# Amazon SQS

> A waiting line for messages between programs, so none of them gets lost.

- Level: Intermediate
- Topics: AWS
- URL: https://tidbit.legios.com.ar/amazon-sqs/
- In Spanish: [Amazon SQS](https://tidbit.legios.com.ar/es/amazon-sqs/)

**Amazon SQS** (Simple Queue Service) is a managed message queue. One program (the producer) drops messages into the queue; another (the consumer) picks them up when it is ready. The two never talk directly and do not even need to be running at the same time.

It solves a common problem: one part of a system is faster than another. An online shop takes 5,000 orders in a minute during a sale, but the service that prints shipping labels handles 50 per minute. With a queue in the middle, orders wait in line instead of failing, and the label printer works through them at its own pace.

SQS keeps each message until a consumer deletes it. If the consumer crashes halfway, the message becomes visible again after a timeout and another consumer retries it. Messages that fail too many times are moved to a **dead-letter queue** for a human to inspect. It pairs naturally with AWS Lambda: a function can be triggered for every batch of messages that arrives.

There are two kinds: standard queues (very high throughput, order mostly preserved, a message may rarely be delivered twice) and FIFO queues (strict order, no duplicates, lower throughput).

## An analogy

The number dispenser at a deli counter. Customers take a ticket and wait; the staff calls the next number when free. Nobody gets lost, even when the line is long.

## Keep in mind

- Producer and consumer are decoupled: neither needs to know about the other or be online at the same time.
- A message stays in the queue until it is explicitly deleted, so a crashed consumer does not lose it.
- Standard queues favor throughput; FIFO queues guarantee order and no duplicates.
- Messages can wait in a queue for up to 14 days (4 by default).

## Go deeper

- [What is Amazon SQS? (official docs)](https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/welcome.html)
- [Amazon SQS product page](https://aws.amazon.com/sqs/)

## Related terms

- [AWS Lambda](https://tidbit.legios.com.ar/aws-lambda/): Run a piece of code only when something happens, with no server to manage.