Cases2 cases

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1. IL-0435 Error rates rose as Aurora MySQL killed connections and declined queries to avoid OOM while load metrics stayed flat

   CyberAgent2026

   Symptoms

   An alert fired one day because an application's error rate had suddenly risen. The error log of the Aurora MySQL RDS instance for that time showed Aurora killing connections and declining queries to avoid running out of memory. The killed queries also made application latency worse.

   High error rateMemory pressure

   Trigger

   An application release added a batch job run every minute by a Kubernetes CronJob. Each run's short-lived pod connected to the database from a new pod IP.

   Root cause

   Capacity exhaustion

   Component

   DatabaseJobs and CronJobs

   Contributing factor

   Unreviewed defaultsChange with a delayed effect

   Detection

   An alert fired on the application's error rate.

   Remediation

   • Set performance_schema to manual management and capped two related Performance Schema parameters, unlimited by default, at 100 in the parameter group, then restarted the RDS instance

   Lessons

   When system alerts fire but the database's load metrics look calm, Performance Schema growth is worth suspecting. The two parameters that were capped are unlimited by default, so connections from ever-changing source addresses keep adding to the records. Capping them has a cost: once the cap is exceeded, per-host figures in Performance Insights are missing. Putting RDS Proxy between the application and Aurora MySQL to keep source hosts stable is another possible measure.

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2. IL-0055 A failing CronJob with the default concurrency policy piled up thousands of restarting pods and took down a cluster

   Target2018

   Symptoms

   A Kubernetes cluster went down after one CronJob kept failing and its pods piled up. Because each failed run kept restarting instead of being replaced by the next one, the pods accumulated into the thousands. The speaker's arithmetic for a job scheduled every minute that fails for three days is 4,320 restarting pods.

   Service outage

   Trigger

   A CronJob kept failing.

   Root cause

   Configuration change

   Component

   Jobs and CronJobsNode pool

   Contributing factor

   Unreviewed defaultsAutomation amplification

   Lessons

   When deploying a CronJob, set its concurrency policy to Replace or Forbid; with the default, failing runs accumulate and restart together. An application should handle a missing dependency with probes and bounded back-off rather than crash in a loop, which also keeps it from hammering a dependency that is already struggling.

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