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1. IL-0383 PagerDuty's shared Cassandra cluster degraded during a repair, delaying notifications and losing incoming events

   PagerDuty2014sev1

   Symptoms

   On 3 June 2014, PagerDuty's notification pipeline performed poorly and some notifications were delayed after the Cassandra cluster behind it became heavily degraded. About 3% of events sent to the integration API could not be received.

   Database overloadHigh error rateHigh latency

   Trigger

   On 3 June 2014 the Cassandra background repair process started its normal run on one node of the cluster while other high load was on the cluster; on 4 June the team restarted the postponed repair by hand.

   Root cause

   Capacity exhaustion

   Component

   Database

   Contributing factor

   No headroomExcessive blast radiusHarmful manual actionIgnored early signal

   Remediation

   • Reduced the load on the Cassandra cluster
   • Stopped the Cassandra repair process
   • Cut communication between some Cassandra nodes to stabilise the cluster
   • On 4 June, deleted all in-flight data in the notification pipeline in a "factory reset"
   • Brought downstream systems back into a consistent state with the emptied pipeline by hand

   Action items

   • Scale the existing Cassandra nodes vertically to bigger, faster servers
   • Set up multiple Cassandra clusters and spread the workloads across them
   • Set system load thresholds at which the Cassandra clusters are scaled out horizontally ahead of need
   • Upgrade the current and new clusters to a more recent Cassandra version
   • Add more load shedding techniques to control Cassandra under high load
   • Bring more Cassandra expertise in-house

   Lessons

   PagerDuty concluded that its Cassandra cluster was underscaled and shared by too many services with different load and usage patterns, and that the scaling work should have come before the planned Cassandra version upgrade.

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