Cases2 cases

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1. IL-0493 Netlify's Google Cloud TCP load balancer stopped serving about 5% of global traffic as all backends failed health checks

   Netlify2021

   Symptoms

   At 14:39 UTC on March 25, 2021, Netlify's monitors alerted that the health check of one of its load balancers was failing. That Google Cloud TCP load balancer served the single static IP that customers use for A records, and about 5% of Netlify's global traffic stopped being served until 18:46 UTC.

   Probe failuresHigh error rate

   Trigger

   Unattended operating system updates, whose timing Netlify randomises across production nodes, restarted the systemd-networkd unit on each node over several hours, and each restart removed the custom route table entry that held the load balancer IP.

   Root cause

   Software defect

   Component

   Cloud load balancersystemd-networkd

   Contributing factor

   Monitoring blind spotOverlapping failures

   Detection

   Netlify's monitors alerted at 14:39 UTC that the load balancer health check was failing. The load balancer's own backend health checks were not part of Netlify's alerting, so the backends dropping out over the preceding hours raised no alert.

   Remediation

   • Gave enterprise customers a separate range of IPs on a new load balancer for enterprise traffic
   • Provisioned an AWS network load balancer backed by Global Accelerator and published its IP for self-serve customers
   • Restarted the GCP networking daemon on all affected backend nodes, which restored traffic on the Google load balancer

   Action items

   • Patch the google-guest-agent systemd configuration on all GCP CDN nodes to react to systemd-networkd restarts
   • Report the bug to Google and open an issue in the google-guest-agent repository
   • Include the load balancer's backend health checks in alerting
   • Add a canary node to the backend to test updates and watch for issues over several days
   • Decouple traffic tiers and move to multiple regions
   • Offer customers more ways to route apex domains to Netlify, such as ALIAS and ANAME records
   • Route enterprise traffic through its own IP

   Lessons

   Netlify's own node-level health checks could not see that the backends were failing the load balancer's health check, and alerting on the load balancer's backend health checks would have given more time to diagnose before the outage.

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2. IL-0333 A downsized RDS PostgreSQL database maxed out its CPU at US peak and failing ELB health checks took buildkite.com down

   Buildkite2016

   Symptoms

   On 22 August 2016 at 17:21 UTC buildkite.com went offline: nobody could log in, view build logs or read the documentation, although builds kept running and GitHub and Bitbucket pull request statuses kept updating. The Buildkite team in Melbourne was asleep and woke at 21:00 UTC, almost 4 hours later, to emails, tweets and Slack messages saying Buildkite was down.

   Service outageDatabase overloadProbe failuresCloud API rate limit exhausted

   Trigger

   Over a weekend maintenance window, Buildkite downgraded its m4.10xlarge Multi-AZ RDS PostgreSQL database to an r3.2xlarge Multi-AZ instance to cut its AWS bill, and moved to a centralised pgbouncer to pool connections. The first US weekday peak after the change pushed the new instance's CPU to its maximum at about 16:00 UTC.

   Root cause

   Capacity exhaustion

   Component

   DatabaseCloud load balancerLiveness and readiness probesNode launch template

   Contributing factor

   Unverified assumptionHealth check tied to a dependencyAutomation amplificationCircular dependencyPages not deliveredOverlapping failures

   Detection

   New Relic, Pingdom, Datadog, Calibre and CloudWatch raised PagerDuty alerts, but the on-call phone call never came and only silent push notifications arrived. The team noticed at 21:00 UTC when they woke to emails, tweets and Slack messages from users.

   Remediation

   • Moved servers from the Agent API to the dashboard ELB to bring buildkite.com back online
   • Switched the Auto Scaling Groups to EC2 health checks and locked in desired counts
   • Upgraded the database to an instance type that could handle the load, using the aws-cli
   • Built a new AMI that did not depend on buildkite.com/_secret/version and rolled out servers that could self-deploy
   • Removed the centralised pgbouncer and went back to pgbouncer on each host

   Action items

   • Plan load testing after significant infrastructure changes without affecting other users of the system
   • Make the ELB health check return OK when the app answers, without checking database connectivity
   • Consider a separate health check endpoint for Pingdom that checks the database and other dependencies
   • Store the latest deployed revision as a file on S3 for newly launched servers to self-deploy from
   • Test by hand that PagerDuty calls reach every on-call team member
   • Investigate regular company-wide tests of everyone's paging setup
   • Ask PagerDuty about calling members when they go on call or requiring an immediate phone rule

   Lessons

   Buildkite listed six lessons. Watch AWS credits, because billing reports show $0 while credits are spent, so the real cost is hidden and big changes end up rushed. Load test after significant infrastructure changes. Think about what a health check should do and what happens when every server behind an ELB fails it. Do not make new servers look up the deployed revision from the service they are replacing. Make sure every on-call member has the right PagerDuty and phone settings before going on call. When servers churn in AWS, stop health checks and autoscaling, for example with the Auto Scaling feature to suspend and resume processes, so the EC2 request limit is not used up.

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