Answer: Amazon Simple Queue Service (Amazon SQS) FIFO queues

## Explanation **Correct Answer: D - Amazon Simple Queue Service (Amazon SQS) FIFO queues** ### Why this is correct: 1. **FIFO (First-In-First-Out) guarantee**: Amazon SQS FIFO queues provide exactly-once processing and preserve the order of messages. This is crucial for the requirement to "guarantee the order of the events." 2. **Concurrent processing**: The system needs to send events to multiple services (leaderboard, matchmaking, authentication) concurrently. With SQS FIFO queues, you can have multiple consumers processing messages in order. 3. **Event-driven architecture**: SQS is designed for decoupled, event-driven architectures where services can process messages asynchronously. ### Why other options are incorrect: **A. Amazon EventBridge event bus**: - EventBridge is excellent for event routing and integration with AWS services - However, it doesn't guarantee strict ordering of events across multiple consumers - Best for event-driven architectures where ordering isn't critical **B. Amazon Simple Notification Service (Amazon SNS) FIFO topics**: - SNS FIFO topics do provide ordering guarantees - However, they are designed for point-to-point messaging with ordering - The requirement mentions sending to "separate" services concurrently, which suggests multiple consumers - SNS FIFO topics with SQS FIFO queues as subscribers could work, but the question asks for the simplest solution **C. Amazon Simple Notification Service (Amazon SNS) standard topics**: - Standard topics don't guarantee ordering - Messages can be delivered out of order - Doesn't meet the requirement to "guarantee the order of the events" ### Key AWS Service Characteristics: - **SQS FIFO Queues**: Guaranteed ordering, exactly-once processing, message deduplication - **SNS FIFO Topics**: Ordering with message deduplication, but typically used with SQS FIFO queues as subscribers - **EventBridge**: Event routing without guaranteed ordering - **SNS Standard Topics**: No ordering guarantees ### Architecture Recommendation: For this scenario where you need to: 1. Send unique events 2. Process them in order 3. Distribute to multiple services concurrently The best solution is to use **Amazon SQS FIFO queues** where each service can have its own consumer processing messages in the correct order.

Author: LeetQuiz Editorial Team