AWS Serverless Messaging with Exactly-Once Delivery

SAA-C03 By islever Editorial Last updated

Design scalable and loosely coupled architectures. Design cost-optimized compute solutions.

Short answer

AD Use AWS Lambda for compute and Amazon SQS FIFO queues for messaging to ensure exactly-once delivery with minimal management.

Question

A company has a multi-tier payment processing application that is based on virtual machines (VMs). The communication between the tiers occurs asynchronously through a third-party middleware solution that guarantees exactly-once delivery. The company needs a solution that requires the least amount of infrastructure management. The solution must guarantee exactly-once delivery for application messaging. Which combination of actions will meet these requirements? (Choose two.)
  1. Use AWS Lambda for the compute layers in the architecture. Correct Answer
  2. Use Amazon EC2 instances for the compute layers in the architecture.
  3. Use Amazon Simple Notification Service (Amazon SNS) as the messaging component between the compute layers.
  4. Use Amazon Simple Queue Service (Amazon SQS) FIFO queues as the messaging component between the compute layers. Correct Answer
  5. Use containers that are based on Amazon Elastic Kubernetes Service (Amazon EKS) for the compute layers in the architecture.

Key Takeaways

The core trap is confusing SNS's broadcast capability with SQS FIFO's strict ordering and deduplication guarantees required for payment processing.

This question tests the design of a serverless architecture using AWS Lambda and Amazon SQS FIFO queues to achieve exactly-once delivery with minimal infrastructure management.

Common mistake: Candidates often select EKS (E) because it feels robust, or SNS (C) because it handles asynchronous messaging, but neither meets the 'least management' or 'exactly-once' criteria as effectively as Lambda + SQS FIFO.

Expert Analysis

Why the Answer Is Correct

To meet the requirement of the least amount of infrastructure management, serverless compute is the optimal choice; AWS Lambda eliminates the need to provision or maintain servers, making Option A superior to EC2 (B) or EKS (E). For messaging, the requirement is strictly 'exactly-once delivery.' While standard SQS offers at-least-once delivery, SQS FIFO (First-In-First-Out) queues are specifically designed to provide message deduplication and ordering, ensuring that each message is processed precisely once. Therefore, combining Lambda (A) with SQS FIFO (D) satisfies both constraints perfectly.

Why the Other Options Are Wrong

Amazon SNS (Option C) is a pub/sub service that does not guarantee message ordering or deduplication by default, meaning messages can be duplicated or lost in transit, failing the 'exactly-once' requirement. Amazon EC2 (Option B) requires significant infrastructure management (patching, scaling groups), violating the 'least management' constraint. Amazon EKS (Option E) involves managing Kubernetes clusters, which is far more complex and management-heavy than Lambda, also violating the infrastructure constraint.

Community Comment Notes

The community overwhelmingly supports AD. As user osmk noted, referencing official documentation confirms SQS FIFO provides exactly-once processing. User MatAlves highlighted that SNS cannot guarantee exactly-once semantics, reinforcing why C is incorrect. User Sivaeas succinctly summarized the pair as 'Lamdba+SQS FIFO'. One dissenting comment suggested CD, but this ignores the critical management overhead of running containers versus serverless functions.

Official Reference

These links open the official vendor documentation in a new tab.

Exam Strategy

Always map 'least infrastructure management' to Serverless (Lambda/Fargate) over VMs or Containers unless specific stateful requirements dictate otherwise. When 'exactly-once' or 'ordering' is mentioned alongside messaging, immediately think SQS FIFO over SNS or Standard SQS.

Frequently Asked Questions

Why isn't SNS sufficient for exactly-once delivery?

SNS is a publish-subscribe service that does not guarantee message ordering or deduplication. It may deliver duplicate messages or lose them under high load, failing the exactly-once requirement.

Can EKS provide exactly-once delivery with less management than Lambda?

No. EKS requires managing Kubernetes control planes, worker nodes, and configurations, which significantly increases infrastructure management compared to the fully managed Lambda service.

More SAA-C03 FAQ →

Community Votes

AD
100%

100% of anonymous learners picked answer AD. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 9 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 9

  1. osmk 👍 6
    https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/FIFO-queues-exactly-once-processing.html
  2. Scheldon 👍 5
    AnswerAD, SQS FIFO will guarantee exactly one time execution for each operation. The problem is with processing as we do not know if whole process will be closed in 15 min (TTL for Lambda). I'm choosing Lambda as it is natural thing for Payment procesing in AWS but I'm not 100% sure
  3. MatAlves 👍 2
    "As its name suggests, exactly-once semantics means that each message is delivered precisely once. The message can neither be lost nor delivered twice (or more times)." - SNS doesn't provide exactly-once delivery. Thus, we need SQS. - To achieve "least amount of intra management", we go with Lambda for compute layer.
  4. PolarFox 👍 2
    someone please explain why the combination of D and E is not the correct?
  5. jaswantn 👍 2
    option A for payment processing. option D for exactly once delivery.
Show 4 more comments
  1. tch 👍 1
    SQS is helpful for Asynchronous processing workflows.
  2. Sivaeas 👍 4
    Lamdba+SQS FIFO
  3. hajra313 👍 3
    a and d
  4. Umuntu 👍 1
    CD IS THE BEST ANSWER

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