Resilient Cloud Solutions
What to know
Resilience questions test multi-AZ vs. multi-Region design, Auto Scaling, load balancing, and failure isolation. Know the difference between high availability (survive AZ failure) and disaster recovery (survive Region failure), and the DR strategies: backup and restore, pilot light, warm standby, multi-site active/active - with RTO/RPO tradeoffs. Understand Auto Scaling groups (launch templates, scaling policies: target tracking, step, scheduled), health checks (EC2 vs. ELB), and how ASG replaces unhealthy instances. Know Route 53 routing policies (failover, weighted, latency, geolocation) and health checks. For data, know RDS Multi-AZ (synchronous standby, failover) vs. read replicas (asynchronous, scaling reads), and S3 durability/replication (CRR/SRR).
Traps
- RDS Multi-AZ is for availability, not read scaling; read replicas are for scaling, not automatic failover.
- ASG with ELB health checks replaces instances the load balancer marks unhealthy; EC2 health checks alone may miss app failures.
- Route 53 failover requires health checks on primary records.
- Cross-Region replication is asynchronous - RPO is nonzero.
Rule of thumb
Match the strategy to the RTO: minutes → warm standby or active/active; hours → pilot light; longest → backup and restore. Always spread across at least two AZs for HA.
Worked example
App must survive an AZ outage with minimal read latency: deploy ASG across 3 AZs behind an ALB, use RDS Multi-AZ for the primary, and add read replicas in other AZs for read traffic.