Kubernetes Troubleshooting #3: Kubernetes CronJob That Floods Your Cluster

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🚨 Fixing a Kubernetes CronJob That Floods Your Cluster: Lessons from a Real Incident

"A single CronJob brought our microservices to a halt. Here's what happened, how we found the issue, and how we fixed it."

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🧩 The Problem

We recently encountered a critical issue in our EKS cluster managed via ArgoCD. One of our Kubernetes CronJobs started spawning multiple pods continuously. Before we noticed, all nodes were filled with CronJob pods, leaving no space for our regular microservices.

Microservices started failing to schedule, and users began reporting degraded performance. The root cause? A runaway CronJob.

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⚠️ Symptoms We Observed

• CronJob was creating pods every minute or even faster.

• Nodes were filled to capacity — CPU/memory exhausted.

• Microservices (Deployments/StatefulSets) showed Pending state.

• Cluster was slowing down; node autoscaler couldn’t keep up.

• ArgoCD showed consistent green (it wasn’t aware of the runtime chaos).

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🧠 Common Causes of CronJob Pod Flooding

If you’ve seen this happen, you’re not alone. These are common scenarios that can cause this issue:

1. Job Duration > Schedule Interval:

   • If a CronJob runs every minute but takes 5 minutes to complete, multiple pods will stack up.

2. Missed Schedules and Backlog:

   • Cluster downtime or CronJob controller issues might result in missed schedules. On recovery, Kubernetes tries to run all missed schedules if startingDeadlineSeconds isn't set.

3. No concurrencyPolicy:

   • Default policy (Allow) lets multiple jobs run concurrently. If previous pods don’t finish, new ones still start.

4. No Resource Limits:

   • Pods consume unbounded resources, choking out other workloads.

5. Job Failures with RestartBackoff:

   • Failed jobs retrying with exponential backoff may flood the system.

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🔍 How We Identified the Root Cause

We used the following tools and steps:

1. Checked the CronJob Behavior

kubectl get cronjob <job-name> -n <namespace> -o yaml

We noticed:

• schedule: "* * * * *"

• No concurrencyPolicy defined

• No startingDeadlineSeconds set

2. Observed Pods

kubectl get pods -n <namespace> --selector=job-name=<job-name>

Hundreds of pods running or stuck in Running or Pending state.

3. Analyzed Logs

kubectl logs <cronjob-pod> -n <namespace>

Found the jobs were taking longer to complete or failing silently.

4. Checked Node Resources

kubectl describe node <node-name>

Revealed that all CPU/memory was consumed by these pods.

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✅ How We Fixed It

🔒 Step 1: Pause the CronJob

kubectl patch cronjob <job-name> -n <namespace> -p '{"spec" : {"suspend" : true }}'

This immediately stopped the creation of new pods.

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🛠 Step 2: Clean Up Old Pods

kubectl delete jobs --selector=job-name=<job-name> -n <namespace>
kubectl delete pods --selector=job-name=<job-name> -n <namespace>

Freed up space on nodes.

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⚙️ Step 3: Update CronJob Configuration

spec:
  schedule: "*/5 * * * *"  # Less frequent
  concurrencyPolicy: Forbid  # Prevents overlapping runs
  startingDeadlineSeconds: 60  # Prevents old job flooding after downtime
  jobTemplate:
    spec:
      backoffLimit: 1
      activeDeadlineSeconds: 300  # Ensure job doesn’t run forever
      template:
        spec:
          containers:
          - name: job
            image: my-image
            resources:
              limits:
                cpu: "500m"
                memory: "512Mi"
              requests:
                cpu: "100m"
                memory: "128Mi"

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🚦 Step 4: Re-enable the CronJob After Fix

kubectl patch cronjob <job-name> -n <namespace> -p '{"spec" : {"suspend" : false }}'

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🔒 Proactive Measures to Avoid This in Future

• ✅ Always set concurrencyPolicy: Forbid or Replace

• ✅ Use startingDeadlineSeconds to limit backlogged job execution

• ✅ Set resource limits and requests

• ✅ Set activeDeadlineSeconds to prevent runaway jobs

• ✅ Enable alerts in Prometheus/Grafana for:

  • Pod count spikes

  • Node resource saturation

  • Job duration anomalies

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📘 Final Thoughts

ArgoCD may show “green” for a CronJob because it doesn't monitor runtime behavior. Always complement GitOps with runtime observability like Prometheus, Loki, and Grafana. This issue taught us the importance of:

• Setting sane defaults for jobs

• Monitoring job behavior post-deployment

• Preparing for edge cases like backlog and concurrency

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💬 Have you faced a similar issue? How did you fix it? Let’s connect in the comments!

🛡️ Stay vigilant, and may your CronJobs be lean and reliable!

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