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4,874 incidents.
Solve them yourself, from root cause to prevention.

CrashLoopBackOff, DNS resolution failures, permission errors, deploys that went green but never shipped — read real-world incidents as scenarios and write the cause, recovery, and prevention yourself.

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One error message, four options. Solve without signing up and read why each option is right or wrong, free.

CI/CD Multiple choice 3 min Self-hosted runner: No space left on device in the middle of builds K8s Multiple choice 3 min Ingress 404 Not Found: only the newly added www host returns 404 Linux Multiple choice 3 min command not found only under cron: the script works when run by hand Network Multiple choice 3 min Connection refused only from other servers: curl works on the host Security Multiple choice 3 min Mixed Content: a chat widget disappears after switching to HTTPS CI/CD Multiple choice 3 min docker build invalid reference format: the image tag is empty in GitHub Actions

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1

Read the scenario

Gauge the blast radius from the logs and symptoms

Run npm ci
Cache restored from key: npm-Linux-9f3c…
npm ERR! `npm ci` can only install packages when your package.json and package-lock.json are in sync.
Error: Process completed with exit code 1.

2

Write three answers

Cause analysis · Recovery plan · Prevention

3

Compare with the model answer

See the signals and ordering you missed

Problems by area

Linux total 1,122 · free 264

FreeBeginner16 min An inode problem where files cannot be created even though disk space is free ProIntermediate18 min A short journalctl retention policy means logs are already gone right after an incident

CI/CD total 1,007 · free 283

FreeIntermediate20 min Finding the real failure point in GitHub Actions build logs ProAdvanced25 min Recovering a container deployment where the latest tag blocks rollback

K8s total 944 · free 270

FreeBeginner18 min Organizing the first checkpoints for a CrashLoopBackOff Pod ProIntermediate22 min Analyzing a service failure where the Pod is Running but receives no traffic

Network total 930 · free 237

FreeBeginner17 min Capturing HTTP traffic with tcpdump and reading the request flow ProAdvanced30 min Port exhaustion in a high-request segment and kernel tuning

Security total 871 · free 226

FreeBeginner16 min The SSH management path is blocked after an overly narrow allowlist change ProIntermediate20 min IAM role rotation leaves one batch worker using stale credentials

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K8s What to check first when CrashLoopBackOff appears K8s The registry and Secret to check first for ImagePullBackOff K8s How to isolate the root cause of ImagePullBackOff K8s When the Pod is Running but only Service traffic fails K8s When CoreDNS is Running but only DNS lookups fail CI/CD When GitHub Actions succeeds but only the rollout fails CI/CD When the CI cache restores the wrong dependency graph K8s How to check Helm values merge and environment override drift

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