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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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Check your cluster for this

This case involves Backbone network (WAN) and BGP routing. kubegap audits a Kubernetes cluster for gaps like these, read-only, from Claude Code or Codex; its checks cite public reports like the ones in this library.

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By root cause1

Dependency failure1 case

Component
Backbone network (WAN)BGP routingIn-memory cacheDatabase replication
Contributing factor
Signal buried in noiseRetry stormSingle point of failure
Organisation
37signals
  1. IL-0255 Basecamp's two redundant links between data centers dropped together and a Redis replica resync loop stalled chat
    37signals2020