SAA-C03 Design Resilient Architectures Practice Question
A patient portal must process every event at least once, but duplicate processing is acceptable if the consumer handles idempotency. Which eventing approach is most suitable? The design must avoid adding custom operational scripts.
⚠ Common exam trap
Watch out — candidates often confuse 'at-least-once' delivery with 'exactly-once' delivery and incorrectly choose a solution like a FIFO queue or a custom retry mechanism, but the question explicitly allows duplicate processing, making the standard queue the correct choice.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use Amazon SQS standard queue and design consumers to be idempotent
Amazon SQS standard queues guarantee at-least-once delivery, which satisfies the requirement that every event is processed at least once. The design avoids custom operational scripts by leveraging a fully managed service, and the acceptance of duplicate processing is handled by making consumers idempotent. This combination provides a scalable, resilient, and cost-effective event-driven architecture without the need for custom infrastructure management.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use an in-memory queue on one EC2 instance
Why it's wrong here
An in-memory queue living on a single EC2 instance cannot satisfy at-least-once delivery because any process crash, instance reboot, or hardware failure permanently clears the buffered events. It also creates a single point of failure that blocks the entire patient portal when that instance becomes unavailable, and scaling requires manual partitioning across additional instances without a durable shared store. For a healthcare workload, you need persistence and replica semantics that a volatile memory-only structure fundamentally lacks.
- ✗
Use UDP messages sent directly to workers
Why it's wrong here
UDP messages sent directly to workers do not provide at-least-once delivery because UDP is a connectionless, best-effort protocol with no acknowledgement, retransmission, or sequence tracking. Packets can be dropped silently by the network or receiver buffer overflow, and the sending service never learns whether a worker actually processed the event. Without built-in delivery guarantees, you would need to implement your own application-layer reliability, which defeats the purpose of using a simple datagram protocol for a critical event stream.
- ✓
Use Amazon SQS standard queue and design consumers to be idempotent
Why this is correct
Using an Amazon SQS standard queue meets the at-least-once requirement: SQS stores messages durably across multiple Availability Zones and redelivers any message that is not deleted before the visibility timeout expires, so every event is processed at least once. Because standard queues can occasionally deliver duplicate copies, your consumer design must be idempotent—e.g., by tracking a unique event ID or a deterministic processing key—so that repeated handling of the same event does not corrupt patient data. This combination gives you high throughput, operational simplicity, and no single point of failure, which is why it is the correct choice.
- ✗
Use CloudFront signed URLs
Why it's wrong here
CloudFront signed URLs are a mechanism for access control and content distribution—they let you restrict who can download objects from a CDN for a limited time. They have nothing to do with event delivery or message queuing, so they cannot provide at-least-once processing guarantees or any delivery semantics at all. Trying to use them for this purpose would be a category error, leaving your patient portal without any reliable channel for event transfer.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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