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1. IL-0255 Basecamp's two redundant links between data centers dropped together and a Redis replica resync loop stalled chat

   37signals2020

   Symptoms

   At 11:59AM CDT on Friday 28 August 2020, Basecamp's Ops team received a flood of alerts from PagerDuty, Nagios and Prometheus. Monitoring showed that the newer of the two metro links between the data centers in Ashburn, VA and Chicago, IL had been lost for about 20-30 seconds, and BC3 kept responding slowly after the link came back.

   Link flappingHigh latencyReplication lag

   Trigger

   Both redundant metro links between Basecamp's Ashburn and Chicago data centers dropped for short periods; monitoring showed the newer link down for about 20-30 seconds.

   Root cause

   Dependency failure

   Component

   Backbone network (WAN)BGP routingIn-memory cacheDatabase replication

   Contributing factor

   Single point of failureRetry stormSignal buried in noise

   Detection

   A flood of alerts from PagerDuty, Nagios and Prometheus, with monitoring showing the lost metro link. A lingering alert for one Redis DB instance later pointed to the replication problem.

   Remediation

   • Turned chat off at the load balancer layer so the rest of BC3 could stabilise
   • Restarted Redis on the replica that was stuck in the resynchronization loop

   Action items

   • Make the documented Redis configuration changes for the known replication issue
   • Make concrete plans to remove the shared physical path under the two metro links
   • Prove the failover systems in production at peak load
   • Make explicit room for post-incident follow-up work and push other work out

   Lessons

   Planned redundancy and live testing had given Basecamp a false sense of security, especially at peak load: moving between redundant network links or shifting load across data centers should work without issue, and Basecamp would rather find out in a controlled test with the full team ready. The follow-up from an outage needs urgency and explicitly cleared time, not just a place on a list.

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