4.1.1. Amazon SQS for Message Queues
First Principle: Amazon SQS provides a fully managed message queuing service that enables developers to decouple application components, improving fault tolerance, scalability, and asynchronous communication.
| Feature | SQS Standard | SQS FIFO | SNS | EventBridge |
|---|---|---|---|---|
| Pattern | Queue (pull) | Queue (pull) | Pub/sub (push) | Event bus (push) |
| Ordering | Best-effort | Guaranteed | No | No |
| Dedup | No | Yes | No | No |
| Throughput | Unlimited | 300 msg/sec* | Unlimited | Varies by target |
| Best for | Decoupling, buffering | Ordered processing | Fan-out notifications | Content-based routing |
*3,000 msg/sec with batching
SQS lets you send messages between application components without them needing to be available at the same time. It's a key tool for decoupling microservices and building event-driven architectures.
- Fully Managed: No servers to provision or manage.
- Decoupling: Producers and consumers can operate independently, even if one is temporarily unavailable.
- Scalable: Handles any volume of messages.
- Reliable: Messages are stored redundantly across multiple servers.
- Standard Queues: (Default) Offer high throughput and at-least-once delivery. Messages can be delivered out of order.
- FIFO (First-In, First-Out) Queues: Guarantee exactly-once processing and maintain message order. Ideal for operations where order and uniqueness are critical (e.g., financial transactions).
- Dead-Letter Queues (DLQs): (A queue that other (source) queues can target for messages that can't be processed successfully.) Messages that fail to be processed by consumers can be automatically moved to a DLQ for later investigation.
Scenario: You're developing an e-commerce application. When a customer places an order, the frontend needs to respond immediately, but payment processing and inventory updates can happen asynchronously. You want to ensure orders are not lost if the backend processing service is temporarily unavailable.
ā ļø Exam Trap: SQS Standard queues allow at least once delivery (duplicates possible). SQS FIFO queues guarantee exactly once processing and order. If a question requires no duplicates, FIFO is the answer ā but it has lower throughput (300 msg/sec without batching).
