What is Kubernetes CrashLoopBackOff Events?
Kubernetes has gained much popularity in recent years and is currently one of the most widely used container orchestration engines. If you are using Kubernetes, you might come across various errors and failures while running containers. One of the commonly occurring errors is the dreaded CrashLoopBackOff error and it may occur because of the incorrect Kubernetes configurations. Such misconfigurations include unavailable volumes, init-container misconfiguration, and more.
In this article, we will majorly focus on the CrashLoopBackOff problem, why it occurs, and how you can debug it.
What is Kubernetes?
Before moving towards the CrashLoopBackOff problem, we will first give you an overview of Kubernetes. In 2014, Google launched Kubernetes as a container orchestration tool and it is also known as k8s. This container orchestration engine is well-suited for cloud-native computing services. Google launched it as its Container as a Service (CaaS) offering and later it is known as Google Container Engine.
It is an open-source system that helps in deploying, scaling, and managing containerized applications. It has extended support from other platforms like OpenShift and Azure. It has a simple and modular API core that makes it a powerful tool for container orchestration. Kubernetes works on pods and a pod is a collection of two or more containers running together.
What is the CrashLoopBackOff Error?
This is a common Kubernetes error that occurs during the deployment of various applications using Kubernetes. If a pod is stuck in the CrashLoopBackOff state, then the pod will keep on crashing at a single point after you deploy and run it on any host. It may occur due to the improper starting of the pod.
Each pod in Kubernetes comes with a specification field that has an associated RestartPolicy field for such errors. There are various values that can be assigned to the RestartPolicy that are namely Always, OnFailure, and Never. These values are applicable for all the containers present within a single pod. The RestartPolicy specifies the restarting of the container by the kubelet within the Kubernetes.
If the RestartPolicy status is “Always or OnFailure”, the kubelet will restart the failed pod every time but with an exponential back-off delay. This delay value is mostly capped at 5 minutes. But, if the pod runs successfully for 10 minutes, this delay value will get resettled to the initial value.
How to Find a CrashLoopBackOff Error?
You can spot this error by running a single command on Kubernetes, which is as follows:
kubectl get pods -n <namespace>
The output of the above command will display the details regarding the pods and you can check the status section for CrashLoopBackOff state.
You can apply the filter for checking the status of the pods and examine if any of them are in CrashLoopBackOff status. If you want to check the status of the pods, you can run the following command:
kubectl describe po <pod name> -n <namespace>
You can use the output of the above-mentioned command to:
- Check the events section for the probes’ status (like liveness, readiness, startup), if they are failing.
- Look for the events section for the particular event mentioned as the OOM Killed.
- Look in the pod status section for any ‘error’ displayed along with the error code.
You will get the output as shown below:
Why Does CrashLoopBackOff Occur?
There can be many reasons for the occurrence of this error. Here are some of the most common reasons:
1. Probe failure
For checking the containers, Kubernetes uses some probes like liveness, readiness, and startup. If the probe with liveness or the startup status fails, the container will get restarted at the same point without delay.
For solving this problem, you need to re-check the configuration settings of the probes, if they are configured properly. Check for the specifications like (endpoint, port, SSL config, timeout, command) and make sure they are mentioned correctly. If you are not able to find any error there, you can look for the logs for more clarification. If the error is not clear yet, then you can use the ephemeral containers and run the curl or other relevant commands to check if the application is running perfectly.
2. Out of memory failure (OOM)
Each pod has an associated memory space. If any pod consumes more memory than the allocated one, the pod tends to be crashing. It may be the case if you have allocated less memory than it requires to run seamlessly. The consumption of memory may increase due to an error in the pod and consumes more memory in its running phase.
On checking the event logs, you will encounter the “OOM killed event” error. It will indicate that the pod has been crashed as it takes up all the RAM that is allocated to it.
For solving this error, you need to allocate more RAM to the pods for successful running. It may work sometimes, but if the pod is consuming way more memory then you need to look at the application for the actual cause of the crash. If you are using Java then you should check the heap configurations.
3. Application failure
As we know, containers store applications and their dependencies together to run as an independent application. The CrashLoopBackOff may occur if that application within the container fails due to some error resulting in a Pod crash.
If you want to check the code of the application for debugging, you can run the following command.
Kubectl logs -n <namespace> <podName> -c <containerName> –previous
You can consider checking the last line of the logs. It will usually help in narrowing down the error source within the application.
How to Alert on Kubernetes CrashLoopBackOff?
If you want to set an alert on the Kubernetes for any failed pods, then you can use the metric Kubernetes.pod.restart.rate. It allows you to do some analysis on the pod’s trend of restarting over time. It will instantly notify the respective team if there’s any type of pod anomaly.
To manage the delay in the environment, you need to change the time settings accordingly. The alert is configured and will trigger if any of the pods has been restarted more than 3 times in the last 4 minutes. Such a situation is the CrashLoopBackOff error and is the default alert for the Kubernetes environment.
You can even enable the Sysdig capture for debugging and troubleshooting the CrashLoopBackOff error. It allows complete recording of every small event that happened on the system when you get the alert. You can look at these captures using Sysdig Inspect for deep insights and troubleshooting the situation for better analysis. This will help the development team to respond quickly and recover from the error.
You can even set the CrashLoopBackOff alert based on the captures generated.
Kubernetes is the most reliable tool for running your containers or pods. However, any application tends to fail due to many reasons running within a container. Such a situation can cause the complete pod to fail. To resolve an issue like CrashLoopBackOff, you need to debug logs and event status for clarity.
In this article, we have mentioned what the CrashLoopBackOff error is and what is the reason for its occurrence. Also, we discussed how you can set up an alert for this error using the Kubernetes UI.