Incident: Redis Cache Failure Causes Cascading Database Load
Incident Summary Date: 2025-09-05 Time: 09:45 UTC Duration: 1 hour 32 minutes Severity: SEV-1 (Critical) Impact: Severe performance degradation affecting 85% of users
Quick Facts Users Affected: ~8,500 active users (85%) Services Affected: Web Application, Mobile API, Admin Dashboard Response Time: P95 latency increased from 200ms to 45 seconds Revenue Impact: ~$18,000 in lost sales and abandoned carts SLO Impact: 70% of monthly error budget consumed Timeline 09:45:00 - Redis cluster health check alert: Node down 09:45:15 - Application latency spiked dramatically 09:45:30 - PagerDuty alert: P95 latency > 10 seconds 09:46:00 - On-call engineer (Sarah) acknowledged alert 09:47:00 - Database CPU spiked to 95% utilization 09:48:00 - Database connection pool approaching limits (180/200) 09:49:00 - User complaints started flooding support channels 09:50:00 - Senior SRE (Marcus) joined incident response 09:52:00 - Checked Redis status: Master node unresponsive 09:54:00 - Identified: Redis master failure, failover not working 09:56:00 - Incident escalated to SEV-1, incident commander assigned 09:58:00 - Attempted automatic failover: Failed 10:00:00 - Decision: Manual promotion of Redis replica to master 10:03:00 - Promoted replica-1 to master manually 10:05:00 - Updated application config to point to new master 10:08:00 - Rolling restart of application pods initiated 10:15:00 - 50% of pods restarted with new Redis endpoint 10:18:00 - Cache warming started for critical keys 10:22:00 - Database load starting to decrease (CPU: 65%) 10:25:00 - P95 latency improved to 3 seconds 10:30:00 - All pods restarted, cache rebuild in progress 10:40:00 - P95 latency down to 800ms 10:50:00 - Cache fully populated, metrics returning to normal 11:05:00 - P95 latency at 220ms (near baseline) 11:17:00 - Incident marked as resolved 11:30:00 - Post-incident monitoring confirmed stability Root Cause Analysis What Happened The production Redis cluster consisted of 1 master and 2 replicas running Redis Sentinel for high availability. On September 5th at 09:45 UTC, the Redis master node experienced a kernel panic due to an underlying infrastructure issue.
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