Reliability Improvement Strategy — AWS Solutions Architect Pro (SAP-C02)
One NAT Gateway Per AZ; Route Tables Must Match Locality
A single NAT Gateway in one AZ handles outbound traffic for private subnets in that AZ without issue. Private subnets in other AZs that route to the same NAT Gateway send traffic cross-AZ, creating an AZ dependency: if that AZ becomes unavailable, all private subnets lose outbound internet access regardless of which AZ they occupy. Adding a second NAT Gateway in the same AZ doubles compute capacity but does nothing to eliminate the cross-AZ routing dependency. True AZ-level resilience requires one NAT Gateway per AZ with each private subnet's route table pointing to the gateway in its own AZ. SAP-C02 tests whether you understand that cross-AZ routing dependencies are themselves single points of failure.
What This Pattern Tests
The exam tests whether you improve reliability through multiple mechanisms. Multi-AZ deployment provides redundancy. ELB health checks detect unhealthy instances. Auto Scaling replaces failed instances automatically. Route 53 health checks trigger DNS failover. AWS Fault Injection Simulator (FIS) tests your resilience by injecting failures (stop instances, increase latency, throttle APIs). The trap is adding redundancy without health checks (failed instances still receive traffic) or health checks without auto-recovery (you detect the problem but do not fix it).
Decision Axis
Reliability requires four layers: redundancy (survive failure), detection (find failure), recovery (fix failure), and testing (prove resilience).
Associated Traps
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Decision Rules
Whether the stated RTO < 30 min and RPO < 5 min targets can be fully satisfied by Multi-AZ redundancy combined with managed auto-scaling within a single Region, or whether cross-Region warm-standby architecture is necessary — with the 'minimal operational overhead' constraint serving as the decisive filter that eliminates cross-Region options before their resilience merits are evaluated.
Domain Coverage
Difficulty Breakdown