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1. IL-0362 PagerDuty could not dispatch notifications for 18 minutes when a peering point shared by two AWS regions degraded

   PagerDuty2013sev1

   Symptoms

   On Saturday 13 April 2013 at 8:11am Pacific Time, PagerDuty's on-call engineer was paged about one node of the notification dispatch system, and an attempt at 8:13 to bring the node back failed. Customers saw delayed notifications and 500 errors on PagerDuty's API endpoints.

   High latencyHigh error rateIntermittent connection failures

   Trigger

   At 7:57am Pacific Time, according to AWS, a peering point in Northern California began to degrade, causing connectivity problems for the two AWS regions that PagerDuty used as two of its three datacenters.

   Root cause

   Cloud provider outage

   Component

   Transit gateway or network interconnectCoordination and key-value store

   Contributing factor

   Single point of failureUnverified assumptionMonitoring blind spot

   Detection

   The PagerDuty on-call engineer was paged at 8:11am about an issue with one node of the notification dispatch system, and at 8:18 monitoring detected multiple-provider failures for notifications.

   Remediation

   • Restarted most of the failed coordinator processes by hand
   • The notification dispatch system reached quorum again and processed all queued notifications

   Action items

   • Add more logging and aggregate logs into a single searchable source
   • Add a process watcher that restarts failed coordinator processes automatically
   • Build a dashboard that shows inter-host connectivity
   • Train engineering staff on Cassandra and ZooKeeper
   • Investigate moving off one of the AWS regions to a new hosting provider and datacenter that avoids a single point of failure

   Lessons

   Datacenters chosen to be independent and physically separate can still share a network peering point, and a design built to survive the loss of one datacenter did not survive losing two at once.

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