Cases2 cases

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1. IL-0027 All CoreDNS pods landed on one node, the node ran out of memory, and DNS for the whole service went down

   Backpackr2025

   Symptoms

   Services on the production Amazon EKS cluster behind the idus marketplace threw intermittent errors whose cause was hard to pin down. Calls to external services such as RDS and Redis failed now and then, and whenever DNS traffic spiked, domain lookups answered slowly or failed outright.

   DNS resolution failuresIntermittent connection failuresHigh latencyOOMKilledService outage

   Trigger

   All CoreDNS pods were scheduled onto the same node, and that node's memory usage spiked until it ran out of memory.

   Root cause

   Capacity exhaustion

   Component

   CoreDNSClient-side DNS resolver and cachekubeletContainer memory limitNode pool

   Contributing factor

   Unreviewed defaultsSingle point of failureNo caching

   Detection

   The write-up does not say how the full outage was noticed. The earlier DNS trouble showed up as intermittent service errors, and investigating failed calls to RDS and Redis led to DNS resolution failures.

   Remediation

   • deployed NodeLocal DNSCache as a DaemonSet in front of CoreDNS
   • added a PodDisruptionBudget (maxUnavailable 1) and topologySpreadConstraints across zones for CoreDNS
   • lowered ndots from 5 to 2 with dnsConfig in the Deployments

   Lessons

   The CoreDNS setup that EKS creates by default (two pods) can be enough for a small or development cluster but becomes a bottleneck in a large production cluster, and DNS is a single point of failure that needs high availability and room to scale. Tuning the CoreDNS cache and buffer size only relieved the symptoms; the real load came from the extra queries that ndots:5 generates for external names. A CoreDNS pod without a memory limit can share a node's fate with everything else on it.

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2. IL-0206 Consul agents in GitLab.com's GKE pods were repeatedly OOM-killed, so the Rails pods lost database replica discovery and sent every query to the primary

   GitLab2022

   Symptoms

   On 2022-04-25 the Sidekiq Apdex (the share of background jobs that finish within a target time) dropped, and an engineer declared an incident in Slack at 15:13 UTC. Users saw intermittent slowness across GitLab.com, and asynchronous work such as pipeline creation and import and export took unusually long.

   High latencyDatabase overloadOOMKilledHigh error rate

   Trigger

   The Consul agent in GitLab.com's GKE pods had memory demand spikes that did not fit in the pods' 500MB memory limit, and the agent was killed again and again.

   Root cause

   Capacity exhaustion

   Component

   Container memory limitService discoveryApplication database

   Contributing factor

   –

   Detection

   An engineer declared the incident in Slack at 15:13 UTC after the Sidekiq Apdex dropped; the second event was found when monitoring detected an elevated error rate at 01:21 UTC on 2022-04-26.

   Remediation

   • planned at 16:13 to raise the per-user connection limit for the gitlab database user and the pgbouncer backend pools for the main database; changes were applied at 17:26 and confirmed at 17:47
   • raised the pod memory limits from 500MB to 1GB so the Consul agents survive memory spikes

   Action items

   • Monitor the Consul agents on GKE pods to see whether the 1GB memory limit keeps them healthy
   • Investigate the Consul memory demand spikes and what can be done beyond raising the hard memory limit

   Lessons

   When the local Consul agent that Rails uses for database service discovery keeps dying, Rails does not fail; it sends every query to the last known primary. What engineers saw was database saturation and slow background jobs, which pointed the investigation first at a recent pgbouncer pool change. The memory limit of the Consul agent in every pod therefore decided the load on the primary database.

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