A developer is designing a serverless application that customers use to select seats for a concert venue. Customers send the ticket requests to an Amazon API Gateway API with an AWS Lambda function th

Question

Options

• ASend the order ID to an Amazon Simple Notification Service (Amazon SNS) FIFO topic that fans
• BChange the Lambda function that generates the order ID to initiate the Lambda function for
• CSend the order ID to an Amazon Simple Notification Service (Amazon SNS) topic. Subscribe the
• DDeliver the order ID to an Amazon Simple Queue Service (Amazon SQS) queue. Configure the

• A
  69% (9)
• B
  8% (1)
• C
  15% (2)
• D
  8% (1)

Explanation

Option A uses an Amazon SNS FIFO topic (with SQS FIFO queues as subscribers for each Lambda function), which is the only option that satisfies both ordering and fan-out simultaneously. FIFO guarantees that the first-received order ID is always delivered first to both the inventory and payment functions, ensuring the seat goes to the rightful winner - and if that first payment fails, the next message in the queue (the second order) is processed instead.

Why the distractors fail:

• Option C (standard SNS topic) lacks ordering guarantees - messages can arrive out of sequence, so the "second" order could be processed before the "first."
• Option D (SQS queue alone) doesn't support fan-out to multiple consumers; a single SQS queue can't simultaneously drive both the inventory Lambda and the payment Lambda independently.
• Option B (Lambda invoking Lambda directly) creates tight synchronous coupling and has no built-in FIFO ordering or retry/fallback mechanism for the payment rejection scenario.

Memory tip: Think "FIFO = First In, First Out = First customer keeps the seat." Whenever an AWS question requires both strict ordering and fan-out to multiple services, the pattern is always SNS FIFO → SQS FIFO queues - SNS handles the broadcast, FIFO preserves the order.

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