In Kubernetes, it’s common to see pods restart unexpectedly or enter a CrashLoopBackOff state, especially during application rollout or configuration changes. In Alibaba Cloud Container Service for Kubernetes (ACK), this behavior typically indicates that a container in a pod crashes repeatedly shortly after it starts.
In this article, we explore how to investigate and resolve pod restart and CrashLoopBackOff issues in an ACK cluster using standard Kubernetes tools and the ACK console.
When Kubernetes tries to start a container, but the container process exits quickly, for example, due to a configuration error, missing dependency, or runtime failure, the kubelet repeatedly restarts it. After several failed attempts, Kubernetes stops trying immediately and sets the pod status to CrashLoopBackOff.
This status indicates the container fails shortly after each start, and Kubernetes delays subsequent restart attempts to avoid consuming unlimited resources, helping to prevent resource exhaustion on nodes and unnecessary scaling activity in the cluster. This behavior is part of Kubernetes’ event and alert management logic that helps reduce repetitive failures and preserve cluster stability.
Begin by checking the general status of the pod:
kubectl get pods -n <namespace>If the status for a pod repeatedly shows CrashLoopBackOff, proceed to detailed inspection.
Pod events often contain Kubernetes system messages that indicate why a container failed:
kubectl describe pod <pod-name> -n <namespace>Check the Events section for information on container exit codes, OOMKills (out-of-memory), and image pull errors. Events provide immediate insight into whether the issue is caused by container crashes, scheduling problems, or environmental errors.
Container logs help you see exactly what the process printed before failing:
kubectl logs <pod-name> -n <namespace> --previousThe --previous flag retrieves logs from the previous container instance before it was restarted, which is critical during CrashLoopBackOff scenarios.
Look for error messages such as:
● Stack traces
● Failed service starts
● Missing configuration
● Permission or dependency issues
If logs do not explain the failure, consider adding more detailed application logging.
If Kubernetes cannot pull the container image, pods may fail early or stay in pull-related failure conditions that look like restarts:
● Confirm image registry address and credentials.
● If using a private registry, verify the image pull secret is correctly referenced in your deployment.
● Ensure node network connectivity allows reaching the image repository.
For example, you can log into a node and check connectivity:
curl -kv https://registry.example.com/v2/If the container exceeds configured CPU or memory limits, the system may kill it abruptly. Such out-of-memory (OOM) events typically result in a CrashLoopBackOff.
In the ACK console or with kubectl, check events for OOM:
kubectl describe pod <pod-name> -n <namespace>Look for entries like:
pod was OOMKilled
If OOM errors occur, you have two options:
● Increase the container’s memory limit
● Investigate and fix memory-intensive behavior in the application itself
Note that at the node level, OS or cgroup limits can also trigger OOM events. For pod troubleshooting, you can refer to the user guide.
Configuration issues in pod manifests can also trigger early container failures:
● Check for typographical errors in keys such as command, args, or environment variable names.
● Validate the YAML with:
kubectl apply --validate -f deployment.yamlIncorrect fields may not stop the pod from being created but can lead to containers exiting immediately due to invalid settings.
You can also compare the applied pod spec with your source manifest:
kubectl get pods <pod-name> -n <namespace> -o yamlThis helps ensure no unexpected changes occurred during deployment.
Sometimes, using the ACK console helps speed up troubleshooting:
● Navigate to Workloads > Pods
● Select the affected pod
● View Events and Logs directly
● Enable Simple Log Service (SLS) for persistent logging
The console also allows terminal access into the container, which can help you inspect local files or run interactive debugging.
Health probes (livenessProbe, readinessProbe) can cause pods to restart if they are too strict or misconfigured. Consider:
● Whether your application startup time is longer than the probe timeout
● If your readiness probe incorrectly marks a healthy container as unhealthy
Misconfigured probes often induce continuous restarts even when the application is functioning correctly.
Adjust probe settings based on your application’s characteristics.
CrashLoopBackOff may happen if the node hosting the pod is under resource pressure:
● Low memory/disk on a node
● Node in NotReady state
Check node conditions:
kubectl describe node <node-name>Look for taints such as:
● node.kubernetes.io/memory-pressure
● node.kubernetes.io/disk-pressure
If the node is under pressure, adjust scheduling or resource requests accordingly.
Pod restarts and CrashLoopBackOff states are common in Kubernetes but often stem from straightforward internal issues, like application failures, misconfiguration, or insufficient resources.
By systematically investigating:
● pod events
● container logs
● image pull issues
● resource limits
● health probes
● node conditions
You can isolate the underlying cause and resolve it effectively. Using the ACK console alongside kubectl commands enhances visibility into cluster behavior, making troubleshooting more efficient.
For persistent or complex cases, such as storage misconfigurations or network issues, refer to the relevant ACK documentation or open a support ticket with detailed logs for deeper analysis.
Disclaimer: The views expressed herein are for reference only and don't necessarily represent the official views of Alibaba Cloud.
1 posts | 0 followers
FollowH Ohara - September 1, 2023
Alibaba Clouder - April 23, 2019
Alibaba Cloud Community - July 5, 2023
Alibaba Clouder - May 21, 2019
Alibaba Clouder - April 23, 2019
Alibaba Cloud Native Community - July 13, 2022
1 posts | 0 followers
Follow
ACK One
Provides a control plane to allow users to manage Kubernetes clusters that run based on different infrastructure resources
Learn More
Function Compute
Alibaba Cloud Function Compute is a fully-managed event-driven compute service. It allows you to focus on writing and uploading code without the need to manage infrastructure such as servers.
Learn More
ECS(Elastic Compute Service)
Elastic and secure virtual cloud servers to cater all your cloud hosting needs.
Learn More
Elastic High Performance Computing Solution
High Performance Computing (HPC) and AI technology helps scientific research institutions to perform viral gene sequencing, conduct new drug research and development, and shorten the research and development cycle.
Learn More