Failing Stuck Pods due to Invalid Images: A Mechanism for Rescue

Core Problem

When a Pod is stuck in the Pending phase due to an invalid image, it can cause significant delays and resource blocks in the cluster. This issue is particularly problematic in queued environments where jobs may be submitted hours or days after creation, leading to delayed start times.

Solution & Analysis

To address this problem, we propose introducing a mechanism that sets a Pod into the Failed phase when the image pull fails for a configurable number of attempts. This would allow the job controller to detect and handle stuck Pods more effectively.

Configurable Image Pull Attempts

We suggest introducing a new config map field, imagePullAttempts, which controls the maximum number of failed attempts allowed before marking a Pod as Failed.

apiVersion: v1
kind: ConfigMap
metadata:
  name: image-pull-attempts
spec:
  data:
    imagePullAttempts: 3

Custom Pod Status Updater

To implement this mechanism, we need to create a custom Pod status updater that checks the number of failed image pull attempts and updates the Pod's phase accordingly.

// Define a custom Pod status updater function
func updatePodStatus(pod *v1.Pod) error {
    // Get the current image pull attempt count
    attempts := pod.Status.ImagePullAttempts

    // Check if the Pod has exceeded the maximum allowed attempts
    if attempts > int32(imagePullAttemptsValue) {
        // Update the Pod's phase to Failed
        pod.Status.Phase = v1.PodPhaseFailed
    }

    return nil
}

// Define a custom Pod status updater webhook
func main() {
    // Register the webhook handler
    webhook := &http.HandlerFunc(updatePodStatus)
    http.Handle("/webhook", webhook)
}

Image Validation

To further improve this mechanism, we can integrate image validation using a new API endpoint that checks the image validity before allowing the Pod to proceed.

// Define a new API endpoint for image validation
func validateImage(image string) (*v1.Image, error) {
    // Check if the image exists and is valid
    if !imageExists && !isValidImage(image) {
        return nil, errors.New("invalid image")
    }
    return &v1.Image{}, nil
}

// Define a new webhook handler for image validation
func validatePodStatus(pod *v1.Pod) error {
    // Validate the Pod's image using the new API endpoint
    image, err := validateImage(pod.Spec.Containers[0].Image)
    if err != nil {
        return err
    }

    // If the image is valid, proceed with updating the Pod's phase
    pod.Status.ImagePullAttempts++
    updatePodStatus(pod)
}

Conclusion

By introducing a configurable mechanism for failing stuck Pods due to invalid images, we can improve the overall reliability and responsiveness of our Kubernetes cluster. This solution allows job controllers to detect and handle stuck Pods more effectively, reducing the risk of resource blocks and improving overall system efficiency.

Reference

• Source