Authentication is required by the image registry.
- Download limit exceeded on the registry.
How to Solve the ImagePullBackOff Error in Kubernetes?
If any of the situations given above occurs, the pod in the cluster ends up in the ImagePullBackOff state. The best way to fix this error is to troubleshoot the Kubernetes cluster. You can troubleshoot by following the instructions below:
Step # 1: Create a Pod and Assign it an Image Name
The pods run on the nodes that run the image container. Each image has a specific name and if you refer to an image name that does not exist or mistakenly enter an incorrect name will result in an ImagePullBackOff error. Here, we will demonstrate the ImagePullBackOff error occurring due to an incorrect image name. So, let us create a pod and assign it a nonsense image name. We can do this by executing the following command:
The ‘kubectl run’ command will create a pod named ‘demo1’ and the image name ‘–image=nonexistentimage/nonexist:bla’ assigned to it.
Step # 2: Display All Pods
The next step is to display all the pods to check their status. Kubectl provides the ‘get’ command to get the list of pods with their associated properties like name, ready, status, age, etc. Use the command given below to display all the pods:
Refer to the output given in the screenshot below:
From the output given above, you can see that there are many pods and each has its status. Some are in the ‘running’ state, some are in the ‘ErrImagePull’ state and some are in the ‘ImagePullBackOff’ state.
Step # 3: Troubleshoot the Pod
Now that we know there are many pods running in the cluster, each having its own status, we can specifically look into the desired pod. This can be done with the help of the command given here:
The ‘demo1’ is the pod that we created earlier, and the ‘describe’ command will give us a detailed description of the ‘demo1’ pod. Refer to the output given below:
Conclusion
We explored the ImagePullBackOff error in the Kubernetes environment. We learned about the Kubernetes cluster, cluster image, and also explored the reasons behind the ImagePullBackOff error. The main and obvious reason for the ImagePullBackOff error is the inability of Kubernetes to pull an image of the container.
