Cases2 cases

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1. IL-0445 Chatwork's service on EKS became unstable as conntrack tables on some nodes overflowed and connections were dropped

   Chatwork2021

   Symptoms

   In 2020 Chatwork moved its legacy system from EC2 to EKS, running both sides in parallel with Route53 weighted records. Once more than a certain share of requests was sent to the EKS side, the service suddenly became unstable. On some nodes the conntrack table had gone past its limit and connections to the application were being dropped.

   Intermittent connection failuresDNS resolution failuresIntermittent timeoutsLoad imbalance

   Trigger

   During the migration from EC2 to EKS, Chatwork sent more than a certain share of requests to the EKS side. Later, applying node-local-dns with force_tcp in its configuration made DNS unstable again.

   Root cause

   Capacity exhaustion

   Component

   conntrack tableCoreDNSkube-proxyClient-side DNS resolver and cacheVPC DNS resolver

   Contributing factor

   Unreviewed defaultsUnrepresentative stagingUnknown limit

   Remediation

   • Raised the conntrack maxPerCore value in the kube-proxy configuration
   • Installed dns-autoscaler so CoreDNS ran with more pods
   • Installed node-local-dns as a DaemonSet on every node
   • Replaced force_tcp with prefer_udp in the node-local-dns configuration

   Lessons

   On EKS, dns-autoscaler and node-local-dns made DNS stable, but node-local-dns must not use force_tcp there. Stable DNS also matters for sizing the number of pods correctly, since blocked DNS queries had made the pod count grow.

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2. IL-0399 Applications on GOV.UK PaaS in AWS eu-west-1 failed DNS lookups intermittently for about two hours overnight

   Government Digital Service2016

   Symptoms

   On 15 December 2016 from 1.17am, VMs of the GOV.UK Platform as a Service (PaaS) in AWS eu-west-1 logged DNS resolution errors, and applications deployed on the PaaS failed DNS queries intermittently. Applications could fail to connect to their databases, backend services and APIs, and some end users of those applications would have seen intermittent slowness and errors between 1am and 3.17am.

   DNS resolution failuresIntermittent connection failuresMonitoring outage

   Root cause

   Cloud provider outage

   Component

   VPC DNS resolver

   Contributing factor

   Signal buried in noise

   Detection

   Datadog and Pingdom alerts fired during the night, but no one was on out-of-hours support. A PaaS team member noticed the warnings at 8am when starting work and checked monitoring to confirm the issue was over.

   Action items

   • Stop the monitoring system from sending emails when disk monitoring data is not being received
   • Make notifying tenants about outages easier and quicker
   • Implement alert notifications for out-of-hours support, shaped by this incident
   • Provide 24/7 support for the PaaS in line with the needs of adopting services

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