Pricing Misconception — AWS Developer (DVA-C02)
The answer assumed a pricing model that doesn't apply. Reserved vs. on-demand, data transfer costs, or tier thresholds tripped you up.
Free Tier Is a Starting Point, Not a Billing Guarantee
A proof-of-concept labelled 'free tier' can still generate real charges once the workload shape is examined. Lambda's free tier covers one million requests and 400,000 GB-seconds of compute per month, applied separately. A function using 1,024 MB of memory for one second consumes one GB-second per invocation, so 400,000 such invocations exhaust the compute allowance regardless of how many requests remain in the one-million allowance. Checking free-tier fit means multiplying memory by duration by invocation count and comparing each product to its own separate limit. DynamoDB, S3, and API Gateway each carry independent free-tier quotas that expire on the same monthly reset cycle.
The Scenario
A startup wants to reduce costs for a development environment running 8 hours per day, 5 days per week. You recommend Reserved Instances for the 40% savings. But RIs commit you to paying 24/7 whether the instance runs or not. For a workload running 29% of the time, you are paying for 71% idle hours. On-Demand with EC2 Instance Scheduler (stop/start automation) or even Spot Instances for fault-tolerant dev workloads cost less. The trap is applying the "RIs always save money" rule without calculating the break-even utilization point — which for 1-year No Upfront RIs is roughly 40-50% utilization.
How to Spot It
- •When you see "reduce costs," do not default to Reserved Instances. Calculate the actual utilization: hours running / hours in period. RIs only save money when utilization exceeds the break-even point for that commitment type. Dev environments, batch jobs, and seasonal workloads rarely qualify.
- •NAT Gateway data processing charges are $0.045/GB — a 1TB/month workload costs $45/month just in NAT processing, on top of the $0.045/hour fixed cost. Questions that describe high-throughput private subnets accessing the internet are testing whether you know NAT Gateway is not "free" infrastructure.
- •Data transfer across AZs is $0.01/GB each way. A chatty microservices architecture with 100GB/month cross-AZ traffic adds $2/month per direction — invisible until you multiply by dozens of services. The exam tests whether you spot cross-AZ transfer costs in architectures that replicate data between availability zones.
Decision Rules
Determine whether increasing memory allocation reduces net GB-second billing cost for a CPU-bound Lambda function by cutting duration proportionally more than it raises the per-unit memory rate.
Domain Coverage
Difficulty Breakdown
Related Patterns