Ensuring Ordered Order Processing with SQS FIFO

SAA-C03 By islever Editorial Last updated

Design scalable and loosely coupled architectures. Design secure workloads and applications.

Short answer

B Integrate the API Gateway REST API with an Amazon SQS FIFO queue that invokes AWS Lambda, guaranteeing orders are processed in arrival order.

Question

A company is building an ecommerce web application on AWS. The application sends information about new orders to an Amazon API Gateway REST API to process. The company wants to ensure that orders are processed in the order that they are received.
Which solution will meet these requirements?
  1. Use an API Gateway integration to publish a message to an Amazon Simple Notification Service (Amazon SNS) topic when the application receives an order. Subscribe an AWS Lambda function to the topic to perform processing.
  2. Use an API Gateway integration to send a message to an Amazon Simple Queue Service (Amazon SQS) FIFO queue when the application receives an order. Configure the SQS FIFO queue to invoke an AWS Lambda function for processing. Correct Answer
  3. Use an API Gateway authorizer to block any requests while the application processes an order.
  4. Use an API Gateway integration to send a message to an Amazon Simple Queue Service (Amazon SQS) standard queue when the application receives an order. Configure the SQS standard queue to invoke an AWS Lambda function for processing.

Key Takeaways

The question tests whether you can recognize that strict ordering requires an SQS FIFO queue, and the trap is picking a standard queue or SNS topic that provide no ordering guarantee at all.

This SAA-C03 question asks how an ecommerce application can guarantee that orders arriving through an Amazon API Gateway REST API are processed in exact arrival order. The correct design routes each order from an API Gateway integration into an Amazon SQS FIFO queue whose queue trigger invokes AWS Lambda, because FIFO queues are the only option listed that preserves strict first-in, first-out ordering.

Common mistake: Option D is the most tempting wrong answer: an SQS standard queue with Lambda invocation looks architecturally identical, but standard queues only offer best-effort, out-of-order delivery, so duplicate or late-arriving orders could be processed out of sequence.

Expert Analysis

Why the Answer Is Correct

Option B sends each new order through an API Gateway integration into an Amazon SQS FIFO queue, which guarantees strict first-in, first-out ordering of messages, and the queue is configured to trigger an AWS Lambda function for processing. FIFO queues support message grouping and ordering, exactly matching the requirement that orders be processed in the order they are received. The queue also decouples intake from processing, providing buffering and retries without blocking the API. This design satisfies the ordering requirement while remaining loosely coupled and scalable.

Why the Other Options Are Wrong

Option A uses a standard Amazon SNS topic, a pub/sub notification service that neither stores messages durably for ordered replay nor guarantees delivery order to subscribers. Option C blocks all requests through an API Gateway authorizer while processing, which serializes the whole API and destroys availability — it neither preserves queue order nor is a sensible architecture. Option D uses an SQS standard queue, which offers high throughput but only best-effort ordering, so messages can be delivered out of order and the stated requirement is not met. As PhucVuu explained, SNS is a notification/pub-sub mechanism that does not retain data in a queue, while the SQS standard queue simply lacks ordering guarantees.

Community Comment Notes

The community converged overwhelmingly on B, with Sinaneos noting "B because FIFO is made for that specific purpose" and rein_chau adding that an "SQS FIFO queue guarantees message order." Guru4Cloud walked through why API Gateway should integrate with the FIFO queue so processing is "guaranteed to be sequential." Andreshere pointed out that even an SNS FIFO option was not what the question described, and that a queue suits this task better than a subscription service. Two commenters, lrraju and darkknight23, asked whether the site's displayed answer could be trusted over the vote majority; here the vote majority and the AWS ordering semantics both point to B.

Official Reference

https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/FIFO-queues.html

This link opens the official vendor documentation in a new tab.

Exam Strategy

When a question demands strict ordering, latency sequencing, or exactly-once processing, scan the options for SQS FIFO — it is the standard AWS answer. Dismiss SNS and standard SQS immediately, since neither guarantees order.

Frequently Asked Questions

Why is an SQS standard queue wrong when it can also trigger Lambda?

Standard queues provide high throughput but only best-effort ordering, so messages may be processed out of sequence. The requirement here is strict arrival-order processing, which only a FIFO queue satisfies.

Why can't an SNS topic preserve order for these orders?

SNS is a pub/sub notification service that fans out messages to subscribers without durable, ordered queuing. A standard SNS topic has no ordering guarantee, so a FIFO queue is required.

More SAA-C03 FAQ →

Discussion 148 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 30 of 148

  1. Sinaneos 👍 70 Answered B
    B because FIFO is made for that specific purpose
  2. rein_chau 👍 28 Answered B
    Should be B because SQS FIFO queue guarantees message order.
  3. PhucVuu 👍 12 Answered B
    Keywords: - Orders are processed in the order that they are received. A: Incorrect - SNS just for notification like send email, SMS. It don't retain the data in the queue and it's used pub-sub pattern. B: Correct - SQS FIFO will help message process in order. FIFO -> first in first out. C: Incorrect - with this solution we will create blocker app not good app =)) D: Incorrect - SQS standard don't guarantee the order.
  4. MoMoCh4n 👍 8 Answered B
    I think the wrong answers are here intentionally as a way for us to deep dive into the topic and gain more knowledge or maybe I'm giving them too much credit.
  5. tuloveu 👍 8 Answered B
    To processed in the order that they are received.
Show 25 more comments
  1. lrraju 👍 7
    All - please help. I am taking this test, but my answers mostly align with most voted. What's the right answer? Should we go with the most voted as correct answer or what the website suggests as the correct answer?
  2. Andreshere 👍 6 Answered B
    A. This option could be correct if we use SNS FIFO option, but this is not the case stated in the question. Additionally, this task is more efficient done by a queue than a subscription service. So, this option is not correct. B. We use a queue, which is efficient processing messages. Additionally, it preserves the order since it is a FIFO queue. Meanwhile we don’t have any kind of messages throughput limitation, this option is correct. C. This option discards any messages that are still processing, which is not a good solution. D. Same option as B but using a normal queue, which does not preserve the order. Incorrect.
  3. Guru4Cloud 👍 6 Answered B
    Explanation: - Amazon API Gateway will be used to receive the orders from the web application. - Instead of directly processing the orders, the API Gateway will integrate with an Amazon SQS FIFO queue. - FIFO (First-In-First-Out) queues in Amazon SQS ensure that messages are processed in the order they are received. - By using a FIFO queue, the order processing is guaranteed to be sequential, ensuring that the first order received is processed before the next one. - An AWS Lambda function can be configured to be triggered by the SQS FIFO queue, processing the orders as they arrive
  4. darkknight23 👍 6 Answered B
    I am new to this site and lost....Will appreciate if someone can explain why is "correct answer" shown in the solution always wrong?
  5. gx2222 👍 5 Answered B
    Explanation: To ensure that orders are processed in the order that they are received, the best solution is to use an Amazon SQS FIFO (First-In-First-Out) queue. This type of queue maintains the exact order in which messages are sent and received. In this case, the application can send information about new orders to an Amazon API Gateway REST API, which can then use an API Gateway integration to send a message to an Amazon SQS FIFO queue for processing. The queue can then be configured to invoke an AWS Lambda function to perform the necessary processing on each order. This ensures that orders are processed in the exact order in which they are received.
  6. cookieMr 👍 5 Answered B
    Option B: Using an API Gateway integration to send a message to an Amazon SQS FIFO queue is the recommended solution. FIFO queues in Amazon SQS guarantee the order of message delivery, ensuring that orders are processed in the order they are received. By configuring the SQS FIFO queue to invoke an AWS Lambda function for processing, the application can process the orders sequentially while maintaining the order in which they were received.
  7. To_mi 👍 5
    Who decides what option is marked as a correct answer? Most of them are incorrect. The correct answer to this question is B.
  8. LuckyAro 👍 4
    I would go with most voted because there have been times when the Admin's answered have been blatantly wrong and makes me question Admin's handle of the subject matter. 100% of us can't be wrong simultaneously.
  9. Buruguduystunstugudunstuy 👍 3 Answered B
    CORRECT ANSWER Option B, using an API Gateway integration to send a message to an Amazon Simple Queue Service (SQS) FIFO queue when the application receives an order and configuring the SQS FIFO queue to invoke an AWS Lambda function for processing, would meet the requirements specified in the prompt. SQS FIFO queues ensure that messages are processed in the order that they are received, and API Gateway integrations allow you to send messages to an SQS queue when certain events occur, such as when the application receives an order. By configuring the SQS FIFO queue to invoke an AWS Lambda function for processing, you can ensure that orders are processed in the order that they are received.
  10. MGKYAING 👍 3 Answered B
    Explanation: FIFO Queues: Amazon SQS FIFO queues are specifically designed to maintain the order of messages. This ensures that messages are processed in the exact sequence they were sent, which is crucial for order processing in an e-commerce application. Why other options are less suitable: A. Amazon SNS: SNS is a publish-subscribe service. It doesn't guarantee message order. C. API Gateway Authorizer: Authorizers are used for authentication and authorization, not for managing message order. D. Amazon SQS Standard Queue: Standard SQS queues do not guarantee message order.
  11. dexpos 👍 3
    Hi folks, I just asked to the support about having different suggested answer vs the one selected from the community and the reply is: "If the provided answer is not correct or against the discussion then you can go with the user voted ones." . Community wins if we agree :). Hope this helps.
  12. Nandan747 👍 3 Answered B
    I don't understand why ALMOST EVERY QUESTION has a wrong answer highlighted. Clearly something is NOT right. And the answer is B, because requirement clearly states order to be followed and SQS FIFO ensures that.
  13. ondan 👍 3
    Standard queues ensure at-least-once message delivery, but due to the highly distributed architecture, more than one copy of a message might be delivered, and messages may occasionally arrive out of order.
  14. MakaylaLearns 👍 3
    I made two short videos explaining why the answer is B Also we should note that sometimes SNS loses the message whilst SQS will hold onto the message until it can send it. These videos are super quick!
  15. dbs6339 👍 3 Answered A
    Answer is A, this is the order process. when ecommerce application an order topic occured. many consumer api subscribe this topic. so is it not proper using SQS. SNS using is the best.
  16. avrk 👍 3
    For most of the questions there is correct answer and votes given by group. Which one is the correct answer to consider as i am getting confused. who has given correct answer?
  17. goodmail 👍 3
    Answer is B SNS has FIFO topics for ordering and deduplication, but the answer does not mention that. So answer A can be assumed to be standard one without ordering feature.
  18. jaradat02 👍 3 Answered B
    what's up with all these strange answers!!!! I chose B, SQS FIFO is designed for this exact use case, why does the author claim that A is the correct answer??
  19. hro 👍 3
    B SNS - PubSub (think pass along) SQS - Queueing (think batch) Kinesis - Real-time SWF - Queuing but with people
  20. smash_aws 👍 3
    SQS queues sure can be configured to trigger functions
  21. Zerotn3 👍 3
    same my confusing :(
  22. dbs6339 👍 2
    addtionally add more detail about the Rest api from question. the SQS FIFO is not proper option. because SQS is porpose one consumer. the rest api is consume many request so this case is correct that the SNS. SNS publish a topic that is fan out for many subscribers through some transfer methods. It's Could be SQS if after the SNS from the API gateway.
  23. Lonojack 👍 2
    I think the highlighted answers are to throw off the Bots or people trying to copy the admins materials and Sell it on another platform. That’s the only thing that makes logical sense to me. Btw, Correct answer is obviously (B) 😉
  24. wacze 👍 2
    Answer is A because SNS is typically used for applications that need real time notifications. considering this (The application sends information about new orders to an Amazon API Gateway REST API to process")
  25. d675329 👍 2 Answered B
    I'll go ahead with B. But my grouse with Option B is it says "Configure the SQS FIFO queue". You configure the Lambda, not the SQS queue. The role for Lambda to access SQS is auto-configured by AWS.

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