SAA-C03 Design Cost-Optimized Architectures practice questions

Design Cost-Optimized Architectures is 20% of the AWS Certified Solutions Architect – Associate (SAA-C03) exam, the lightest of its 4 domains: roughly 10 of the 50 scored questions, and every 10% of accuracy here moves your scaled score about 20 points. Below are 7 free questions from this domain only, each with the answer explained and why every wrong option misses. Last updated 2026-07-03.

Every question is verified against official AWS documentation, linked below each answer.

  1. Question 1Design Cost-Optimized Architectures

    A team stores raw clickstream data in Amazon S3 and queries it with Amazon Athena. They currently store data as uncompressed JSON. Queries scan the entire dataset and monthly Athena charges are high. The dataset is written once and read many times by analysts running column-specific queries. Which change most reduces Athena costs?

    Reveal answer & explanation

    Answer: B. Convert the data to Apache Parquet format, partitioned by date

    Athena bills by the amount of data scanned per query. Converting to columnar Parquet lets Athena read only the columns a query references instead of full rows, and partitioning by date lets it skip whole partitions that don't match the query's filter. Together these cut scanned bytes — and cost — by 90% or more.

    Go deeper: Why does Athena charge based on bytes scanned rather than query complexity? How does partitioning let Athena skip data before it even reads it?

    Source: official AWS documentation · verified 2026-07-03

  2. Question 2Design Cost-Optimized Architectures

    A company runs 500 AWS Lambda functions across three AWS accounts. Functions have diverse memory settings (128 MB to 3 GB) and duration profiles. The team wants a systematic, data-driven way to right-size all Lambda functions to minimize cost without manually profiling each function. Which AWS-native approach is most effective?

    Reveal answer & explanation

    Answer: A. Use AWS Compute Optimizer to review Lambda memory and duration recommendations for each function

    Compute Optimizer analyzes each function's invocation history and recommends a memory size that minimizes cost while preserving performance, accounting for the fact that Lambda bills memory times duration so more memory can sometimes lower total cost by shortening runtime. It works across multiple accounts when integrated with AWS Organizations, giving a systematic, data-driven view instead of manual profiling.

    Go deeper: Why can raising a function's memory sometimes lower its total cost? What would happen to a CPU-bound function's duration if you set it to the minimum memory?

    Source: official AWS documentation · verified 2026-07-03

  3. Question 3Design Cost-Optimized Architectures

    A company runs an Amazon OpenSearch Service domain with 6 data nodes for a logging platform. Logs older than 14 days are rarely queried but must be searchable within minutes if needed. Storage costs for the hot nodes are growing rapidly. What is the most cost-effective way to reduce OpenSearch storage costs without losing searchability?

    Reveal answer & explanation

    Answer: A. Enable UltraWarm nodes and move indices older than 14 days to UltraWarm storage

    UltraWarm moves indices onto S3-backed warm storage that costs significantly less per GB than hot EBS-backed nodes, while keeping those indices searchable — queries are just slower, but still return within minutes.

    Go deeper: Why is S3-backed warm storage so much cheaper than EBS-backed hot storage? What tradeoff are you accepting when data moves from hot to warm storage?

    Source: official AWS documentation · verified 2026-07-03

  4. Question 4Design Cost-Optimized Architectures

    A company is running hundreds of EC2 instances across multiple AWS regions. The billing team notices that several Elastic IP addresses are allocated but not attached to any running instance, generating unnecessary charges. What is the most direct action to eliminate these charges?

    Reveal answer & explanation

    Answer: C. Release all unattached Elastic IP addresses

    AWS charges for Elastic IP addresses that are allocated but not associated with a running instance. Releasing the unattached ones stops those charges immediately — it's the most direct fix.

    Go deeper: Why does AWS charge for an idle Elastic IP but not for one attached to a running instance? What's the difference between an EIP being 'allocated' and being 'associated'?

    Source: official AWS documentation · verified 2026-07-03

  5. Question 5Design Cost-Optimized Architectures

    A company runs an AWS Fargate service with tasks that each require 4 vCPU and 8 GB memory. Tasks run continuously. The engineering team has just determined that actual CPU usage averages 1 vCPU and memory averages 2 GB. Which two changes together minimize Fargate cost? (Choose the single answer that captures both.)

    Reveal answer & explanation

    Answer: A. Right-size tasks to 1 vCPU / 2 GB and purchase Compute Savings Plans

    Right-sizing the tasks from 4 vCPU/8 GB down to 1 vCPU/2 GB cuts the Fargate resource cost by roughly 75%, matching provisioned capacity to actual usage. Layering a Compute Savings Plan on top adds a further discount (up to about 17%) with full flexibility, without introducing interruption risk for a continuously running service.

    Go deeper: Why does right-sizing produce a bigger saving here than a Savings Plan alone? What makes a workload a poor fit for Spot capacity?

    Source: official AWS documentation · verified 2026-07-03

  6. Question 6Design Cost-Optimized Architectures

    A company hosts a popular read-heavy web application backed by Amazon RDS for MySQL. They add multiple read replicas to scale read traffic, but their RDS bill increases significantly. Analysis shows that 70% of read queries repeatedly request the same rows. What is the most cost-effective way to reduce the number of read replicas needed?

    Reveal answer & explanation

    Answer: B. Place an Amazon ElastiCache for Memcached cluster in front of RDS to cache frequent reads

    Since 70% of reads target the same rows, putting a cache in front of RDS lets the majority of reads be served from memory instead of the database, dramatically reducing load and allowing the number of read replicas — and their cost — to be reduced.

    Go deeper: Why does caching repeated reads reduce the number of replicas you need? What kind of read pattern makes a cache especially effective?

    Source: official AWS documentation · verified 2026-07-03

  7. Question 7Design Cost-Optimized Architectures

    A company uses AWS CloudTrail to record API activity across all accounts in an AWS Organization. They enable CloudTrail in every region and every account independently, each writing to its own S3 bucket. The storage and duplication costs are high. What is the most cost-effective way to centralize and reduce CloudTrail costs across the organization?

    Reveal answer & explanation

    Answer: A. Create an organization trail in the management account that logs all accounts and regions to a single S3 bucket

    An AWS Organizations trail is a single CloudTrail configuration in the management account that automatically covers every existing and future account and region, writing to one central S3 bucket. This eliminates the duplicate per-account trails and their storage and API costs, and consolidating into one trail maximizes the free first copy of management events.

    Go deeper: Why does consolidating into one organization trail reduce cost compared to per-account trails? What would you lose by disabling CloudTrail in all but one region?

    Source: official AWS documentation · verified 2026-07-03

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