Designing for failure: Building resilient systems on AWS

This article explains how to build resilient distributed systems on AWS by learning from a real incident where three Cassandra nodes failed simultaneously due to physical collocation on the same hardware.

• Use Partition Placement Groups to distribute database replicas across separate physical racks, preventing correlated failures
• Monitor system status checks with CloudWatch composite alarms to detect hardware failures within minutes
• Automate instance recovery using Auto Scaling groups with lifecycle hooks for stateful systems like Cassandra
• Build automated incident response workflows with AWS Systems Manager to preserve logs while enabling fast replacement
• Invest in observability from day one using CloudWatch agents, synthetics, and X-Ray tracing across all services
• Test resilience regularly with disaster recovery drills and AWS Fault Injection Service to uncover hidden weaknesses

Resilience is an ongoing practice requiring layered defenses, proactive testing, and blameless post-mortems to continuously improve system reliability.